Infrared Lattice Vibrations and Dielectric Dispersion in Single-CrystalCr2O3

Abstract

Reflectivity measurements on single-crystal ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$ have been made at room temperature in the wavelength range 1 to 45 μ. A classical oscillator analysis of the data has determined the frequencies, strengths and line-widths of the optical-lattice vibration modes with dipole moment vibrating parallel and perpendicular to the $c$ axis. The modes vibrating parallel to the $c$ axis occur at 538 and 613 ${\mathrm{cm}}^{-1\mathrm{}}$. Those vibrating perpendicular to the $c$ axis occur at 417, 444, 532, and 613 ${\mathrm{cm}}^{-1\mathrm{}}$. Forbidden modes, believed to be caused by surface shear strains, have been observed. Vibration frequencies of long-wavelength longitudinal modes corresponding to the transverse vibration frequencies quoted above were calculated from the zeros of the real part of the dielectric constant. These values offer excellent experimental verification of the generalized Lyddane-Sachs-Teller equation derived by Kurosawa and by Cochran and Cowley. Comparison of the transverse-optic-mode frequencies of ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$ with those of $\alpha -{\mathrm{Al}}_2{\mathrm{O}}_3$ shows a considerable increase in force constants for the former, probably due to increased covalent bonding in ${\mathrm{Cr}}_2$ ${\mathrm{O}}_3$.