Lattice vibrations and ion transport spectra in β -alumina. I. Infrared spectra

Infrared reflectivity and transmission measurements have been made on the superionic conductors Na β-alumina and Ag β-alumina in the frequency range 20-5000 cm^-1. Polarized spectra show the lattice vibrations of the metal ion and of the spinel block to be well separated and almost completely uncoupled for vibrations normal to the c axis. The lowest-frequency mode for Ag β-alumina shows a splitting which is associated with occupancy of the anti-Beevers-Ross site. The temperature dependence of some of the modes is studied and dielectric parameters are evaluated for use in a microwave study being done concurrently. A simple rigid-ion model is constructed which elucidates the nature of the metal-ion Raman and infrared modes and shows that the mode seen in neutron spectra at 44 cm^-1 is optically inactive and involves no metal-ion motion.