Local crystal chemistry, structured diffuse scattering and the dielectric properties of (Bi 1- x Y x ) 2 ( M III Nb 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 Bi2(MIIINbV)O7 Bi-pyrochlores, Bi2InNbO7 and Bi2FeNbO7, as well as of their A site substituted Bi1.5Y0.5InNbO7 and Bi1.5Y0.5FeNbO7 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′A2 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 Bi2InNbO7 and Bi2FeNbO7 are also investigated along with the effect upon them of A site doping with Y.