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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

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posted on 03.04.2007, 00:00 by Stuart J. Henderson, Olga Shebanova, Andrew L. Hector, Paul F. McMillan, Mark T. Weller
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.