Structural study of an unusual cubic pyrochlore Bi1.5Zn0.92Nb1.5O6.92

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

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Abstract

Single-phase pyrochlore-type specimens of Bi1.5Zn0.92 Nb1.5O6.92 were studied using combined electron, X-ray and neutron powder diffraction techniques. Rietveld refinements using neutron powder diffraction data confirmed an average pyrochlore structure A2B2O6O′ (Fd3m̌m, a = 10.5616(1) Å) 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 the A and O′ sites. The best fit was obtained with a disordered model in which the A-cations were randomly displaced by ∼0.39 Å from the ideal eight-fold coordinated positions. The displacements occur along the six <112> directions perpendicular to the O′-A-O′ links. In addition, the O′ ions were randomly displaced by ∼0.46 Å along all 12 <110> 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 change the coordination sphere of the A-cations from 8 to (5 + 3); the resulting coordination environment of the A-cations bears similarities to that of the (5 + 1)coordinated Zn in zirconolite-like Bi2Zn2/3Nb4/3O7. The observed displacive disorder in the A2O′ network of the Bi1.5Zn0.92Nb1.5O6.92 structure involves atoms associated with the lowest-frequency vibrational bending mode, and is likely responsible for both the high dielectric constant and the dielectric relaxation reported for this compound.

Original languageEnglish (US)
Pages (from-to)69-75
Number of pages7
JournalJournal of Solid State Chemistry
Volume168
Issue number1
DOIs
StatePublished - Oct 2002

All Science Journal Classification (ASJC) codes

  • Electronic, Optical and Magnetic Materials
  • Ceramics and Composites
  • Condensed Matter Physics
  • Physical and Theoretical Chemistry
  • Inorganic Chemistry
  • Materials Chemistry

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