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Structural Changes behind the Diffuse Dielectric Response in AgNbO3

Published

Author(s)

Igor Levin, Victor L. Krayzman, Joseph C. Woicik, J. Karapetrova, T. Proffen, M. G. Tucker, I. M. Reaney

Abstract

Structural changes among the so-called M-polymorphs of AgNbO3 were analyzed using combined high-resolution X-ray diffraction, neutron total scattering, electron diffraction, and X-ray absorption fine structure measurements. These polymorphs crystallize with the Pbcm symmetry and lattice parameters 2ac×2ac×4ac (ac≈4 Å is the lattice parameter of an ideal cubic perovskite) which are determined by a complex octahedral tilt system (a-b-c-)/(a-b-c+) that involves a sequence of two in-phase and two anti-phase rotations around the c-axis. Our results revealed that, similar to KNbO3, the Nb cations in AgNbO3 exhibit local off-center displacements correlated along the Nb-Nb-Nb chains. The displacements appear to be present even in the high-temperature AgNbO3 polymorphs where the Nb cations on average reside on the ideal fixed-coordinate sites. An onset of the (a-b-c-)/(a-b-c+) tilting lifts the symmetry restrictions on the Nb positions and promotes ordering of the local Nb displacements into a long-range antipolar-like array. This ordering preserves the average Pbcm symmetry but is manifested in electron diffuse scattering and corroborated by other local-structure sensitive techniques. Rietveld refinements indicated intimate coupling between the displacive behavior on the oxygen, Nb, and Ag sublattices. The Pbcm symmetry of the octahedral framework precludes a complete ordering of Nb displacements so that some positional disorder is still maintained in the structure. This disorder provides a likely source for the dielectric relaxation that, according to the previous spectroscopic studies, is at the origin of the diffuse dielectric response exhibited by the M-phase of AgNbO3 at ≈250 ºC.
Citation
Physical Review B (Condensed Matter and Materials Physics)
Volume
79
Issue
10

Keywords

local structure, perovskites, phase transitions, total scattering, EXAFS, electron diffraction, diffuse scattering

Citation

Levin, I. , Krayzman, V. , Woicik, J. , Karapetrova, J. , Proffen, T. , Tucker, M. and Reaney, I. (2009), Structural Changes behind the Diffuse Dielectric Response in AgNbO3, Physical Review B (Condensed Matter and Materials Physics), [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=901412 (Accessed April 17, 2024)
Created March 13, 2009, Updated February 19, 2017