Structural Study of an Unusual Cubic Pyrochlore Bi1.5Zn0.92Nb1.5O6.92

✍ Scribed by I. Levin; T.G. Amos; J.C. Nino; T.A. Vanderah; C.A. Randall; M.T. Lanagan

• Publisher: Elsevier Science
• Year: 2002
• Tongue: English
• Weight: 286 KB
• Volume: 168
• Category: Article
• ISSN: 0022-4596
• DOI: 10.1006/jssc.2002.9681

✦ Synopsis

Single-phase pyrochlore-type specimens of Bi 1.5 Zn 0.92 Nb 1.5 O 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 A 2 B 2 O 6 O 0 (Fd % 3m; a=10.5616(1) ( A) 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 0 sites. The best fit was obtained with a disordered model in which the A-cations were randomly displaced by B0.39 ( A from the ideal eight-fold coordinated positions. The displacements occur along the six /112S directions perpendicular to the O 0 -A-O 0 links. In addition, the O 0 ions were randomly displaced by B0.46 ( A along all 12 /110S directions. Crystal-chemical considerations suggest the existence of short-range correlations between the O 0 displacements and both the occupancy of the A-sites (i.e., Bi or Zn) and the directions of the A-cation displacements. The combined Acation and O 0 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 Bi 2 Zn 2/3 Nb 4/3 O 7 . The observed displacive disorder in the A 2 O 0 network of the Bi 1.5 Zn 0.92 Nb 1.5 O 6.92 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. # 2002 Elsevier Science (USA)