Optical Phonons in BeO Crystals

Abstract

The optical phonons of BeO crystals have been studied by measurements of polarized infrared reflection spectra in the range 400-4000 ${\mathrm{cm}}^{-1\mathrm{}}$. Raman spectra for shifts in the range 100-1500 ${\mathrm{cm}}^{-1\mathrm{}}$ at room temperature were analyzed. Oscillator fits to the reflectivity spectra lead to values for the frequency, linewidth, and strength of the $A_1$ and $E_1$ vibrations. The infrared data are confirmed by the Raman measurements, which also give the Raman-active-only $E_2$ vibrations. Within experimental error, the high-frequency $E_2$ mode at 684 ${\mathrm{cm}}^{-1\mathrm{}}$ coincides with the $A_1$ transverse mode and is at about twice the frequency of the low-frequency $E_2$ mode. The multiple-phonon absorptions at zone boundaries have also been measured at room temperature on the high-energy side of the reststrahlen band up to 4000 ${\mathrm{cm}}^{-1\mathrm{}}$.