Kurt E. Sickafus Los Alamos National Laboratory
Alexandra Navrotsky University of California-Davis
Simon R. Phillpot University of Florida
O1: Fluorites: General Structure/Property Relationships I
Tuesday PM, December 02, 2008
Fairfax B (Sheraton)
9:30 AM - **O1.1
Defect Chemistries in Fluorites and Related Materials.
Robin Grimes 1 , Kurt Sickafus 2 , Cleave Antony 1 , Blas Uberuaga 2 , Christopher Stanek 2 Show Abstract
1 Materials, Imperial College London, London United Kingdom, 2 MST-8, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Fluorite-related ceramics have attracted considerable attention as potential host materials for the immobilisation of radionuclides. Here we have used computer simulation to investigate a range of materials properties, as a function of composition, with a particular emphasis on defect processes. In particular, we have determined the relative energies of basic defect processes that occur in these materials as a consequence of radiation damage. Other factors important to defect evolution, such as the activation energy for oxygen migration, which is crucial in the process of damage recovery, have also been considered. Although there are many fluorite related structures, here we consider in detail three: fluorite itself (MO2), pyrochlore (A2B2O7) and the delta (δ) phase (A4B3O12). Each of the structures can be formed with a variety of compositions, which together provide a broad range of possible materials. In terms of trivalent cations, these range in size from Sc to La and in terms of tetravalent cations from Ti to Pb. Consequently, the results are presented in the form of property composition maps that are used to identify materials which show greatest overall potential.
10:00 AM - **O1.2
Neutron Scattering Studies of Fluorite-Structured Ordering of Hydrogen in Rare-Earth Hydrides.
Terrence Udovic 1 Show Abstract
1 NIST Center for Neutron Research, National Institute of Standards & Technology, Gaithersburg, Maryland, United States
The novel properties of the neutron such as its large scattering cross section for hydrogen can be routinely exploited by a variety of experimental neutron methods in order to probe the amount, location, bonding states, and motion of hydrogen in condensed-phase materials. This talk will provide an overview of neutron scattering studies that have been undertaken at the NIST Center for Neutron Research to explore the interesting dynamics, bonding, and structural arrangements of hydrogen in the fluorite-type rare-earth hydride systems. The combination of neutron diffraction and spectroscopic measurements and the use of H and D mixed-isotope hydride samples have proved to be beneficial for characterizing the diverse, temperature- and concentration-dependent H sublattice ordering associated with these fluorite-type hydrides.
10:30 AM - **O1.3
Flourite-related Oxygen-Deficient Nonstiochoimetric Rare Earth Higher Oxides and Its Applications For IT-SOFC.
Zhen Kang 1 Show Abstract
1 , RE Power-Tech, Scottsdale, Arizona, United States
Understanding fluorite-related oxygen-deficient nonstoichiometric rare earth higher oxides was taken for almost half of a century since nonstoichiometry of the oxides discovered at middle of 20 century. Wadsley related the nonstoichiometry of transition metal oxides with its coordinated polyhedral structures and promoted direct observations of the defects in the crystal oxides and understanding the nonstoichiometry of provskite-related transition metal oxides, using high resolution transmission electron microscopy . However, the nonstoichiometry of fluorite-related oxygen deficient homologous series of the rare earth higher oxides had been understood by different way , which based on understanding of fluorite structure as the edges shared tetrahedra having oxygen at each tetrahedral center. The nonstoichiometry of fluorite-related oxygen-deficient rare earth higher oxides is due to the composition micro-domains with different homologous phases having different oxygen content, which directly related to the oxygen partial pressure in the vicinity of its surfaces. This unique characteristic make the rare earth higher oxides to have composition hysteresis, lattice oxygen releasing or absorbing, oxygen fast migration, ionic and electron mixed conduction, and so on . These properties make the rare earth higher oxides (nowadays the most attention paid to the cerium higher oxides) to be very useful for environmental and IT-SOFC technology. This talk will present our view on the fluorite structure, fluorite modules theory of the all homologous series phases RnO2n-2m, structure and quantum origin of the oxygen fast migration and electron conduction, oxygen absorbing or releasing with the valence variations of Ce, Pr, Tb, nonstoichiometry of the ROx (x=2(1-m/n) m. n is integer numbers) and thermal process, and finally we discuss the possibility of designing a system having fluorite-related structure with compositional gradient having the functions of cathode, electrolyte, and anode of solid oxide fuel cell for frequent switch on or off without any failure to overcome the inborn weakness of SOFC------two porous layers laminated with a dense layer. B. G. Hyde David Wadsley’s Science unpublished Z.C.Kang and L. Eyring, 1997, Aust. J. Chem., 49, 981. Z. C. Kang Handbook on Physics and Chemistry of Rare Earths, edited by K. A. Gschneinder Jr et al. 2008, Vol. 38 , pp.1
11:30 AM - **O1.4
Hydrogen Ordering And Metal-Semiconductor Transitions In Rare Earth Hydrides.
Peter Vajda 1 Show Abstract
1 Lab. des Solides Irradiés, Ecole polytechnique, Palaiseau France
The rare earth metals (R) La through Lu, including Sc and Y, form with hydrogen generally fluorite structured dihydrides, with H occupying the two available tetrahedral (T) sites. Most of them possess a rather broad existence range in the phase diagram, forming over-stoichiometric compounds of the type RH2+x where the additional x-atoms are sitting in the octahedral (O) interstitial sites of the unit cell. Now, these systems exhibit a great wealth of structural and electronic phenomena, due to the interaction of the x H-atoms with each other, on one hand, and with the R atoms, on the other. (For a review, see e.g. ref. .) Thus, the stoichiometric dihydrides, RH2, are good monovalent metals, with an order of magnitude lower residual resistivity than that of the H-free pure metals. The O-site added x atoms, on the other hand, behave like impurities contributing to conduction-electron scattering but also - and principally – to a decrease of the carrier density, pumping them off towards low-lying bands below the Fermi level: finally yielding insulators or semiconductors. An additional important property of these supernumerary x atoms is their high mobility which permits, for high enough concentrations and at low enough temperatures, to create ordered compounds of the form RH2.25, RH2.33, RH2.5, etc. (characterized by neutron scattering), which can interact with other basic physical properties. Thus, the ordering can lead to a tetragonal distortion of the cubic fluorite cell - readily observable by X-ray diffraction. Furthermore, the magnetism existing in the rare earths with a partially filled 4f shell is strongly influenced, either vanishing more or less completely or reappearing but with a modified structure. And, most exciting, some semiconducting RH2+x systems, with x-values near their phase boundary, turn metallic below 200 - 250 K when the x atoms are allowed to form a super-lattice giving rise to a delocalized band at the Fermi energy.P. Vajda, “Hydrogen in Rare Earth Metals”, in Handbook on the Physics and Chemistry of Rare Earths (K.A. Gschneidner, Ed.), vol. 20, Elsevier (1995).
12:00 PM - O1.5
Energetics of Defect Formation in Pyrochlore, Gd(Ti,Zr)2O7, at High Pressure.
Jianwei Wang 1 , Fuxiang Zhang 1 , Jie Lian 1 2 , Rodney Ewing 1 3 , Udo Becker 1 Show Abstract
1 Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, United States, 2 Department of Mechanical, Aerospace and Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 3 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
12:15 PM - O1.6
Micro- and Nano-scale Characterisation of Fluorite-related Structures in the Gd2(Zr2-xCex)O7 System.
Martin Stennett 1 , Daniel Reid 1 , Neil Hyatt 1 Show Abstract
1 Engineering Materials, The University of Sheffield, Sheffield, South Yorkshire, United Kingdom
The isometric pyrochlore structure A2B2O7, with space group Fd3m, is closely related to the fluorite structure except that there are two cation sites and one eighth of the anions are absent. Recent interest has been shown in the use of the pyrochlore structure type in the management of nuclear waste, due to its ability to incorporate lanthanides and actinides. Much research has focussed on the use of titanate pyrochlores, due to their chemical durability and ability to support a wide range of cation substitutions. However, recent ion beam irradiation studies have revealed the improved radiation resistance of zirconate pyrochlores, which resist amorphization, remaining crystalline as a defect fluorite structure type due to disordering of the cations.The focus of this work was the synthesis and characterisation of pyrochlore ceramics, based on Gd2Zr2O7, for potential immobilisation of plutonium. The solid solution of cerium (as a Pu surrogate) was investigated in the Gd2(Zr2-xCex)O7 system and high quality, dense, ceramic samples (>93% theoretical) were prepared using a high temperature solid state reaction and sintering process. A number of different fluorite-related structure types were shown to be stabilised as a function of Ce content. Phase transformations from the pyrochlore, through defect fluorite, to C-type rare earth oxide structure with increasing substitution (x), were observed by X-ray diffraction (XRD), although accurate phase boundaries were determined by electron diffraction (ED). Nano-scale domains were observed by imaging in the transmission electron microscope in some compositions and high resolution lattice imaging was used to investigate the nature of this domain texture. Analysis of X-ray absorption spectra demonstrated the presence of Ce4+ as the dominant oxidation states across the Gd2(Zr2-xCex)O7 system. In general, these results support the consensus regarding Gd2Zr2O7 as a suitable host for plutonium immobilisation.
12:30 PM - O1.7
Fluorite and Pyrochlore Phases in the HfO2 - La2O3 - Gd2O3 System: Characterization and Calorimetric Study of Samples Quenched from Melts Formed by Laser Heating and Aerodynamic Levitation.
Sergey Ushakov 1 , Alexandra Navrotsky 1 , Jean Tangeman 2 Show Abstract
1 , University of California at Davis, Davis, California, United States, 2 , Corporate Research Materials Laboratory 3M Center, St. Paul, Minnesota, United States
Aerodynamic levitation combined with laser heating was used to prepare melts in the HfO2-La2O3 -Gd2O3 systems by quenching in oxygen from ~3000 °C. All melts crystallized on cooling. Hf2La2O7 pyrochlore and Gd0.5Hf0.5O1.75 fluorite phases were identified. Gd0.5Hf0.5O1.75 fluorite was transformed into the pyrochlore structure by annealing at 1450 °C. Pyrochlore that crystallized from HfO2 - La2O3 melts contained 31.6-34.2 mol% La2O3. The unit cell parameter increased linearly with La content from 10.736 to 10.789 Å. Drop solution calorimetric experiments were performed in 3Na2O4MoO3 melt at 702 °C. Formation enthalpies for lanthanum and gadolinium hafnate pyrochlores were measured ΔH°fr.ox. (La2Hf2O7)= -107.0 ± 5.0 kJ/mol, ΔH°fr.ox. (Gd2Hf2O7) = -48.8 ± 4.7 kJ/mol). The enthalpy of the order-disorder phase transition in Hf2Gd2O7 from pyrochlore in fluorite structure was derived from calorimetric data as 23.6 ±3.1 kJ/mol of Hf2Gd2O7, suggestive of local ordering in the fluorite phase and/or residual disorder in the pyrochlore phase. Applications of laser heating and aerodynamic levitation for calorimetric studies are discussed.
12:45 PM - O1.8
First-principles Study of Displacement Pathways in Bismuth Pyrochlores.
Beverly Hinojosa 1 , Juan Nino 2 , Aravind Asthagiri 1 Show Abstract
1 Chemical Engineering, University of Florida, Gainesville, Florida, United States, 2 Materials Science and Engineering , University of Florida, Gainesville, Florida, United States
There is considerable interest in pyrochlore systems (A2B2O7) for use in high-permittivity dielectrics, capacitors, and high-frequency filter applications. The properties of these materials can be tuned through substitutions on the A and B cation sites, resulting in an extensive parameter space. Better understanding of the role of the local atomic structure and dynamics on the macroscopic properties will enable rational design within the vast number of possibilities. Using density functional theory (DFT), quantum mechanical calculations were performed to examine several Bi-containing pyrochlores with the Fd-3m (No. 227) space group to determine the role of chemical substitutions on the local geometric and electronic structure. We considered six simple bismuth pyrochlores (Bi2B4+2O7 with B4+ = Ti, Ru, Rh, Ir, Os, and Pt) and four pyrochlores with cation substitution selected based on availability of experimental IR and Raman spectroscopy (Bi3/2M2+Nb3/2O7 and Bi3/2M2+Ta3/2O7 with M2+ = Zn, Mg). We will present DFT results on the structural properties of the simple pyrochlores including equilibrium lattice constants, oxygen positional parameters, and atomic displacement patterns. For the pyrochlores with cation substitution, energetics related to cation ordering will be presented, as well as atomic displacement magnitudes and pathways. Where possible, our DFT results will be compared to available experimental studies to understand the role of the atomic substitutions on the material properties.
O2: Fluorites: General Structure/Property Relationships II
Tuesday PM, December 02, 2008
Fairfax B (Sheraton)
2:30 PM - **O2.1
Interfacial Proximity Effects on Structure and Conduction Behavior of ZrO2 – In2O3 Heterostructures.
Jeffrey Eastman 1 , D. Fong 1 , H. Iddir 1 , P. Zapol 1 , B. Kabius 1 , M. Highland 1 , P. Fuoss 1 , T. Fister 1 , M. Richard 1 , P. Baldo 1 Show Abstract
1 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, United States
3:00 PM - O2.2
Structure of δ-Bi2O3 from Simulation: A Systematic Crystallographic Analysis.
Dilpuneet Aidhy 1 , Eric Wachsman 1 , Susan Sinnott 1 , Simon Phillpot 1 , Juan Nino 1 Show Abstract
1 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States
3:30 PM - O2.4
Surface and Bulk Structure Characterization and Redox Properties of Ceria-zirconia Mixed Oxides.
Jose Perez-Omil 1 , Juan Hernandez 1 , Ana Hungria 1 3 , Maria Yeste 1 , Serafin Bernal 1 , Ali Alavi 2 , Jose Calvino 1 Show Abstract
1 Materials Science and Metallurgic Engineering and Inorganic Chemistry, University of Cadiz, Puerto Real, Cadiz, Spain, 3 Materials Science, University of Cambridge, Cambridge United Kingdom, 2 Department of Chemistry, University of Cambridge, Cambridge United Kingdom
Some recent results on the surface, structural properties and the redox behaviour of thermally aged ceria-zirconia mixed oxide are presented. High Resolution Electron Microscopy, High Angle Annular Dark Field-Scanning Transmission Electron Microscopy, and Molecular Dynamics calculations - in the frame of the Density Functional Theory – together with chemical studies reveal that the presence/absence of a pyrochlore related phase in cerium-zirconium mixed oxides plays a key role on their redox response.An enhancement in the reducibility is observed after a treatment consisting in a strong reduction in hydrogen (1173K) followed by a mild oxidation (773K), the so called SR-MO treatment. This change in the chemical behaviour seems to be associated with the presence of well-faceted oxidized pyrochlore-related mixed oxide nucleus exposing Zr-ended (111) surfaces, created during the reduction treatment. Theoretical calculations, based on electron microscopy results, have allowed us to refine the structure of the oxidized pyrochlore.Further experiments show that the destruction and recovery of pyrochlore phase start at the surface of the oxide particles. As a result, the redox behaviour of these oxides at low temperature, which is related with activation phenomena taking place on the surface, is greatly influenced by oxidizing and reducing treatments. Nevertheless the redox behaviour at high temperature is still controlled by the bulk structure of the mixed oxides.These findings lend further support to the idea that the enhanced reducibility exhibited by the mixed oxide after a SR-MO treatment is due to parallel modifications occurred in its surface structure and surface chemistry. On the other hand we would like to highlight the importance of using DFT calculations for a better interpretation of the experimental results.
3:45 PM - O2.5
Near Room-Temperature Preparation of Nanocrystalline Ceria-supported Metallic/ Metallic Oxide Catalysts.
Hongying Liang 1 , Joan Raitano 1 , Siu-Wai Chan 1 Show Abstract
1 Applied Physics & Applied Math., Columbia University, New York, New York, United States
Cerium oxide (CeO[SUB]2[/SUB]) or ceria-supported catalysts have been getting increased attention for water-gas-shift (WGS) and selective removal of CO from hydrogen-rich reformate streams. One of the most intriguing scientific aspects of ceria catalysts is that its activity and stability depend strongly on preparation conditions. Our group had developed a simple method to co-precipitate nanoparticle ceria with a variety of active metals and metal oxides. In this work, x-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), x-ray absorbance near edge spectroscopy (XANES), and extended x-ray absorption fine structure (EXAFS) were used to characterize the nanoparticles. For Cu-CeO[SUB]2[/SUB], the average crystallite size is 6-8nm. Only peaks corresponding to cubic ceria were observed in XRD. No extraneous phases of Cu, Cu[SUB]2[/SUB]O or CuO were observed. However, these peaks are slightly shifted to higher angles relative to pure ceria nanoparticles of the same size. HRTEM images of the 8%Cu-CeO[SUB]2[/SUB] sample shows that the nanoparticles exhibit the octahedral or truncated octahedral morphology of pure ceria nanoparticles prepared using a similar method, but, unlike our pure ceria analogs, these particles are characterized by a high density of extended defects including extra partial planes of atoms. XANES reveals that the copper exists in the 2+ state. Overall, this method should be easy to scale up and applicable to the many redox reactions for which ceria acts as an oxygen storage component.
4:00 PM - O2: FluoritesII
4:30 PM - O2.6
Structure and Dielectric Properties of Weberite-type Compounds Ln3NbO7 (Ln = La3+, Nd3+ and Gd3+)
Lu Cai 1 , Juan Nino 1 Show Abstract
1 , University of Florida, Gainesville, Florida, United States
4:45 PM - **O2.7
Fluorite Related Oxides with Exceptionally Low Thermal Conductivity.
David Clarke 1 Show Abstract
1 , UC Santa Barbara, Santa Barbara, California, United States
It has long been known that yttria-stabilized zirconia has the lowest thermal conductivity at high temperatures of any oxide, even fused silica. Another unusual feature of its thermal conductivity is that it is almost independent of temperature above room temperature: most oxides exhibit classic 1/T dependence. In recent years there has been growing interest in identifying oxides with even lower thermal conductivity that also have the capability of withstanding temperatures well in excess of 1000oC. This has spurred renewed interest in the mechanisms responsible for the unusual thermal conductivity behavior of zirconia and using the insights gained to guide the search for lower conductivity oxides.Based on these new insights, many oxides, including several fluorite-related, have been found to have exceptionally low, temperature independent thermal conductivity. These new discoveries and the relationship between thermal conductivity and both the defect structure and crystal structures will be described.
5:15 PM - **O2.8
Defects in Scintillators: Using Atomic-scale Simulation to Identify and Compensate.
Christopher Stanek 1 , Blas Uberuaga 1 , Ken McClellan 1 , Kurt Sickafus 1 , Mark Levy 2 , Ankoor Patel 2 , Robin Grimes 2 Show Abstract
1 Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 Dept. of Materials, Imperial College London, London United Kingdom
Many years of thorough empirical searches for new scintillator compounds have significantly reduced the likelihood of discovering a new compound with superior properties (e.g. high light output, dense, fast decay, etc.). Nevertheless, improvements in scintillator performance are still required by a range of communities, including medical imaging, high-energy physics and threat reduction. Atomic scale point defects are known to limit the performance of scintillators by either preventing or delaying luminescence by trapping electrons. However, our understanding of defect related phenomena in scintillators is incomplete for many materials of interest. Therefore, rather than attempt an unlikely discovery of a new scintillator compound, we propose to improve the performance of existing materials by ultimately controlling the performance limiting defects.To control defects requires the ability to both identify and compensate. Experimentally, it is difficult to determine the type of defects present in many scintillator materials, which makes compensation impossible. In this talk, we use a range of atomic scale simulation techniques to aid in the identification and compensation of defects in scintillators. The identification component consists of (1) using pair potential calculations to predict the defect composition (i.e. what types of defects are present and in what relative concentrations) and (2) using density functional theory to determine the electronic properties of the defects present, which allows us to determine whether or not a particular defect will strongly trap electrons. Once having identified defects of interest, we use similar methods to investigate schemes to remove or exchange them. Specific, fluorite-related examples to be discussed include site preference in RE2O3 bixbyites and antisite defects in RE3Al5O12 garnets. Where possible we compare our theoretical predictions to experimental data. It is through these comparisons that we are able to improve our understanding of defects in scintillators, and in doing so, improve their performance.
5:45 PM - O2.9
Structural Properties that Determine Performance of Cubic Oxide Scintillators.
Nerine Cherepy 1 , Jeffery Roberts 1 , Alexander Gash 1 , Joshua Kuntz 1 , Thaddeus Norman 2 1 , Stephen Payne 1 Show Abstract
1 Chemistry and Materials Science, Lawrence Livermore National Lab, Livermore , California, United States, 2 Applied Optics Laboratory, SRI, Menlo Park, California, United States
O3: Poster Session: Fluorites
Wednesday AM, December 03, 2008
Exhibition Hall D (Hynes)
9:00 PM - O3.1
Pyrochlore To Fluorite Transitions - Ordering In Fluorites?
Karl Whittle 1 2 , Lachlan Cranswick 3 , Simon A. Redfern 1 , Ian Swainson 3 , Gregory Lumpkin 1 Show Abstract
1 Institute of Materials Engineering, ANSTO, Sydney, New South Wales, Australia, 2 Earth Sciences, University of Cambridge, Cambridge United Kingdom, 3 Canadian Neutron Beam Centre, Chalk River Laboratories, Chalk River, Ontario, Canada
As part of ongoing research into radiation damage, two systems have been studied La2-xYxZr2O7 and La2-xYxHf2O7. The structural effects of increasing Y content have been studied with neutron diffraction and electron microscopy. Results have shown a difference in structural stability for both the pyrochlore and fluorite phases within each system.In addition to the differences in phase stability, electron diffraction, predominantly down the  zone axis, has shown evidence for ordering within the fluorite phases. The ordering length is incommensurate with unit cell size, and decreases as a function of increasing Y content. Evidence for this will be shown and potential structural models will be presented.
9:00 PM - O3.10
Mechanosynthesis, Radiation-Thermal Modification and Characterization of Nanostructured Scandia Stabilized Zirconia Ceramics.
Vladimir Zyryanov 1 , Nikolay Uvarov 1 , Artem Ulihin 1 , Vladislav Sadykov 2 Show Abstract
1 Russian Academy of Sciences, Siberian Branch, Institute of Solid State Chemistry, Novosibirsk Russian Federation, 2 Russian Academy of Sciences, Siberian Branch, Boreskov Institute of Catalysis, Novosibirsk Russian Federation
SSZ-based ceramics derived by the fast thermal sintering at 1633 K with subsequent RT-modification at 1563 K from mechanochemical powders were studied by complex impedance method and characterized by XRD, Raman, SEM, TEM, SIMS techniques. It was established a strong affect of admixtures on conducting properties through microstructure of ceramics. The modification of ceramics by 2.5 MeV electrons results to additional densification of ceramics and reveals the interaction of admixtures with a formation of intergrain phases influencing on conducting properties. The phase composition of ceramics is very close to cubic structure but better fitting of XRD patterns was obtained for rhombohedral lattice. According to Raman spectroscopy data, all obtained ceramics are not single cubic phase. However, conductivities at T = 500-1000 K don’t display any peculiarities as in ceramics prepared from commercially available SSZ powders. Obtained ceramics from refined technical grade ZrO2 nano-precursors display very high sinterability, nanostructuring, high mechanical strength. However, the conductivities of ceramics are lowered at operation temperature T ~ 1000 K. This effect is related to interaction of admixtures and free volume (vacancy defects) in the bulk of crystallites originated from mechanochemical powders with very low relative density ~0.90. RT-modification results to the change of conductivities in 2 and more times.This work was supported by RFBR, 06-03-32131, and SB RAS + NAS of Ukraine, Integration Project 95.
9:00 PM - O3.11
A Potential Generation Toolkit for Actinide Oxides.
Pratyush Tiwary 1 , Axel van de Walle 1 Show Abstract
1 Materials Science, California Institute of Technology, Pasadena, California, United States
The development of accurate empirical potential models for actinide oxides is the focus of intense and ongoing research, with clear applications to the modeling of radiation damage in nuclear fuels. We develop new potential models that extend existing work in various directions by 1) stabilizing the correct ground state of UO_2 (a distorted fluorite structure) 2) accounting for the mixed valence state of uranium 3) properly reflecting the relative stability of the various polymorphs of UO_2 and UO_3. Our approach goes beyond the development of a single general purpose potential by providing a potential generation toolkit and an associated database of ab initio structural energies which let researchers generate new potentials "on-the-fly" by selecting which properties are crucial to reproduce, based on the intended application. This toolkit is also extensible, thus facilitating the inclusion of new actinides in the database to develop mutually consistent and transferable interaction parameters.
9:00 PM - O3.3
Prospects for Co-Doping in Yttria-Stabilized Zirconia.
Richard Darby 1 , Ian Farnan 2 , R. Kumar 1 Show Abstract
1 Department of Materials Science and Metallurgy, University of Cambridge, Cambridge United Kingdom, 2 Department of Earth Sciences, University of Cambridge, Cambridge United Kingdom
Further doping of the yttria-stabilized zirconia system with alternative dopants offers a possible route for increasing the oxygen ion conductivity of this widely-utilised solid-electrolyte. To this end, a range of co-dopants were added to yttria-stabilised zirconia (including gallium, scandium and calcium oxide) to produce single and multi-phase conductors. The ionic conductivities of these new compositions were measured via ac impedance spectroscopy and the 4-point dc method. The resulting conductivities give an insight into the factors affecting ion conduction in the cubic fluorite structure. The local environment around the yttrium ions was monitored using solid-state magic angle sample spinning 89Y nuclear magnetic resonance. These measurements allowed inferences regarding the changing nature of defect clusters to be made.
9:00 PM - O3.4
Using AFM to Study Grooving at Phase Boundaries.
Jessica Riesterer 1 2 , Sanjit Bhowmick 1 , C. Barry Carter 1 Show Abstract
1 Chemical, Materials and Biomolecular Engineering, University of Connecticut, Storrs, Connecticut, United States, 2 Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States
Now that fuel-cell research is growing, more information is needed on how the ceramic electrolyte behaves. Of particular interest is the surface/phase boundary/vapor triple junction. Gas and vapor absorption, the essential step in the operation of the cell, is thought to occur most easily along this junction. Grain boundary grooves form when the surface tensions at this triple junction are balanced. Grain boundary grooving has been extensively studied in ceramic materials but grooving at phase boundaries between two oxides has received little attention. The present study examines several two-phase systems of interest to the fuel-cell community; in particular, both ZrO2 and CeO2 based-ceramics are discussed. After polishing, the orientations of the various grains are examined using electron backscatter diffraction (EBSD). Atomic-force microscopy (AFM) is then used to monitor changes in the topography of the surfaces of these materials through a series of heat treatments. The grooves form upon heating the sample and continue to develop where the phase boundaries intersected the initially flat surfaces. The same sets of grooves are examined after each heat treatment. The groove formation, migration and healing processes will be discussed, along with implications for the performance of fuel cells.
9:00 PM - O3.5
Studies on Interface Structure and Defects in Crystalline Yttria and Lanthanum Oxide Films Grown on Sapphire Single Crystals by Molecular Beam Synthesis.
Masaru Tsuchiya 1 , Nestor Bojarczuk 2 , Supratik Guha 2 , Shriram Ramanathan 1 Show Abstract
1 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States, 2 , IBM T. J. Watson Research Center, Yorktown Heights, New York, United States
Recent advances in oxide thin film synthesis have opened up routes to manipulate functional properties, such as ionic conductivity, ferroelectricity and high temperature superconductivity. A detailed understanding on growth and structural properties is crucial towards to further advancement of oxide thin film technologies. Among several approaches to synthesize oxide thin films, molecular beam synthesis (MBS) is an elegant method to grow highly crystalline films. The low background pressure used in MBS offers atomic-level control of the film growth process and very low impurity concentration in the grown films.In this presentation, we report on growth and structural characterization of crystalline yttrium oxide (yttria) films grown on sapphire c-plane substrates. Yttria is an important material owing to its high permittivity (κ ~ 12-15), large band gap (~ 6 eV) and protonic conduction. Furthermore, yttria on transparent substrate, such as sapphire, is particularly important because rare-earth-doped (such as Yb3+ and Er3+) yttria can be used as lasers and amplifiers. The 99.99 % pure Y materials were deposited from high temperature Knudsen cells at 1013 K at 5 x 10-5 Torr of molecular oxygen at flow rate of 1 sccm. The base pressure of the growth chamber was in the low 10-10 Torr range. Detailed microstructure study was performed by transmission electron microscopy (TEM, JEOL 2100), scanning transmission electron microscopy (STEM, JEOL 2010 F) and X-ray diffraction.From x-ray θ-2θ and ψ scans and plan-view TEM imaging, it is found that Y2O3 is preferably oriented in (111) direction on Al2O3  substrate. The orientation relationships were (111) Y2O3 || (0001) Al2O3 and  Y2O3 || [1-210] Al2O3, corresponding to the large lattice mismatch between Y2O3 and Al2O3 of ~ 20%. Cross-sectional TEM micrographs showed that the mismatch was accommodated by introducing stacking faults and dislocations in Y2O3 layer, while Y2O3 / Al2O3 interface was sharp. No significant intermixing at the interface was observed. Interestingly, La2O3 layer acts as buffer layer for Y2O3 (111) film integration on Al2O3 (0001) surface, where the full width at half maximum (FWHM) of Y2O3 (222) peak improved from 3.12° to 1.43° by introducing a buffer layer of ~ 50 nm. Detailed analysis and discussion on the origin of highly mismatched interface formation between fluorite-derivative oxides and sapphire, point defect ordering in Y2O3 films will be presented in this presentation.
9:00 PM - O3.6
Surface Energetics of Nanocrystalline YSZ Powders.
Gustavo Costa 2 1 , Alexandra Navrotsky 1 , Sergey Ushakov 1 , Reginaldo Muccillo 2 Show Abstract
2 CCTM, IPEN, Sao Paulo, Sao Paulo, Brazil, 1 Neat ORU, UCDavis, Davis, California, United States
Nanocrystalline 8 mol % yttria stabilized zirconia (YSZ) powders with fluorite-type structure were synthesized by Pechini, polyacrylamide and precipitation methods. Powders were characterized by X-ray diffraction, differential scanning calorimetry, thermogravimetry and nitrogen adsorption analyses. Precipitation method produced amorphous powder which crystallized at 600-800 °C into a fluorite phase with a crystallization enthalpy of -19.7 ± 1.0 kJ/mol. The measured surface areas for powders after calcinations at 650-800 °C were 61 m2/g, 27 m2/g, 110 m2/g for Pechini, polyacrylamide and precipitation methods, respectively.Heats of water adsorption at room temperature were measured on samples obtained by sol-gel techniques using a Setaram Calvet microcalorimeter and a Micromeritics gas dosing system. Differential adsorption enthalpy of gaseous water decreased in magnitude with coverage from ~180 kJ/mol for 1 H2O/nm2, reaching the enthalpy of condensation (-44 kJ/mol) at a coverage of 4.6 H2O/nm2 with an integral adsorption enthalpy of -95.6 ± 0.7 kJ/mol H2O. Samples with different surface areas were prepared by annealing the synthesized powders at 650-1500 °C. Drop solution calorimetry experiments were performed in a custom made Calvet twin calorimeter using sodium molybdate 3Na2O.4MoO3 solvent. These experiments, combined with water adsorption calorimetry, allow deriving, through a thermodynamic cycle, surface enthalpies for hydrous and anhydrous surfaces.
9:00 PM - O3.7
Optical Properties and Thermal Effects in CaSZ and YSZ Single Crystal by Raman Scattering, Photoluminescence and Absorption Spectroscopy.
Dorcas Torres 1 , José Llopis 2 Show Abstract
1 Natural Science, Inter American University, Bayamon, Puerto Rico, United States, 2 Fisica de Materiales FCC Fisicas, Universidad Complutense, Madrid Spain
Temperature dependence of Photoluminescence (PL) and Raman spectra have been used to study the defect structure of untreated and thermochemically reduced calcia stabilized zirconia (CaSZ) and yttria stabilized zirconia (YSZ) single crystals. Over the whole temperature range the emission (EM) spectrum of the untreated crystals can be decompose into three broad bands, related to radiative electron transitions which can be associated with anion vacancies centers. Results points out to intrinsic F type centers and extrinsic F type centers (FA,FAA) as the main defect generated in the stabilization process. Effect of thermal reduction on the Raman activity caused a decreased in the acoustic mode region and a shift in the maximum of the excitation (EX) spectra. These variations can be related to a broad absorption band centered at approximately 365 nm. This is consistent with a nonrandom arrangement of vacancies, which produces the superposition of periodic sequences of vacancies within domains. The concentration of the color centers was determined by optical absorption in the UV-VIS region. Concentration dependence on the reduced heat temperature is coherent with previously reported results that ascribed these centers to an ESR active impurity. However, our results of the PL temperature dependence study in colored samples suggest that no significant new type of defect was generated in the reduction process, but an increased of anion vacancies and a recombination of the charges states of the initial intrinsic and extrinsic F centers.
9:00 PM - O3.8
Oxygen Vacancy Migration in CSZ using Density Functional Theory.
Byung-Un Lee 1 , Dae-Hee Kim 1 , Yeong-Cheol Kim 1 Show Abstract
1 Materials Engineering, Korea University of Technology and Education, Chonan Korea (the Republic of)
We study oxygen migration in CaO stabilized cubic zirconia using density functional theory. A Zr atom is substituted by a Ca atom in a 2x2x2 ZrO2 cubic supercell, and an oxygen vacancy is produced to satisfy the charge neutrality condition. We find that the energy of oxygen vacancy as a function of its location with respect to the Ca atom is varied. The relative energies of the oxygen vacancies located at the first, second, third, and fourth nearest neighbors are 0, -0.3, 0.7, and 0.7 eV, respectively. Therefore, the oxygen vacancy located at the second nearest neighbor site of the Ca atom is the most favorable, the oxygen vacancy located at the first nearest neighbor site the second, and the oxygen vacancies at the third and fourth neighbor sites the least favorable. We also calculate the energy barriers for the oxygen vacancy migration between two oxygen sites. The energy barriers between the first and the second nearest sites, and between the third and the second nearest sites are 0.4 and 1.2 eV, respectively. Therefore, the oxygen vacancies favor the first and second nearest neighbor oxygen sites when they drift under an electric field.
9:00 PM - O3.9
Neutron Rietveld Analyses and Prediction of Structures for Fast-Ion Conducting Pyrochlore Solid Solutions.
Bernhardt Wuensch 1 Show Abstract
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
Bernhardt J. Wuensch, Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139Neutron Rietveld analysis has been performed as a function of composition for solid-solution series A2 (B2-x B’x) O7 and, to a lesser extent, (A2-x A’x)2 B2 O7. Cations A and A’ are trivalent species for which A’ has a radius smaller than A. B and B’ are tetravalent species for which B’ is larger than B. The oxygen ions form a face-centered cubic array from which one-eighth of the sites are vacant to maintain charge balance. Initiation of solid solution of either sort (that is, A-site or B-site) serves to reduce the average size difference between the 3+ and 4+ cations and immediately induces disorder in the oxygen ion array. This introduces vacancies into normally-occupied anion sites with an attendant increase in oxygen fast-ion conduction by two or more orders of magnitude. The cation arrangement remains fully ordered until anion disorder is about halfway complete. The cations then rapidly mix over a small range of compositions in a fashion reminiscent of Bragg-Williams disorder.The independent onset of disorder in the cation and anion arrays is surprising as one would expect them to be coupled by Coulombic interactions. Indeed, Pauling’s rule concerning bond-strength summations fails badly for the disordered structures. However, application of bond-valence summations provides excellent agreement with the average value of charge at either the cation or anion sites. There are two independent structural parameters for an ideal pyrochlore structure: the lattice constant and a single positional coordinate for one of the oxygen species. It was found that bond valence summations especially closely matched ionic charge, independent of the state of disorder, for two bond valence summations: that for oxygen coordinated by two A3+ and two B4+, and for the B4+ cation which is surrounded by six O2- with variable positions. Significantly, the summation for both of these ions involves coordination that is not fixed by symmetry. These conditions have been used to compute the structures of two large series of pyrochlores, each having the same B4+ cation, with a sequence of A3+ with progressively larger A3+. The computed parameters are in excellent agreement with the experimental results.
Kurt E. Sickafus Los Alamos National Laboratory
Alexandra Navrotsky University of California-Davis
Simon R. Phillpot University of Florida
O4: Fluorites: Electromagnetic & Optical Properties
Wednesday AM, December 03, 2008
Fairfax B (Sheraton)
9:30 AM - **O4.1
Is Hafnia the New Silica?
Alex Demkov 1 Show Abstract
1 Department of Physics, The University of Texas, Austin, Texas, United States
Hafnium dioxide or hafnia has recently taken the place of silica as a gate dielectric in Si-based field effect transistors. After the introduction of Cu this is arguably the most drastic departure from the traditional CMOS process. Hafnia belongs to a class of metal oxides with a high dielectric constant or high-k dielectrics. Thanks to the presence of d-electrons thee physics and chemistry of these materials is much more complicated than that of Si3N4 or SiO2, and theoretical calculations have proven to be extremely useful in both process development and device engineering. The work horse of the modern computational materials science is density functional theory within the local density and pseudopotential approximations. In this talk I will give a brief introduction of the basic concepts of this theoretical approach and an overview of the recent results in the theory of high-k dielectrics obtained in my group. I will discuss the structural and electronic properties of crystalline and amorphous hafnia, but the main focus of the talk will be on the interfaces of hafnia with metals, oxides and semiconductors that are crucial in gate stack engineering. I will discus the role of oxygen-related defects on the interface stability and band alignment, and the practical ways of band gap engineering to tune the alignment. The microscopic information obtained via first principles calculations will be related to the electric properties and phenomena.
10:00 AM - **O4.2
Dielectric Properties of Compounds with Fluorite-Related Superstructures.
Juan Claudio Nino 1 Show Abstract
1 Materials Science & Eng., University of Florida, Gainesville, Florida, United States
In recent years ceramics with fluorite-related superstructures (like pyrochlore, zirconolite, weberite, etc.) have attracted significant attention from the electronic ceramics research community due to the wide variety of properties exhibited by these compounds. Current and potential applications of pyrochlore electroceramics include capacitors, cathodes, ionic conductors, gas membranes, high frequency filters, resistors, ferromagnets, tunable dielectrics, and semiconductors. With a nominal composition A2B2O7, pyrochlores are usually cubic; however the crystal structure allows for a broad range of atomic substitutions at the A-, B- and O-sites and can also easily accommodate cation and anion non-stoichiometry. As a consequence of this enormous crystallochemical flexibility the properties and functionality of pyrochlores can be engineered in unique ways. In this invited contribution, a review of the pyrochlore crystallography and a summary of known structure-dielectric property relationships are presented. This will be followed by a survey of the most promising pyrochlore compositions for dielectric and cathode applications. In addition, recent advances in the understanding of the nature of the dielectric relaxation, typically observed in pyrochlores will be presented. An outlook on potential future research directions aimed at expanding the fundamental understanding of structure-property relationships of pyrochlore electroceramics will be discussed. Finally, towards the development of advanced dielectric compounds with tailored properties, crystallographic and dielectric relationships between fluorites, pyrochlores and weberites will be outlined.
10:30 AM - **O4.3
Fluorite-derivative high-κ dielectrics on InGaAs and Ge for technologies beyond Si CMOS – interfacial properties and high-performance devices.
Minghwei Hong 1 , Raynien Kwo 2 , Chia-Hung Hsu 3 , Wei-Chin Lee 1 Show Abstract
1 Dept. of Materials Sci. and Eng., National Tsing Hua University, Hsinchu Taiwan, 2 Dept. of Physics, National Tsing Hua University, Hsinchu Taiwan, 3 Research Division, National Synchrotron Radiation Research Center, Hsinchu Taiwan
The semiconductor industry is now facing unprecedented challenges in materials, physics, devices, and processing due to the limitations of Si CMOS transistor scaling arising from nonscaling of matters, namely gate dielectrics and channel mobility. The drive for alternative high κ dielectrics (for replacing conventional SiO2) and metal gates initiated a decade ago has led to the present production of 45 nm microprocessors using high κ + metal gate transistor breakthrough, truly a remarkable achievement in science and technology. As driven by continual demands of even faster speed of enhanced transport in channels and reducing power dissipation beyond the 22 nm node, the current consensus is to pursue urgently the research on employing Ge and III-V semiconductors as high-mobility channels to be integrated with high κ gate dielectrics for future CMOS technology. The realization of these new MOSFET’s has posted great tasks not only for material scientists and processing engineers, but also for condensed matter and device physicists as a whole. In this talk, a review will be given on recent advances of our research work on InGaAs and Ge MOS and MOSFET devices based on fluorite-derivative high-κ dielectrics, including the interfacial structural, chemical, and electrical characteristics, as well as the fundamental mechanism responsible for Fermi level unpinning. The very recent world-record device performances will be presented.
11:00 AM - O4: FluorEMamp;O
O5: Fluorites: Ion Transport I
Wednesday PM, December 03, 2008
Fairfax B (Sheraton)
11:30 AM - **O5.1
Fluorites : Superionics and Conduction Processes.
Stephen Hull 1 Show Abstract
1 The ISIS Facility, STFC Rutherford Appleton Laboratory, Didcot, Oxfordshire, United Kingdom
Anomalously high electrical conductivity within fluorite structured halides such as CaF2, β-PbF2 and SrCl2 at temperatures well below their melting points was first observed by Faraday in the first half of the 19th century . Subsequent research demonstrated that this ‘superionic’ conduction arises from increasing concentrations of dynamic Frenkel defects within the anion sublattice and, despite the difficulties inherent in both experimental and computational studies of such heavily disordered crystalline systems, the structural and ionic diffusion properties of these stoichiometric halides are now largely understood (for a review, see ). The presence of an equivalent superionic transition within oxide fluorites has been demonstrated for the case of UO2 , but the significantly higher temperatures required (over 2500K in the case of UO2) has restricted the number of experimental studies reported. Instead, attention has focussed on anion-deficient fluorites, motivated by the need for oxide ceramics showing high ionic conductivities at temperatures in the range 800-1200K for use as solid electrolytes within Solid Oxide Fuel Cells (SOFCs). High concentrations of anion vacancies can be accommodated within the fluorite lattice, either as a direct consequence of the stoichiometry (such as the high temperature δ phase of Bi2O3), a reduction in the valence state of some of the cations (reduction of Ce4+ to Ce3+ to form CeO2-x) or by aliovalent cation doping. In the latter case, the addition of divalent or trivalent cations substitutionally onto the host cation sites generates charge compensating O2- vacancies, forming a family of highly conducting oxides which includes the current ‘best’ solid electrolyte material, ZrO2 doped with Y2O3 .This presentation will focus on structure-property relationships within anion-deficient fluorite-structured oxides, using selected examples taken from recent work on systems such as δ-Bi2O3, CeO2-x, Y3NbO7 and ZrO2-M2O3 (M=Y and Sc). Specifically, the use of neutron diffraction studies analysing both the Bragg and diffuse scattering components will be described, since this approach gives a consistent picture of both the long-range (time-averaged) structure and the short-range ion-ion correlations which characterise the defects. The complementary use of computational studies, validated with reference to the experimental work, will also be highlighted, providing a direct insight into the nature of the O2- disorder within these highly conducting phases and the role of both host and dopant cations in promoting (or hindering) extensive anion diffusion. M. Faraday, Phil. Trans. R. Soc., 90 (1838).  S. Hull, Rep. Prog. Phys., 67, 1233-1314 (2004). J. Ralph and G.J. Hyland, J. Nucl. Mater., 132, 76-79 (1985). R.M. Ormerod, Chem. Soc. Rev., 32, 17-28 (2003).
12:00 PM - O5.2
La- and Gd-doped Ceria: Correlation Between Energetics and Conductivity.
Hugo Avila-Paredes 1 , Tatiana Shvareva 2 , Weiqun Chen 2 , Alexandra Navrotsky 2 , Sangtae Kim 1 Show Abstract
1 Department of Chemical Engineering and Materials Science, University of California, Davis, Davis, California, United States, 2 Peter A. Rock Thermochemistry laboratory and NEAT ORU, University of California, Davis, Davis, California, United States
Oxides with fluorite structure, such as CeO2, when doped with trivalent cations, particularly La and Gd, form good ionic conductors due to the creation of oxygen vacancies. The measured conductivity, however, does not linearly increase as a function of dopant concentration, but reaches a maximum at 5 mol % LaO1.5 and 10 mol % GdO1.5 respectively. This effect is related to the formation of defect associates between the positively charged oxygen vacancies and the negatively charged dopant cations. The enthalpies of formation of (1-x)CeO2-xLaO1.5 and (1-x)CeO2-xGdO1.5 solid solutions from the constituent oxides (prepared by sol-gel method with x = 0.02 – 0.3) were measured by oxide melt solution calorimetry at 700 °C using sodium molybdate as a calorimetric solvent. For both La- and Gd-doped CeO2, enthalpies of formation show a maximum (7.6 ± 2.3 kJ/mol and 10.4 ± 1.6 kJ/mol respectively) at the dopant level that corresponds to the highest conductivity. As the concentration of the dopant increases, formation enthalpies become more exothermic (2.6 – 4.5 kJ/mol for (1-x)CeO2-xLaO1.5 and 0.6 – 5.1 kJ/mol for (1-x)CeO2-xGdO1.5). These energetics imply an upper limit of freely mobile oxygen vacancies at 5 mol % La and 10 mol % Gd doping, and the formation of the thermodynamically favorable long-range interactions between charged defects and oxygen vacancies as dopant level increases. Thus we have shown a strong correlation between conductivity and formation enthalpies of La- and Gd-doped CeO2. These findings suggest that the critical dopant concentration associated with the conductivity maximum in doped ceria systems could be predicted from the thermodynamics of the solid solutions.
12:15 PM - O5.3
Real Structure - Oxygen Mobility Relationship in Nanocrystalline Doped Ceria-Zirconia Fluorite-Like Solid Solutions Promoted by Pt.
Vladislav Sadykov 1 , Nathalia Mezentseva 1 , Vitalii Muzykantov 1 , Dmitrii Efremov 1 , Elena Gubanova 1 , Eugenii Paukshtis 1 , Arcady Ishchenko 1 , Martin Fedotov 1 , Vladimir Voronin 2 , Julian Ross 3 , Claude Mirodatos 4 , Andre van Veen 4 Show Abstract
1 , Boreskov Institute of catalysis, Novosibirsk Russian Federation, 2 , Institute of Metals Physics , Yekaterinburg Russian Federation, 3 Centre of Environmental Research, University of Limerick, Limerick Ireland, 4 , Institut de Recherches sur la Catalyse et l’Environnement de Lyon, Lyon France
Nanocrystalline ceria-zirconia oxide solid solutions doped with rare-earth cations and promoted by precious metals are efficient anode catalysts for SOFC, components of gas sensors etc. The oxygen mobility and state of supported Pt are important factors ensuring their high and stable performance. This work considers effect of type and content of dopant on the real structure, state of surface Pt species and oxygen mobility of nanocrystalline Lnx(Ce0.5Zr0.5)O2-y (Ln=La3+, Sm3+, Gd3+, Pr3+/4+) solid solutions prepared by Pechini route.The real structure of these nanocrystalline (domain sizes 10-50 nm) systems was studied by TEM with EDX, neutron and X-ray diffraction, WAX, EXAFS, Raman, magnetic measurements, FTIRS of lattice modes. Progressive rearrangement of oxygen polyhedra with dopant content removes oxygen vacancies from coordination spheres of Ce and Zr cations making them more symmetric. The bigger is the size of doping cation, the higher is the lattice disordering counteracting this rearrangement. Segregation of dopants within domain boundaries and on the surface of domains favors formation of layers with specific ordered structures. Pr4+ share was found to increase with Pr content, thus favoring formation of Pr3+-O-Pr4+ clusters within domain boundaries. FTIRS of adsorbed CO test molecules revealed several types of Lewis acid sites – Zr4+, Ce3+, Pr3+/4+and Pt species (Pto, Pt+, Pt2+) differing by clustering degree. Cationic Pt2+ species were found to be mainly located at domain boundaries being stabilized by interaction with basic doping cations and clustered Ce3+ cations. Detailed analysis of oxygen isotope exchange kinetics in these systems by Monte Carlo modeling allowed to estimate kinetic parameters of oxygen diffusion. Oxygen diffusion along domain boundaries (characteristic time t~ 1 s at ~ 700 0C) occurs much faster than within domains (~ 10 s). Incorporation of Pt into domain boundaries increases oxygen mobility and its storage capacity. The oxygen transfer from Pt to support (t< 10-2 s) is very fast. The rate constants of oxygen diffusion along different paths as a function of dopant type and content correlate with the density of “free” bulk/surface anion vacancies, distortion of Zr –O polyhedra, content of cationic Pt2+ species and Pr3+-O-Pr4+ clusters within domain boundaries.Specific catalytic activity of these systems in methane selective oxidation into syngas by gas/lattice oxygen correlates both with the domain/surface oxygen mobility and concentration of Pt2+ species. Detailed TAP studies allowed to explain this correlation by primary CH4 activation on Pt2+ sites, reactive oxygen species being replenished by the oxygen spillover from support. This research is supported by RFBR-CNRS 05-03-34761, INTAS 05-1000005-7663 and YSF 06-1000014-5773, ISTC 3234 Projects.
12:30 PM - O5.4
Effect of Macroscopic Tensile Stress on Ionic Conductivity of Polycrystalline Zirconia Stabilized with Yttria.
Wakako Araki 1 , Yoshinori Imai 2 , Tadaharu Adachi 2 Show Abstract
1 Mechanical Engineering and Science, Saitama University, Saitama Japan, 2 Mechanical Sciences and Engineering, Tokyo Institute of Technology, Tokyo Japan
It is know that internal stress such as residual stress and thermal stress occurs in solid oxide fuel cells (SOFC). The effect of the internal stress on the cell performance has not been examined. To clarify the effect of the elastic stress on the ionic conductivity can lead to safe operation of SOFC and also to innovative structure design of SOFC with high performance. In this study, the ionic conductivity of polycrystalline zirconia stabilized with 8 mol%-yttria (8YSZ) was investigated under tensile loading at high temperatures. The 8YSZ samples were cut into dog-bone shapes for tensile test and platinum electrodes were put on the parallel part of them for impedance analysis. The tensile test was conducted with the material testing machine and the infrared furnace. The specimen was hung on the jigs of the machine so as not to be broken by clamping. The applied tensile loading was varied from 1.5 N to 20 N (i.e. 0.4 MPa to 5.5 MPa). The test temperature was from 973 K to 1273 K. The ionic conductivity under the tensile stress was determined from the impedance analysis, where the direction of the applied electric field was coincident with that of the stress. The impedance test was conducted twice under each temperature and loading condition. The conductivity at any temperature linearly increased with the tensile loading up to 10 N and it retained the improved value from 10 N to 20 N. The conductivity at 973 K increased by nearly 20 % with 10 N. The conductivity was generally governed by that within the grain under any loading condition. The unloading removed the improvement and the reloading increased the conductivity again as well as elastic deformations. Thus, the improvement in the conductivity could be attributed to the elastic strain of the crystal lattice in each grain.The activation energy under each loading condition was derived from the temperature dependence of the conductivity. Although the activation energy was relatively higher with smaller loading, it was almost constant at about 75 kJ/mol regardless of the loading. It can be considered, therefore, that the tensile stress did not facilitate the existing ionic migration but did give additional migration paths. The migration path in fluorite crystals is mainly along the <100> direction, which has a small activation energy. The samples used in this study was polycrystalline so that the random effect of the tensile stress on the migration along <100> could be balanced out. Another possible path is along the <111> direction, which has slightly higher activation energy. In addition, the (111) plane has small elastic modulus so that the stress preferentially could cause the strain in the planes close to (111), which could give the additional migration path along <111> perpendicular to the planes and facilitate that. This mechanism could explain the change in the activation energy but further study will follow to clarify the mechanism.
12:45 PM - O5.5
Synthesis Of A New Fluorite-Related Structure Type A3BO6 as a Potential Fast-ion Conductor.
Bernhardt Wuensch 1 , Emily Walton 1 , Jarred Schantz 1 , Daniel Montana 1 Show Abstract
1 Materials Science and Engineering, MIT, Cambridge, Massachusetts, United States
O6: Fluorites: Ion Transport II
Wednesday PM, December 03, 2008
Fairfax B (Sheraton)
2:30 PM - **O6.1
Conductivity in Highly Defective Fluorites.
Eric Wachsman 1 Show Abstract
1 Florida Institute for Sustainable Energy, University of Florida, Gainesville, Florida, United States
The study of ionic transport in cubic bismuth oxides is important technically because these materials exhibit the highest oxygen–ion conductivity of any material known to date. From a scientific point of view, the study of ionic transport in cubic bismuth oxides also provides an understanding of how anion transport in oxides with the fluorite structure is influenced by high vacancy concentration and how this is influenced by local structure. Bismuth oxide doped with isovalent rare earth cations retains the high temperature defective fluorite structure upon cooling down to room temperature. However, these doped materials undergo an order–disorder transition of the oxygen sublattice at about 600 C. When annealed at temperatures less than the transition temperature the oxygen sublattice continues to order, and consequently oxygen ion conductivity undergoes a decay. However, theconductivity activation energies of the ordered structures after extended aging at 500 C were observed to be lower than those of the structures prior to aging. Modeling of ordered structures based on TEM diffraction patterns indicates a<111> vacancy ordering in the anion sublattice (occupancy ordering). Neutron diffraction studies show additional structural changes in the oxygen sublattice due to positional ordering. These studies indicate that the ionic conductivity is dependent on the distribution of oxygen ionsbetween the regular 8c sites and the interstitial 32f sites in the fluorite structure. Based on the TEM and neutron diffraction studies and conductivity of ordered and disordered structures the influence of local structure on conductivity is described. These results indicate that ordering of anion vacancies in <111> is common to fluorite oxides at high vacancy concentrations. Further, that the tendency to order depends on the dopant radii and polarizability.
3:00 PM - O6.2
Ionic Conductivity in Nanocrystalline Gd Doped Ceria.
Gianguido Baldinozzi 1 2 , David Simeone 2 1 , Dominique Gosset 2 1 , Georgette Petot-Ervas 1 2 , Claude Petot 1 2 Show Abstract
1 SPMS, CNRS, Chatenay-Malabry France, 2 DEN/DANS/DMN/SRMA/LA2M, CEA, Gif-sur-Yvette France
We have synthesized Gd-doped Ceria polycrystalline samples (5, 10, 15 %mol), having relative densities exceeding 95% and grain sizes between 30 and 50 nm after axial hot pressing (750C, 250-750MPa). The samples were prepared by sintering nanopowders obtained by sol-gel chemistry methods having a very narrow size distribution centered at about 16 nm. SEM and X-ray diffraction were performed to characterize the sample microstructures and to assess their structures. We report ionic conductivity measurements using impedance spectroscopy. Evidence of Gd segregation at the grain boundaries is given and the impact on the ionic conductivity, as a function of the grain size and Gd composition, is discussed and compared to microcrystalline samples.
3:15 PM - O6.3
Molecular Dynamics Simulations of Diffusion and Mechanical Properties in Ceria-Based Electrolytes.
Haixuan Xu 1 , Rakesh Behera 1 , Yanli Wang 1 , Fereshteh Ebrahimi 1 , Susan Sinnott 1 , Eric Wachsman 1 2 , Simon Phillpot 1 Show Abstract
1 Department of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 High Temperature Electrochemistry Center, University of Florida, Gainesville, Florida, United States
Ceria based materials are used as electrolytes in solid oxide fuel cells (SOFCs) due to their excellent ionic conductivity. The performance of SOFCs depends on the ease of diffusion of oxygen ions from cathode to anode and the overall stability of the structure. Therefore, it is important to understand this diffusion process at the atomic level. Here, molecular dynamics simulations are used to examine the oxygen diffusion of Ce1-2xM2xO2-x (where M = In3+, Y3+, Gd3+, La3+) at different temperatures and dopant concentrations, where the forces are determined using several different potentials that are available for these systems. The activation energy for diffusion is obtained from the Arrhenius fit of the simulation data and compared with experimental results. The diffusion mechanism is elucidated. The structure and mechanical properties of stoichiometric and sub-stoichiometric ceria and of aliovalently doped ceria are also investigated. Furthermore, the reliabilities of various empirical interatomic potentials for the accurate description of the properties of CeO2 are evaluated. This work is supported by NASA under the grant NAG3 – 2930 and the High Temperature Electrochemistry Center (HiTEC) at the University of Florida.
3:30 PM - O6:FLUOII
O7/Q6: Joint Session: Fluorites: Actinide Fuel and Waste Forms
Wednesday PM, December 03, 2008
Back Bay D (Sheraton)
3:45 PM - **O7.1/Q6.1
Computational Approaches to UO2 Defects and Radiation Damage.
Mark Asta 1 , Sergey Barabash 2 , Anurag Chaudhry 3 , Niels Gronbech-Jensen 3 , Benjamin Hanken 1 , Byoungseon Jeon 3 , Yongduo Liu 2 , Vidvuds Ozolins 2 , Alex Thompson 4 , Pratyush Tiwary 5 , Axel van de Walle 5 , Chris Wolverton 4 Show Abstract
1 Chemical Engineering and Materials Science, University of California at Davis, Davis, California, United States, 2 Materials Science and Engineering, University of California, Los Angeles, Los Angeles, California, United States, 3 Applied Science, University of California at Davis, Davis, California, United States, 4 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 5 Materials Science, California Institute of Technology, Pasadena, California, United States
4:15 PM - O7.2/Q6.2
Combined First Principles and Thermodynamic Calculation of Defect Formation Energies in UO2.
Pankaj Nerikar 1 , Simon Phillpot 1 , Susan Sinnott 1 Show Abstract
1 Materials Science & Engineering, University of Florida, Gainesville, Florida, United States
Uranium oxide is used as the standard nuclear fuel in pressurized water reactors, and defects are created over time by several mechanisms, including self-irradiation. These defects have an important effect on the physical properties of the fuel as they can cause swelling of the material and change the crystal structure thereby reducing the efficiency of the process. Despite their importance, the effect of temperature and oxygen partial pressure on the formation of these defects is not well understood. Here, density functional theory calculations using the Hubbard U correction term is used in combination with thermodynamic approaches to calculate the formation energies associated with intrinsic point defects and fission products. The predicted equilibrium properties and the defect formation energies match trends in the experimental literature quite well. The formation of oxygen interstitials is predicted to become increasingly difficult as higher temperatures and reducing conditions are approached. In addition, the stability of charged defects is predicted to depend to a substantial degree on the position of the Fermi level in the system. This work is supported by a DOE-NERI (DE-FC07-05ID14649).
4:30 PM - O7.3/Q6.3
Oxygen Lattice Distortions and U Oxidation State in UO2+x Fluorite Structures.
Lionel Desgranges 1 , Gianguido Baldinozzi 2 3 Show Abstract
1 DEN/CAD/DEC, CEA, St. Paul-lez-Durance France, 2 SPMS, CNRS, Chatenay-Malabry France, 3 DEN/DANS/DMN/SRMA/LA2M, CEA, Gif-sur-Yvette France
Whether UO2 has a simple fluorite structure, the specific chemical characteristic of oxygen and uranium are largely responsible for the complex features observed in the UO2+x system. In spite of the wide technological interest of uranium oxides, most of the structural features of this system are still unsettled. This is maybe the reason why a lot of speculations about the effective charge of U are still discussed overlooking some the fine structural modifications of the oxygen sub-lattice. Often, the U oxidation state changes are interpreted taking into account only a direct relationship between the lattice parameters and the U ionic radius. Obviously, a better description of the relation between lattice, structure and electronic properties is desirable. In this context, we would like to address structural features induced by different U oxidation states thanks to two examples.The first example deals with oxygen insertion in the fluorite structure during UO2 oxidation at low or intermediate temperature (less than 600K). Following Bevan and Willis pioneering works, the O increase leads only to slight modifications of the X-Ray diffraction pattern, although significant local distortions of the oxygen sub-lattice exist, as it is witnessed by neutron diffraction. Our analysis of U4O9 and U3O7 crystalline structures evidences the deformation of the oxygen coordination polyhedron linked to U5+ and U6+ oxidation state. The second example deals with electron-hole pair formation in UO2 at higher temperature (above 1200 K). In this temperature range, electrical conductivity measurements evidenced the formation of intrinsic carriers, resulting from the formal reaction 2U4+↔U3+ + U5+. Thanks to a coupled analysis of heat capacity experiments, diffraction and in agreement with other results in the literature, evidence is obtained that the U3+-U5+ formation is associated with a significant increase of the fluorite unit cell parameter (about 10%). This unit cell local deformation is in agreement with the structural features expected from the existence U3+ and U5+ coordination polyhedra, replacing the usual U4+ ones.
4:45 PM - O7.4/Q6.4
Theory of Defect Clustering in AnO2+x (An=U, Np or Pu).
David Andersson 1 , Juan Lezama 1 , Steven Conradson 1 , Blas Uberuaga 1 Show Abstract
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
5:00 PM - O7.5/Q6.5
Stress-induced Phase Transformation in Nanocrystalline UO2.
Tapan Desai 1 , Blas Uberuaga 2 , Paul Millett 1 , Dieter Wolf 1 Show Abstract
1 Material Sciences, Idaho National Laboratory, Idaho Falls, Idaho, United States, 2 Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
We have performed Molecular Dynamics (MD) simulations using an empirical potential to study stress-induced phase transformation in nanocrystalline UO2 at T = 1000K. The columnar UO2 microstructure consists of 6 grains of identical hexagonal shape and diameter (d = 20 nm) in a three-dimensional periodic simulation cell. Under constant-stress tensile loading conditions, we found a phase transformation from the fluorite to a-PbO2 structure. The heterogeneous nucleation process of this new phase (a-PbO2) occurs at the grain boundaries and the new phase then grows toward the interior of the grain. To verify that this phase transformation seen in MD simulations is physically reasonable, density functional theory (DFT) calculations were performed. The DFT calculations agree that the a-PbO2 structure is energetically favored over the fluorite structure under certain tensile conditions. According to our knowledge, experimental validation of this phase transformation is not yet available. This work was supported by the DOE-BES Computational Materials Science Network.
5:15 PM - O7.6/Q6.6
Helium Behaviour in UO2 doped with 10 wt% of 238PuO2.
Emilio Maugeri 1 , Thierry Wiss 1 , Jean-paul Hiernaut 1 , Jean-Yves Colle 1 , Hartmut Thiele 1 , C. Sabathier 2 , Vincenzo Rondinella 1 , Rudy Konings 1 Show Abstract
1 , Institute for Transuranium Elements, Karlsruhe Germany, 2 , Commissariat à l'Energie Atomique, Centre de Cadarache, St-Paul-lez-Durance France
5:30 PM - O7.7/Q6.7
Cerium Dioxide Surface Characterization.
Nieves Rodriguez 1 , Juan Carlos Marugan 1 , Eduardo Iglesias 1 , Juan Manuel Nieto 1 , Tiziana Missana 1 , Nairoby Albarran 1 , Joaquin Cobos 2 , Javier Quinones 1 Show Abstract
1 , CIEMAT, Madrid Spain, 2 , ITU-JRC. European Commission, Karlsruhe Germany
5:45 PM - O7.8/Q6.8
Order-disorder Phase Transformation and Amorphization of Nd2Zr2O7 in Gd2Zr2O7 in the Electronic Stopping Regime.
Maulik Patel 1 , V. Vijayakumar 1 , Swaminathan Kailas 2 , Devesh Avasthi 3 , Jean-Claud Pivin 4 , Avesh Tyagi 5 Show Abstract
1 High Pressure Physics Division, Bhabha Atomic Research Center, Mumbai, Maharashtra, India, 2 Physics Group, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India, 3 Materials Science and Radiation Biology, Inter-University Accelerator Center , New Delhi India, 4 , Centre De Spectrometrie Nucleairie Et De Spectrometrie De Masse-In2P3, Paris, Orsay Campus France, 5 Chemistry Division, Bhabha Atomic Research Centre, Mumbai India
Kurt E. Sickafus Los Alamos National Laboratory
Alexandra Navrotsky University of California-Davis
Simon R. Phillpot University of Florida
O8: Fluorites: Radiation Effects I
Thursday AM, December 04, 2008
Fairfax B (Sheraton)
9:45 AM - **O8.1
Radiation-Induced Transformations in a Fluorite-Derivative Structure - Pyrochlore.
Rodney Ewing 1 Show Abstract
1 Geological Sciences, University of Michigan, Ann Arbor, Michigan, United States
10:15 AM - O8.2
Anion Defect Manipulation and Structural Properties of Thin Film Zirconia and Ceria Utilizing Photon Irradiation.
Masaru Tsuchiya 1 , Shriram Ramanathan 1 Show Abstract
1 School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts, United States
Ultra-thin ceria and zirconia films (< 100 nm) are of great interest in electronics and energy applications. However, many structural and functional properties in nanoscale films are markedly different from that observed in the bulk, thereby it is necessary to develop an deep understanding of size effects on functional properties. In addition, thin film systems are extremely sensitive to small differences in materials properties, thus it is expected to play an important role to give insights into some long standing questions in fluorite-class oxides, such as phase stability in zirconia and defect association in ceria. In this presentation, we show our unique approach to control various properties in nanoscale oxide thin films using ultraviolet (UV) photon irradiation. The use of UV light is an attractive option for oxide thin films owing to the proximity of photon energy to the bond energy of oxygen molecules. UV irradiation introduces photochemical reactions that create activated oxygen species, enabling enhanced kinetics of oxygen transport. This process is very effective even at near room temperature; thereby we can use this as a selective method to manipulate anion point defects in fluorite oxides. Zr-Y and Ce-Y metal alloys were grown by dc-magnetron co-sputtering on various substrates at room temperature. The Zr/Y and Ce/Y ratio was controlled by changing the growth parameters during the deposition. The samples were transferred to an in-situ oxidation chamber and exposed to UV irradiation from a Hg vapor lamp in 100% O2 ambient for 15 minutes. The films used in this study are about 50-70 nm thick. The films with similar thickness and composition were grown by thermal oxidation of Zr-Y and Ce-Y alloy with same thickness, and rf-sputering and electron beam evaporation of ceramic targets. From high-resolution transmission electron microscopy studies on interfacial layer growth kinetics, we found that oxygen diffusivity maximum in UV-grown yttria-doped-zirconia film is located at higher yttria concentration (~ 10-12 mol% yttria ) than that in thermally oxidized films. Interestingly, grain mobility in UV-grown oxide in narrow dopant concentration ranges is significantly enhanced.In cerium oxide, UV-grown films were preferably oriented along  direction even at room temperature on various substrates. After 900oC annealing in air, weak in-plane texture was observed in UV ceria films grown on sapphire substrates. This is particularly unique because the films grown from ceria target at room temperature were randomly oriented even after annealing at 900oC for more than 50 hours. These results clearly demonstrate that UV irradiation can be used as an elegant approach to modulate structure, interfacial phenomena, and defect chemistry in zirconia and ceria thin films. We anticipate our approach opens up new routes to investigate structure-property relationships in fluorite derivative oxide thin films.
10:30 AM - O8.3
Ion Irradiation of Ternary Pyrochlores.
Karl Whittle 1 , Katherine Smith 1 , Mark Blackford 1 , Simon A. Redfern 2 , Elizabeth Harvey 2 , Nestor Zaluzec 3 , Gregory Lumpkin 1 Show Abstract
1 Institute of Materials Engineering, ANSTO, Sydney, New South Wales, Australia, 2 Dept of Earth Sciences, University of Cambridge, Cambridge United Kingdom, 3 Materials Science Division, Argonne National Laboratory, Chicago, Illinois, United States
Synthetic pyrochlore samples Y2Ti2-xSnxO7 (x=0.4, 0.8, 1.2, 1.6), Nd2Zr2O7, Nd2Zr1.2Ti0.8O7, and La1.6Y0.4Hf2O7, were irradiated in-situ using the IVEM-TANDEM microscope facility at the Argonne National Laboratory. The critical temperatures for amorphisation have revealed a dramatic increase in tolerance with increasing Sn content for the Y2Ti2-xSnxO7 series. This change has also found to be linear with increasing Sn content. Nd2Zr1.2Ti0.8O7 and La1.6Y0.4Hf2O7 were both found to amorphise, while Nd2Zr2O7 was found to be stable to high doses (2.5x10^15 ions cm-2). The observed results are presented with respect to previously published results for irradiation stability predictions and structural disorder.
10:45 AM - O8: FluorRadEffI
O9: Fluorites: Radiation Effects II
Thursday PM, December 04, 2008
Fairfax B (Sheraton)
11:15 AM - O9.1
First-principles Prediction of Disordering Tendencies in Complex Oxides and Implication for Radiation Resistance.
Chao Jiang 1 , Blas Uberuaga 1 , Chris Stanek 1 , Kurt Sickafus 1 Show Abstract
1 MST-8, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
One important criterion in selecting host materials for nuclear applications is that the material must be highly resistant to radiation damage. This is because the radioactive atoms that are incorporated into the host will decay to create atomic scale defects, which with time will accumulate, possibly leading to amorphization and subsequent micro-cracking of the crystalline waste forms. Here we use pyrochlore-type oxides as an example to demonstrate how first-principles electronic structure calculations can be used to predict the susceptibility of complex oxides to amorphization upon radiation. Our key concept is the disordering energy, i.e., the total energy difference between the fully ordered pyrochlore-type A2B2O7 compound and the fully disordered fluorite-type (A,B)(O,Va)2 solid solution with the same chemical composition (Va is a structural oxygen vacancy in the pyrochlore compound, relative to the parent fluorite). A large disordering energy indicates that the crystalline lattice will become energetically very unfavorable upon the formation of irradiation-induced defects and will therefore collapse into aperiodic amorphous structures in an effort to reduce its energy. On the contrary, a small disordering energy indicates that the crystalline lattice can readily accommodate those irradiation-induced defects with only little increase in its energy. In such a case, very high irradiation doses are required to induce amorphization and the material thus exhibits good radiation tolerance. In this study, we have performed first-principles calculations to rank the radiation resistance of a wide range of pyrochlore-type oxides in terms of their disordering energies. The special quasirandom structure (SQS) approach is employed to model the fluorite alloy in which both the cation and anion sublattices are completely disordered. As a way to validate the accuracy of our SQS calculations, we have also estimated the order-disorder transition temperatures of those pyrochlore-type oxides and our results show excellent agreement with the existing phase diagrams in the literature.
11:30 AM - O9.2
Radiation Damage in Gd2Zr2O7 Induced by MeV Fullerene (C60) Projectiles.
Jiaming Zhang 1 , Maik Lang 1 , Jie Lian 2 , Jie Liu 3 , Christina Trautmann 4 , Rodney Ewing 1 Show Abstract
1 Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan, United States, 2 Department of Mechanical, Aerospace & Nuclear Engineering, Rensselaer Polytechnic Institute, Troy, New York, United States, 3 , Institute of Modern Physics, Lanzhou China, 4 , Gesellschaft für Schwerionenforschung, Darmstadt Germany
Pyrochlore (A2B2O7), an important fluorite-derivative structure, has received considerable attention because this structure is a potential phase for the incorporation of actinides in nuclear waste forms and inert matrix fuels. Certain pyrochlore compositions (e.g., B = Zr, Hf) are highly resistant to low energy ion-induced amorphization through elastic collisions (even to doses of 100 dpa upon 200 keV Ti+); instead, these pyrochlores form a disordered, defect-fluorite structure. In order to investigate the response of zirconate pyrochlore to highly ionizing irradiation, Gd2Zr2O7 bulk samples were irradiated with fullerene ions (C60), at energies of 12 and 30 MeV. Characterization of the sample using cross-sectional transmission electron microscopy revealed nanocrystals of the disordered, defect fluorite-structure and amorphous domains. The observations differ from those that result from irradiations with swift heavy monoatomic projectiles, (e.g. Xe or U ions with energies of 1-3 GeV), indicating both energy deposition density and electronic energy loss are critical to determining the structural transformations that result.
11:45 AM - O9.3
Electron Scattering and the Order-disorder Problem in Ion Irradiated Pyrochlore Compounds.
Gregory Lumpkin 1 , Katherine Smith 1 , Mark Blackford 1 , Nestor Zaluzec 2 , Karl Whittle 1 Show Abstract
1 Institute of Materials Engineering, ANSTO, Sydney, New South Wales, Australia, 2 Materials Science Division, Argonne National Laboratory, Chicago, Illinois, United States
Selected area electron diffraction patterns obtained from thin samples in a TEM are routinely used to determine the effects of radiation damage. Using zone axis orientations, Bragg diffraction intensities change dramatically as the amorphous fraction increases. In studies of pyrochlore phases this has been attributed to the change from an ordered pyrochlore to a disordered fluorite. This work proposes that the change in intensities is due to not only potential disorder, but coupled with amorphous regions degrading intensities. When there is a significant amorphous fraction it is found that dynamical diffraction changes to kinematical in nature. This change in scattering conditions implies that the absence of intensity alone cannot be used to determine degree of disorder within a material.This explanation is used to explain the disordering/damage processes in Gd2Ti2O7 and Y2Sn1.6Ti0.4O7, two examples pertinent in radiation damage studies.
12:00 PM - O9.4
Study of Defects and Structural Transformations Induced by Ion Implantation of Y2O3 Thin Films Deposited by Ion Beam Sputtering.
Bertrand Lacroix 1 , Fabien Paumier 1 , Michael Jublot 1 , Jérôme Pacaud 1 , Rolly Gaboriaud 1 Show Abstract
1 Phymat, University, Chasseneuil France
12:15 PM - O9.5
Irradiation-induced Microstructural Evolution in Delta Phase Oxide Compounds: Y6U1O12 and Y6W1O12, and Yb6W1O12.
Ming Tang 1 , James Valdez 1 , Blas Uberuaga 1 , Yongqiang Wang 1 , Kurt Sickafus 1 , Kiel Holliday 2 , Ken Czerwinski 2 Show Abstract
1 Materials Science & Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 Radiochemistry Group, University of Nevada, Las Vegas, Las Vegas, Nevada, United States
12:30 PM - **O9.6
Structural Chemistry of Pu/UO2+x for x≤0.25.
Steven Conradson 1 Show Abstract
1 Materials Science and Technology Division , Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Despite their widespread technological applications, the structural chemistry of Pu/UO2+x for 0≤x≤0.25 has recently become controversial. The primary issue here has been the nature of the adventitious O in the superstoichiometric compound and whether it forms oxo groups or not. Using thermogravimetric methods that constitute an easier route to obtaining intermediate compositions has allowed us to measure the U L3 XAFS oF series of compounds that show as their most prominent change the monotonic growth of a feature fit with an O atom around 1.74 Å. We have also performed neutron and x-ray pdf, inelastic x-ray scattering, O K edge XAS, Raman measurements, and some preliminary variable temperature XAFS below 200 K, as well as DFT calculations to explore the relative energies of various types of O cluster configurations. The results indicate not only that collective behavior in the form of strong interactions between the O defects resulting in local composition fluctuations and nanoscale heterogeneity is important but also point to a strong role for dynamical processes, even at low temperature.