Infrared properties of the bismuth pyrochlores*

We measured the temperature dependence of the reflectivity spectra of cubic bismuth pyrochlores Bi$_{3/2}$ZnTa$_{3/2}$O$_{7 }$(BZT), Bi$_{3/2}$MgNb$_{3/2}$O$_{7}$ (BMN), Bi$_{3/2}$MgTa$_{3/2}$O$_{7} $ (BMT) and Bi$_{3/2}$(Zn$_{0.92}$Nb$_{1.5})$O$_{6.92}$ (BZN) by infrared spectroscopy from 30 to 3300 cm$^{-1}$ between 50 and 300K. The optical constants were estimated by Kramers-Kronig analysis and classical dispersion theory. Infrared-active phonon modes were assigned to specific bending and stretching vibrational modes. Of particular interest is a previously unassigned IR mode at $\sim $850 cm$^{-1 }$which may suggest a complex structure. Splitting of the B-O and O-B-O stretching phonons is attributed to mixed cation occupancy. The temperature dependences of the phonon frequencies and the damping coefficients indicate a decrease of lattice constant and orientation disorder at low temperatures.