The local crystal chemistry and dielectric properties of the cubic pyrochlore phase in the Bi 2O 3[sbnd]M2+O [sbnd]Nb 2O 5 ( M2+=Ni 2+ and Mg 2+) systems

Abstract

The composition, dielectric properties and inherent displacive disorder of a Bi-based, misplaced-displacive cubic pyrochlore phase found in two ternary Bi 2O 3 M 2+O Nb 2O 5 ( 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) 2O 7 and (Bi 0.835Mg 0.085□ 0.08) 2(Mg 0.235Nb 0.765) 2O 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, β-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.