Local crystal structure of antiferroelectric Bi₂Mn _4/3Ni_2/3O₆ in commensurate and incommensurate phases described by pair distribution function (PDF) and reverse Monte Carlo (RMC) modeling

The functional properties of materials can arise from local structural features that are not well determined or described by crystallographic methods based on long-range average structural models. The room temperature (RT) structure of the Bi perovskite Bi₂Mn_4/3Ni _2/3O₆ has previously been modeled as a locally polar structure where polarization is suppressed by a long-range incommensurate antiferroelectric modulation. In this study we investigate the short-range local structure of Bi₂Mn_4/3Ni_2/3O₆, determined through reverse Monte Carlo (RMC) modeling of neutron total scattering data, and compare the results with the long-range incommensurate structure description. While the incommensurate structure has equivalent B site environments for Mn and Ni, the local structure displays a significantly Jahn-Teller distorted environment for Mn³⁺. The local structure displays the rock-salt-type Mn/Ni ordering of the related Bi ₂MnNiO₆ high pressure phase, as opposed to Mn/Ni clustering observed in the long-range average incommensurate model. RMC modeling reveals short-range ferroelectric correlations between Bi³⁺ cations, giving rise to polar regions that are quantified for the first time as existing within a distance of approximately 12 Å. These local correlations persist in the commensurate high temperature (HT) phase, where the long-range average structure is nonpolar. The local structure thus provides information about cation ordering and B site structural flexibility that may stabilize Bi ³⁺ on the A site of the perovskite structure and reveals the extent of the local polar regions created by this cation.