Hidden symmetry lowering, nanoscale order-disorder transition and ionic conductivity in Na1/2-xLa1/2-xBa2xZrO3

Abstract

This work concerns the local/nano-scale effects of Ba²⁺ substitution on the structure and ionic conductivity of Na_1/2-xLa_1/2-xBa_2xZrO₃, 2/32 ≤ x ≤ 8/32. Samples were investigated in detail by neutron total scattering analysis in combination with synchrotron X-ray and neutron powder crystallography, X-ray absorption spectroscopy, and solid-state nuclear magnetic resonance spectroscopy. Substitution of Ba²⁺ onto the perovskite A site causes all members of the series to adopt a crystallographic average I4/mcm symmetry analogous to the high temperature phase of Na_1/3La_1/3Sr_1/3ZrO₃, but a deviation from Vegard’s law at x = 6/32 indicates the presence of more complex nanoscale structural features. Above this deviation, analysis of neutron total scattering and extended X-ray absorption fine structure data show that the structure can only be modelled accurately by lowering the symmetry from tetragonal I4/mcm to orthorhombic P4₂/nmc space group to capture the local features. Further refinements conducted using reverse Monte Carlo methods allowed for the quantification of cation ordering, octahedral tilting angles, and element-specific thermal motion. Based on this model, we used a combination of bond valence sum energy mapping and molecular dynamics simulations to develop a detailed model of the effects of Ba²⁺ substitution on Na⁺ conductivity in Na_1/2-xLa_1/2-xBa_2xZrO₃.