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

Single-phase pyrochlore-type specimens of Bi1.5Zn0.92Nb1.5O6.92 were studied using combined electron, X-ray and neutron powder diffraction techniques. Rietveld refinements A using neutron powder diffraction data confirmed an average pyrochlore structure A2B2O6Oʹ (Fd&3macr;m, 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 the A and Oʹ sites. The best fit was obtained with a disordered model in which the A-cations were randomly displaced by ~0.39 ? from the ideal eight-fold coordinated positions. The displacements occur along the six <112 > directions perpendicular to the Oʹ-A-Oʹ links. In addition, the Oʹ ions were randomly displaced by ~0.46 ? along all 12 <110> directions. Crystal-chemical considerations suggest the existence of short-range correlations between the Oʹ 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 Oʹ 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 A2Oʹ 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.