Displacive Order–Disorder Behavior and Intrinsic Clustering of Lattice Distortions in Bi‐Substituted NaNbO3. (4th June 2020)

• Record Type: Journal Article
• Authors: Levin, Igor; Yang, Fan; Maier, Russell; Laws, William J.; Keeble, Dean S.; Cibin, Giannantonio; Sinclair, Derek C.
• Is Part Of: Advanced functional materials. Volume 30:Number 30(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-06-04
• Subjects: dielectric -- disorder -- pair distribution functions -- perovskites -- solid solutions -- X‐ray scattering; Materials -- Periodicals; Chemical vapor deposition -- Periodicals; 620.11
• DOI: 10.1002/adfm.202001840
• Languages: English
• ISSNs: 1616-301X

Abstract

Abstract: Perovskite‐like NaNbO3 ‐Bi1/3 NbO3 solid solutions are studied to understand the interactions between octahedral rotations, which dominate the structural behavior of NaNbO3 and displacive disorder of Bi present in Bi1/3 NbO3 . Models of instantaneous structures for representative compositions are obtained by refining atomic coordinates against X‐ray total scattering and extended X‐ray‐absorption fine structure data, with additional input obtained from transmission electron microscopy. A mixture of distinct cations and vacancies on the cuboctahedral A‐sites in Na1−3 x Bi x NbO3 ( x ≤ 0.2) results in 3D nanoscale modulations of structural distortions. This phenomenon is determined by the inevitable correlations in the chemical composition of adjacent unit cells according to the structure type—an intrinsic property of any nonmolecular crystals. Octahedral rotations become suppressed as x increases. Out‐of‐phase rotations vanish for x > 0.1, whereas in‐phase tilts persist up to x = 0.2, although for this composition their correlation length becomes limited to the nanoscale. The loss of out‐of‐phase tilting is accompanied by qualitative changes in the probability density distributions for Bi and Nb, with both species becoming disordered over loci offset from the centers of their respective oxygen cages. Symmetry arguments are used to attribute this effect to different strengths of the coupling between the cation displacements and out‐of‐phase versus in‐phase rotations. The displacive disorder of Bi and Nb combined with nanoscale clustering of lattice distortions are primarily responsible for the anomalous broadening of the temperature dependence of the dielectric constant. Abstract : Interactions between octahedral rotations and Bi displacements are elucidated for a representative perovskite system. Out‐of‐phase rotations promote the ordering of these displacements, whereas in‐phase rotations support their disorder. A mixture of distinct species on the same crystallographic site yields nanoscale clustering of lattice distortions. This clustering, combined with the Bi displacive disorder, flattens the temperature dependence of the dielectric constant.