Infrared Lattice Vibrations in Calcium Tungstate and Calcium Molybdate

The infrared active lattice vibrations of single-crystal CaW${\mathrm{O}}_4$ and CaMo${\mathrm{O}}_4$ have been studied by taking polarized reflection spectra in the region 1 to 130 μ. Four infrared phonon modes vibrate with the electric vector $E$ parallel to the $c$ axis and four with $E$ perpendicular to the $c$ axis in each material. A classical oscillator dispersion analysis is carried out to give the mode strengths, frequencies, and linewidths of the eight infrared modes. The frequencies of the eight corresponding longitudinal optic modes are found from the dispersion analysis. A symmetry study of the zero wave vector vibrations is carried out for the Scheelite structure which predicts four infrared modes for $E\parallel c$ axis and four for $E\perp c$ axis as well as 13 Raman modes and 3 inactive modes. A study is made of apparent failure of the classical oscillator model near one of the strong modes.