Atomic Displacive Disorder in Bi₂Ti₂O₇

The recently synthesized cubic pyrochlore Bi 2 Ti 2 O 7 has been shown to possess displacive disorder in both the anion and cation sublattices. Here, the nature and characteristics of the displacive disorder are further investigated via vibrational spectroscopy. The infrared reflectance was measured over 30–3300 cm –1 at temperatures between 20 and 300 K, while the Raman spectra were collected from 50 to 3500 cm –1 at room temperature. It is found that Bi 2 Ti 2 O 7 exhibits more than the six modes expected in the Raman spectrum for the ideal pyrochlore structure. In addition, infrared-active modes are also present in the Raman spectra. These two results suggest displacements in the atomic positions of bismuth and oxygen away from their higher symmetry conventional pyrochlore Wyckoff positions and are strong and surprising evidence of disorder at the titanium site. Infrared-active phonon modes have been assigned to specific bending and stretching vibrational modes. The effect of the Bi ion on the O–Bi–O and O′–Bi–O′ phonon modes is discussed.