The local crystal chemistry and dielectric properties of the cubic pyrochlore phase in the Bi2O3MONb2O5 (M=Ni and Mg) systems

The composition, dielectric properties and inherent displacive disorder of a Bi-based, misplaced-displacive cubic pyrochlore phase found in two ternary Bi₂O₃M²⁺ONb₂O₅ (M=Ni and Mg) systems has been investigated. The dielectric permittivities (up to 1 MHz) of (Bi_0.825Ni_0.125□_0.05)₂(Ni_0.25Nb_0.75)₂O₇ and (Bi_0.835Mg_0.085□_0.08)₂(Mg_0.235Nb_0.765)₂O₇ 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, 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 investigate the local crystal chemistry responsible for this displacive disorder.