Local Crystal Structure of Antiferroelectric Bi<sub>2</sub>Mn<sub>4/3</sub>Ni<sub>2/3</sub>O<sub>6</sub> in Commensurate
and Incommensurate Phases Described by Pair Distribution Function
(PDF) and Reverse Monte Carlo (RMC) Modeling
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Abstract
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 Bi<sub>2</sub>Mn<sub>4/3</sub>Ni<sub>2/3</sub>O<sub>6</sub> 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 Bi<sub>2</sub>Mn<sub>4/3</sub>Ni<sub>2/3</sub>O<sub>6</sub>, 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 Mn<sup>3+</sup>. The local structure displays the rock-salt-type
Mn/Ni ordering of the related Bi<sub>2</sub>MnNiO<sub>6</sub> 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 Bi<sup>3+</sup> 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<sup>3+</sup> on the A site of the perovskite structure and reveals
the extent of the local polar regions created by this cation