Displacive disorder and dielectric relaxation in the stoichiometric bismuth-containing pyrochlores, Bi2MIIINbO7 (M=In and Sc)

The structural disorder and temperature-dependent dielectric properties of two Bi-based niobate pyrochlore systems which have both previously been reported to occur at the ideal Bi2(MIIINbV)O7 stoichiometry without any compositional disorder on the pyrochlore A site, namely the Bi2InNbO7 (BIN) and Bi2ScNbO7 (BSN) pyrochlore systems, have been carefully re-investigated. It is established that A site stoichiometric, Bi-based niobate pyrochlores can indeed exist. Electron diffraction is used to investigate the nature of the displacive disorder therein both at room temperature as well as at close to liquid nitrogen temperature. The characteristic structured diffuse scattering observed arises from β-cristobalite-like, 1-d correlated rotations and associated translations of chains of corner-connected O′Bi4 tetrahedra. The temperature-dependent dielectric properties including the low temperature dielectric relaxation properties of these A site stoichiometric, Bi-based niobate pyrochlores are also reported as are the micro-Raman spectra thereof. The experimental results suggest that the dipoles as well as the glassy relaxation behaviour in these Bi-based pyrochlores are directly related to these β-cristobalite-like, correlated rotations of 〈110〉 chains of corner-connected O′Bi4 tetrahedra.