Infrared study of the phonon modes in bismuth pyrochlores

The temperature dependence of the reflectance of the cubic bismuth pyrochlores Bi3/2ZnTa3/2O7, Bi3/2MgNb3/2O7, Bi3/2MgTa3/2O7, and Bi3/2Zn0.92Nb1.5O6.92 (BZN) has been investigated by infrared spectroscopy. Spectra were collected from 30 to 3300 cm(-1) between 50 and 300 K, and the optical constants were estimated by Kramers-Kronig analysis and classical dispersion theory. In addition, BZN was studied by terahertz techniques to lower frequencies. Infrared-active phonon modes have been assigned to specific bending and stretching vibrational modes. A previously unassigned infrared mode at similar to 850 cm(-1) is discussed. A splitting of the B-O stretching phonon modes and O-B-O bending modes is assigned to mixed-cation occupancy. The temperature dependence of the phonon frequencies and the damping coefficients are consistent with a decrease of both lattice constant and orientational disorder at low temperatures.