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 Bi2Mn4/3Ni2/3O6 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 Bi2Mn4/3Ni2/3O6, 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 Bi2MnNiO6 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 Bi3+ on the A site of the perovskite structure and reveals the extent of the local polar regions created by this cation.
Highlights
The crystal structures of materials are traditionally investigated by the long-range phenomenon of Bragg diffraction and described by unit cells that represent the average structure.Atoms in the structure are described in terms of positions and thermal vibrations that are time averaged over all unit cells
3.234(9) (3.84:2.89:5.26)b aDistances for the average incommensurate model and the reverse Monte Carlo (RMC) local structure were obtained by fitting Gaussian functions to the observed partial pair distributions functions (Figure 3), while distances listed for the commensurate (HT) phase are the 12 nearest Bi−O distances from the average crystallographic model. bApproximate ratio of bond distances obtained from the integrated areas of peaks in gBi−O(r) for the average incommensurate model and the RMC local structure model in the room temperature (RT) and high temperature (HT) structures
The B site environment at RT is significantly different in the RMC local structure and average incommensurate model
Summary
The crystal structures of materials are traditionally investigated by the long-range phenomenon of Bragg diffraction and described by unit cells that represent the average structure. The long-range average high temperature (HT, measured here at 300 °C) structure is commensurate and antiferrodistortive (GdFeO3 structure, space group Pcmn in the orthorhombic setting consistent with the other structures discussed here, unit cell size √2ap × 2ap × √2ap) and has a single regular A site environment Both the incommensurate (RT) and commensurate (HT) structures of BMN described above are derived from diffraction phenomena and represent a long-range average of the true local structure that may not adequately describe the diversity of short-range structural features that can be responsible for the observed chemical stability and physical properties. The relationships between these models can be rate structures as a class are rare; PDF analysis of found, and the generation of the models is described in incommensurate charge-density waves in CeTe2 revealed significantly larger amplitudes of local atomic distortions in the Te coordination environment than in the crystallographic more detail in Supporting Information section S2. ADistances for the average incommensurate model and the RMC local structure were obtained by fitting Gaussian functions to the observed partial pair distributions functions (Figure 3), while distances listed for the commensurate (HT) phase are the 12 nearest Bi−O distances from the average crystallographic model. bApproximate ratio of bond distances (summed to 12 oxygen coordinate system) obtained from the integrated areas of peaks in gBi−O(r) for the average incommensurate model and the RMC local structure model in the RT and HT structures
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