Spectroscopy of rare earth oxide systems. I. Far infrared spectra of the rare earth sesquioxides, cerium dioxide and nonstoichiometric praseodymium and terbium oxides

The far infrared spectra of the rare earth sesquioxides, cerium dioxide, and nonstoichiometric praseodymium and terbium oxides have been observed over the range 10-250 cm^-1 at liquid helium temperatures. Absorption due to the excitation of phonons and low-lying electronic states of the rare earth ions occur in this region. The long wavelength phonons in the sesquioxides involve motions of the rare earth ions in an effectively rigid oxygen lattice. To a first approximation, the absorptions can be described using the rare earth ion site symmetries. This, together with observations of the line widths and frequencies in mixed crystals, suggests that these phonons and the electronic excitations of the rare earth ions have been observed. In general, the observed electronic energies agree with those obtained by less direct methods; however, in a few cases significant differences were found. The spectra of the other oxides indicate the presence of complex or mixed phases.