Local crystal chemistry, structured diffuse scattering and the dielectric properties of (Bi{sub 1-} {sub x} Y {sub x} ){sub 2}(M {sup III}Nb{sup V})O{sub 7} (M=Fe{sup 3+}, In{sup 3+}) Bi-pyrochlores

Electron diffraction is used to investigate the large amplitude displacive disorder characteristic of the Bi{sub 2}(M {sup III}Nb{sup V})O{sub 7} Bi-pyrochlores, Bi{sub 2}InNbO{sub 7} and Bi{sub 2}FeNbO{sub 7}, as well as of their A site substituted Bi{sub 1.5}Y{sub 0.5}InNbO{sub 7} and Bi{sub 1.5}Y{sub 0.5}FeNbO{sub 7} variants. Highly structured diffuse distributions in the form of {l_brace}110{r_brace}* sheets of diffuse intensity perpendicular to the six directions of real space along with * rods of diffuse intensity perpendicular to the four {l_brace}111{r_brace} real space planes are observed. The existence of this structured diffuse scattering is interpreted in terms of large amplitude, {beta}-cristobalite-type tetrahedral rotations of the O'A {sub 2} tetrahedral framework sub-structure of the ideal pyrochlore structure type. Bond valence sum calculations are used to understand the local crystal chemistry responsible for such displacive disorder. The frequency-dependent dielectric properties of Bi{sub 2}InNbO{sub 7} and Bi{sub 2}FeNbO{sub 7} are also investigated along with the effect upon them of A site doping with Y. - Graphical abstract: The pattern of correlated O'Bi{sub 4} tetrahedral rotation around the axis responsible for the observed * rods of diffuse intensity in Bi-pyrochlores.