Nguyen, Hai BinhLiu, YunWithers, Raymond2015-12-070022-4596http://hdl.handle.net/1885/22737The composition, dielectric properties and inherent displacive disorder of a Bi-based, misplaced-displacive cubic pyrochlore phase found in two ternary Bi2O3{single bond}M2+O{single bond}Nb2O5 (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.Keywords: Chemical bonds; Crystal structure; Dielectric losses; Electron diffraction; Permittivity; Crystal chemistry; Dielectric permittivities; Diffuse intensity distribution; Misplaced displacive cubic pyrochlores; Bismuth compounds Misplaced-displacive cubic pyrochlores200710.1016/j.jssc.2006.10.0392015-12-07