The local crystal chemistry and dielectric properties of the cubic pyrochlore phase in the Bi2O3M2+ONb2O5 (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 Bi2O3M2+ONb2O5 (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.

The characteristic β-cristobalite-type displacive disorder of the O’A2 sub-structure (of the ideal pyrochlore structure type) drawn relative to the surrounding B2O6 octahedral framework sub-structure and responsible for the observed structured diffuse intensity distribution in the Bi1.65Ni0.75Nb1.50O7 and Bi1.67Mg0.64Nb1.53O7 misplaced-displacive cubic pyrochlores.Figure optionsDownload as PowerPoint slide