The complete sets of elastic constants of 4H and 6H silicon carbide single crystals were determined by Brillouin scattering. The elastic constants of 6H SiC are C11=501±4, C33=553±4, C44=163±4, C12=111±5, and C13=52±9 GPa; the corresponding ones of 4H SiC are the same within experimental uncertainties. The compressibility, 4.5×10−3 GPa, is about 3–5 times smaller than those reported for polycrystalline SiC materials.

We report the measured phonon density of states of a bulk GaN powder by time-of-flight neutron spectroscopy. The observed one-phonon excitation spectrum consists of two broad bands centered at about 23 and 39 meV corresponding to the acoustic and the first group of optical phonons; two sharp bands of upper optic modes at about 75 and 86 meV; and a gap of 45-65 meV. The phonon dispersion curves, lattice specific heat, and Debye temperature are calculated from fitting the data with a rigid-ion model.

The phonon density of states of aluminum nitride was determined by time-of-flight neutron spectroscopy using a polycrystalline sample. The observed phonon excitation spectrum consists of a broadband centered at about 35 meV, a small gap in the 75-80 meV region, and two sharp bands at approximately 85 and 92 meV. A rigid-ion model was applied to the interpretation of the data. After optimization, the model provided a satisfactory description of the neutron results as well as the Raman and IR data, sound-velocity measurements, and the lattice specific heat reported in the literature. The partial and total DOS and the phonon-dispersion curves along major symmetry directions of the Brillouin zone were calculated, and the contribution of phonons to the Debye behavior of the low-temperature thermal conductivity was discussed.

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