Local crystal chemistry, structured diffuse scattering and the dielectric properties of (Bi 1−xY x) 2( MIIINb V)O 7 ( M=Fe 3+, In 3+) Bi-pyrochlores

Abstract

Electron diffraction is used to investigate the large amplitude displacive disorder characteristic of the Bi 2( M IIINb V)O 7 Bi-pyrochlores, Bi 2InNbO 7 and Bi 2FeNbO 7, as well as of their A site substituted Bi 1.5Y 0.5InNbO 7 and Bi 1.5Y 0.5FeNbO 7 variants. Highly structured diffuse distributions in the form of {110}* sheets of diffuse intensity perpendicular to the six 〈110〉 directions of real space along with 〈111〉* rods of diffuse intensity perpendicular to the four {111} real space planes are observed. The existence of this structured diffuse scattering is interpreted in terms of large amplitude, β-cristobalite-type tetrahedral rotations of the O′ A 2 tetrahedral framework sub-structure of the ideal pyrochlore structure type. Bond valence sum calculations are used to understand the local crystal chemistry responsible for such displacive disorder. The frequency-dependent dielectric properties of Bi 2InNbO 7 and Bi 2FeNbO 7 are also investigated along with the effect upon them of A site doping with Y.