Optical Phonons in BeO Crystals

The optical phonons of BeO crystals have been studied by measurements of polarized infrared reflection spectra in the range 400-4000 cm^-1. Raman spectra for shifts in the range 100-1500 cm^-1 at room temperature were analyzed. Oscillator fits to the reflectivity spectra lead to values for the frequency, linewidth, and strength of the A₁ and E₁ vibrations. The infrared data are confirmed by the Raman measurements, which also give the Raman-active-only E₂ vibrations. Within experimental error, the high-frequency E₂ mode at 684 cm^-1 coincides with the A₁ transverse mode and is at about twice the frequency of the low-frequency E₂ 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 cm^-1.