OPTICAL LATTICE SPECTRA OF $Cr_{2}O_{3}$ AND $Al_{2}O_{3}^{\ast}$

$^{\ast}$ Supported by U. S. Air Force Cambridge Research Laboratories. $^{1}$ S. S. Mitra, J. Chem. Phys. 59, 3031, (1963).Author Institution: Physics Division, IIT Research Institute; Air Force Cambridge Research Laboratories, Bedford, Massachusetts, 01731.Reflection spectra of single crystal $Cr_{2}O_{3}$ and $\alpha-Al_{2}O_{3}$ were investigated in the region $250 cm^{-1}$ to $1200 cm^{-1}$. Kramers-Kronig analyses of the spectra gave the infrared active factor group $(k \simeq 0)$ fundamentals, consistent with the $D_{3d}^{6}$ point group to which the crystals belong. Transmission spectra of both compounds were studied up to $2000 cm^{-1}$ and a number of absorption maxima recorded, which were assigned to combinations of $k \neq 0$ normal $modes.^{1}$ The assignments are consistent with the polarization and temperature dependence of the spectra