Local crystal chemistry, structured diffuse scattering and the dielectric properties of (Bi1-xYx)(2)((MNbV)-Nb-III)O-7 (M=Fe3+, In3+) Bi-pyrochlores

摘要

Electron diffraction is used to investigate the large amplitude displacive disorder characteristic of the Bi-2((MNbV)-Nb-111)O-7, 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 < III >* 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, beta-cristobalite-type tetrahedral rotations of the O ' A(2) tetrahedral framework substructure 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. (c) 2006 Elsevier Inc. All rights reserved.