Defect Chemistry, Structure and Property Evolution During Amorphous-to-Crystalline Transformation of Mixed Conducting Oxides

Point defects impact multiple properties of mixed ionic/electronic conducting oxides (MIECs) that are relevant for energy storage and conversion applications. At the surface they help to catalyze electrochemical reactions such as oxygen reduction/evolution, and in the bulk they enable transport of both mass and charge. While the high-temperature defect chemistry of certain prototypical ceramic MIECs is well-established, non-equilibrium and low-temperature/low-crystallinity scenarios remain largely uncharted territory. In this presentation, I will describe our work on amorphous thin film MIECs deposited by pulsed laser deposition and the multiscale, coupled evolution of their structure, defect chemistry, and properties that arises as they are crystallized.<br/>We have applied <i>in situ</i> isothermal optical and electrical monitoring of dynamic defect-mediated properties, along with <i>ex situ</i> diffraction, high resolution (scanning) transmission electron microscopy with spectroscopy, and synchrotron X-ray absorption spectroscopy to correlate behavior to multiscale structural and chemical changes as a function of degree of crystallinity. Our results on perovskite-structured and related MIECs in the titanate family demonstrate that crystallization is chemo-mechanically coupled to oxygen incorporation in this class of materials that can “breathe.” This coupled crystallization-oxidation process results in: local structural changes including increases in valence states, cation coordination numbers, polyhedra symmetry, and neighboring polyhedra alignment; the corresponding increases in hole concentration and mobility that underpin an orders-of-magnitude conductivity rise; the associated dramatic appearance of high catalytic activity for surface oxygen reduction/evolution; and solid phase contraction resulting in formation of highly porous, hierarchical structures with uniform composition [1-4].<br/>The upshot is that this non-equilibrium, low-temperature processing route generates high electrical conductivity and unprecedented oxygen surface exchange kinetics – two of the key properties needed for implementation of MIECs in devices. Along with the benefits of low-thermal-budget processing (choice of substrates, flexible architectures possible, less energy intensive, avoidance of surface segregation), these results on hierarchical, <i>in situ</i> crystallized MIECs may enable new applications in the low-to-intermediate temperature range.<br/>[1] H.B. Buckner, Q. Ma, J. Simpson-Gomez, E.J. Skiba, and N.H. Perry, “Multi-scale Chemo-Mechanical Evolution During Crystallization of Mixed Conducting SrTi_0.65Fe_0.35O_3-d Films and Correlation to Electrical Conductivity” J. Mater. Chem. A (2021) DOI: 10.1039/D1TA06455J<br/>[2] E. Skiba, T. Chen, and N.H. Perry, “Simultaneous Electrical, Electrochemical, and Optical Relaxation Measurements of Oxygen Surface Exchange Coefficients: Sr(Ti,Fe)O_3-d Film Crystallization Case Study,” ACS Appl. Mater. Interfaces (2020) DOI: 10.1021/acsami.0c14265<br/>[3] T. Chen, G.F. Harrington, J. Masood, K. Sasaki, and N.H. Perry, “Emergence of Rapid Oxygen Surface Exchange Kinetics During In Situ Crystallization of Mixed Conducting Thin Film Oxides” ACS Appl. Mater. Interfaces (2019) DOI: 10.1021/acsami.8b21285<br/>[4] T. Chen, G.F. Harrington, K. Sasaki, and N.H. Perry, “Impact of Microstructure and Crystallinity on Surface Exchange Kinetics of Strontium Titanium Iron Oxide Perovskite by In Situ Optical Transmission Relaxation Approach” J. Mater. Chem. A (2017) DOI: 10.1039/C7TA04940D