Defect Clustering and Vacancy Ordering in Gadolinium Doped Ceria—A Combined Reverse Monte Carlo Study

Lanthanide-doped cerias exhibit fast oxide ion conduction, making them an effective electrolyte for solid oxide fuel cells operating at intermediate temperatures (<i>c</i><i>a</i>. 500-700 °C).¹ Among all the lanthanides, ceria doped with gadolinium (GDC, Ce_1-<i>x</i>Gd<i>_x</i>O_2-<i>x</i>/2) offers the best conductivity system and has already been adopted commercially.² This high ionic conductivity arises from the creation of high concentrations of oxide ion vacancies along with 3-dimensional conduction pathways within the cubic fluorite structure when Ce⁴⁺ substituted by Gd³⁺. However, a lack of homogeneity or disruptions in local atomic arrangements in these systems, can impede O^2- ion diffusion, potentially suppressing ionic conductivity.<br/><br/>To address the structural complexity, recent advancements in neutron total scattering data analysis now allow for a detailed atomic arrangement through the combination of Bragg and diffuse scattering, providing a more complete picture of both long-range and short-range structures, respectively.³In this study, samples prepared with isotopically enriched ¹⁶⁰Gd were used to overcome the high neutron absorption coefficient of naturally abundant Gd, enabling us to access previously inaccessible local details in the defect structure of GDC by analysing total neutron scattering data. The total scattering data of Ce_0.8¹⁶⁰Gd_0.2O_1.9 sample were successfully modelled through reverse Monte Carlo (RMC). The variation from the average structure, a complex local structure, including different defect clusters or associations and vacancy ordering patterns, was observed in the final RMC configurations. Dopant cation-oxide ion vacancy association is thought to play an important role at lower temperatures, leading to higher activation energies for conductivity. These findings will also help to uncover local details of the conduction mechanism in other doped ceria systems.<br/><br/><b>References </b><br/>1. H. Inaba and H. Tagawa, Ceria-based solid electrolytes, <i>Solid State Ionics</i>, 1996, <b>83</b>, 1–16.<br/>2. B. C. H. Steele and A. Heinzel, Materials for fuel-cell technologies, <i>Nature</i>, 2001, <b>414</b>, 345–352.<br/>3. J. Ming, M. Leszczynska-Redek, M. Malys, W. Wrobel, J. Jamroz, M. Struzik, S. Hull, F. Krok and I. Abrahams, Dopant clustering and vacancy ordering in neodymium-doped ceria, <i>Journal of Materials Chemistry A</i>, DOI:10.1039/D3TA07668G