Fast oxide-ion conducting solids are used as electrolytes in solid oxide fuel cells (SOFCs), solid oxide electrolyser cells (SOECs), gas sensors, and oxygen pumps. Commercial devices are typically based on electrolytes showing a defect fluorite structure. The fluorite structure is well-known as a host for fast oxide-ion conduction, with archetypal examples including yttrium doped zirconia, delta-Bi₂O₃ and lanthanide doped ceria. High oxide-ion conductivity in these systems is facilitated by large concentrations of oxide-ion vacancies within the cubic fluorite structure. While the long-range structures of these materials are similar and essentially based on a cubic close packed array of cations with anions in the tetrahedral sites, local structure can vary significantly, particularly with respect to dopant cations and oxide-ion vacancies.

Until fairly recently, analysis of local structure in such systems relied on careful examination of average structural models, derived from diffraction data, to speculate on local coordination environments, in some cases supported by local structural probes such as solid-state NMR and EXAFS as well as computational modelling. However, recent developments in reverse Monte Carlo (RMC) analysis of total neutron scattering data have allowed for a more detailed analysis of local structure based on physical data using a combination of Bragg and diffuse scattering. The former is associated with the long-range order while the latter arises from short-range order. The large box approach of RMC modelling uniquely allows for examination of the resulting model for local vacancy ordering as well as preferred vacancy association and dopant clustering. 

Using this approach we have examined local structure in a number of fluorite structured systems. In the case of delta-Bi₂O₃, the highly conducting fluorite phase is only stable above ca. 730 °C but can be preserved to room temperature through incorporation of dopant cations. However, even for isovalent dopants, where the nominal vacancy concentration is unaltered through doping, conductivity is lower than in the parent delta-Bi₂O₃, and has been associated with oxide-ion vacancy trapping by the dopant cation. In early work, using RMC modelling of total neutron scattering data we were able to show that local ordering of oxide-ion vacancy pairs occurs in these systems with a statistically non-random preference for ordering in the <100> direction.^1,2,3  

In seeking to lower the operating temperature of SOFCs, electrolytes based on lanthanide doped cerias, such as gadolinium doped ceria (GDC), have been utilised. Substitution of tetravalent Ce⁴⁺ by trivalent lanthanide (Ln³⁺) ions (Ce_1-xLn_xO_2-x/2) introduces oxide ion vacancies into the fluorite lattice. We have carried out RMC analysis of neutron total scattering data from Nd doped ceria and GDC. In the latter case, samples prepared with isotopically enriched ¹⁶⁰Gd were used to overcome the high neutron absorption coefficient of naturally abundant Gd. 

The study reveals three distinct features in the local structure of lanthanide doped cerias. Firstly, clustering of the dopant cations, secondly preferred association between oxide-ion vacancies and the dopant cations and finally oxide-ion vacancy clustering. Details of these findings will be presented.