The local crystal chemistry and dielectric properties of the cubic pyrochlore phase in the Bi2O3-M2+ O-Nb2O5 (M2+ = Ni2+ and Mg2+) systems

The composition, dielectric properties and inherent displacive disorder of a Bi-based, misplaced-displacive cubic pyrochlore phase found in two ternary Bi2O1-M2+O-Nb2O5 (M = Ni and Mg) systems has been investigated. The dielectric permittivities (up to 1 MHz) of (Bi0.825Ni0.125 square(0.05))2(Ni0.25Nb0.75)(2)O-7 and (Bi0.835Mg0.085 square(0.08))(2)(Mg0.235Nb0.765)(2)O-7 at room temperature are found to be 116 and 151, respectively, while the dielectric loss tangents are 0.00065 and 0.00042, respectively, at 100 kHz. A highly structured characteristic diffuse intensity distribution apparent in electron diffraction is reported in both cases and partially interpreted in terms of large amplitude, beta-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 investigate the local crystal chemistry responsible for this displacive disorder. (c) 2006 Elsevier Inc. All rights reserved.