The local crystal chemistry and dielectric properties of the cubic pyrochlore phase in the Bi2O3single bondM2+Osingle bondNb2O5 (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 Bi2O3single bondM2+Osingle bondNb2O5 (M=Ni and Mg) systems has been investigated. The dielectric permittivities (up to 1 MHz) of (Bi0.825Ni0.125□0.05)2(Ni0.25Nb0.75)2O7 and (Bi0.835Mg0.085□0.08)2(Mg0.235Nb0.765)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.