Structural Variations in Pyrochlore-Structured Bi₂Hf₂O₇, Bi₂Ti₂O₇ and Bi₂Hf₂₋xTixO₇ Solid Solutions as a Function of Composition and Temperature by Neutron and X-ray Diffraction and Raman Spectroscopy

The structural behavior of the pyrochlore Bi2Hf2O7 with varying temperature has been studied by powder neutron diffraction X-ray diffraction, and Raman scattering. The unit cell undergoes phase changes that are determined by diffraction methods to occur between α-, β-, γ- and δ-polymorphs at around 400, 550 and 900 °C. The δ-Bi2Hf2O7 polymorph corresponds to the ideal cubic pyrochlore phase. Three of the four polymorphs have previously been reported to occur for Bi2Sn2O7. The need to accommodate the active Bi3+ lone pair in the presence of large HfO6 octahedra results in an “icelike” ordering of Bi positions in Bi2Hf2O7 at room temperature. A similar sequence of phase transitions is observed at room temperature with increasing Ti4+ for Hf4+ substitution in the compositional series Bi2Hf2-xTixO7. The structure changes as the space available to the Bi3+ ion increases represent a gradual increase in disorder and symmetry.