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The effect of inhomogeneities on the infrared reststrahlen band of beryllium oxide

We have measured the infrared-optical properties of ceramic beryllium oxide and observed a pronounced reflectance minimum in the reststrahlen band. This feature can be due to phonon absorption in voids in the material or in surface bumps and pits. Calculated reflectance spectra for each of these mechanisms show qualitative agreement with experiment and both effects are probably present in our experimental situation. A small reflectance dip corresponding to the longitudinal optical phonon mode was observed in our experiments. For a bulk sample, this mode can only be excited by bumps at the surface. This shows that phonon absorption in surface features is of great importance for the reststrahlen band in beryllium oxide.