Structural Variations in Pyrochlore-Structured Bi₂Hf₂O₇, Bi₂Ti₂O₇ and Bi₂Hf_2-xTi_xO₇ Solid Solutions as a Function of Composition and Temperature by Neutron and X-ray Diffraction and Raman Spectroscopy

The structural behavior of the pyrochlore Bi 2 Hf 2 O 7 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-Bi 2 Hf 2 O 7 polymorph corresponds to the ideal cubic pyrochlore phase. Three of the four polymorphs have previously been reported to occur for Bi 2 Sn 2 O 7 . The need to accommodate the active Bi 3+ lone pair in the presence of large HfO 6 octahedra results in an “icelike” ordering of Bi positions in Bi 2 Hf 2 O 7 at room temperature. A similar sequence of phase transitions is observed at room temperature with increasing Ti 4+ for Hf 4+ substitution in the compositional series Bi 2 Hf 2 - x Ti x O 7 . The structure changes as the space available to the Bi 3+ ion increases represent a gradual increase in disorder and symmetry.