Reverse Monte Carlo refinements of local displacive order in perovskites: AgNbO3 case study

Reverse Monte Carlo refinements with combined input from neutron/x-ray total scattering and extended x-ray absorption fine structure (EXAFS) were applied to analyze local atomic displacements in perovskite-related AgNbO3. This compound exhibits complex displacive order-disorder behavior that determines the unique dielectric properties of AgNbO3-based ceramics. Our results revealed that the total scattering and EXAFS data collected from polycrystalline samples were insufficient to reproduce the correlated local Nb displacements which are evident from the diffuse scattering observed in electron diffraction. A credible representation of the AgNbO3 structure was obtained by enforcing these correlations via restraints on the atomic moves, thus ensuring that the refined atomic ensemble not only satisfied the total scattering and EXAFS data (a total of five datasets) but also reproduced the shape of the observed electron diffuse scattering. Such constrained RMC refinements revealed two-site positional disorder for each of the two non-equivalent Ag crystallographic positions. The refinements confirmed that all Ag cations in AgNbO3 exhibit similar local Ag-O coordination environments as opposed to the two distinct Ag coordinations in the average Pbcm structure. These results support the existence of Ag order-disorder behavior similar to that observed for Nb.