Structural Study of an Unusual Cubic Pyrochlore Bi1.5Zno.92Nb1.5O6.92

Published

October 1, 2002

Author(s)

Igor Levin, T G. Amos, J Nino, Terrell A. Vanderah, C A. Randall, M T. Lanagan

Abstract

Single-phase pyrochlore specimens of Bi_1.5Zn_0.92Nb_1.5O_6.92 were studied using combined electron, X-ray and neutron powder diffraction techniques. Rietveld refinements using neutron powder diffraction data confirmed an average pyrochlore structure A₂B₂O₆ O' (Fd3m, a=10.5616(1) ) with both Bi and Zn mixed on the A-sites. However, refinements revealed significant local deviations from the ideal pyrochlore arrangement which were caused by apparent displacive disorder on both A- and O' sites. The best fit was obtained in the disordered model, where A-cations were randomly displaced by about 0.39 from the ideal eight-fold coordinated positions along the six , directions perpendicular to the O'-A-O' links. In contrast, the O' ions were randomly displaced by about 0.46 along all twelve directions. Crystal-chemical considerations suggest the existence of short-range correlations between the O' displacements and both the occupancy of the A-sites (i.e. Bi or Zn) and the directions of the A-cation displacements. The combined A-cation and O' displacements reduce the coordination number of the A-cations from 8 to (5+3); the resulting coordination environment of the A-cations in the pyrochlore bears similarities to that of (5+1)- coordinated Zn in zirconolite-like Bi₂Zn_2/3Nb_4/3O₇. The observed displacive disorder in the A₂O' network of the pyrochlore structure involves atoms associated with the lowest-frequency vibrational bending mode and, thus, is likely responsible for both the high-dielectric constant and the dielectric relaxation reported for this compound.

Citation

Journal of Solid State Chemistry

Volume

168

Issue

No. 1

Pub Type

Journals

Keywords

Bismuth zinc niobate, crystal structure, dielectrics, neutron diffraction, pyrochlore, rietveld analysis

Citation

Levin, I. , Amos, T. , Nino, J. , Vanderah, T. , Randall, C. and Lanagan, M. (2002), Structural Study of an Unusual Cubic Pyrochlore Bi_1.5Zn_o.92Nb_1.5O_6.92, Journal of Solid State Chemistry (Accessed May 3, 2024)