Zone-Boundary Acoustic Phonons in Adamantine Compounds from Far-Infrared Absorption Measurements

Optical-absorption studies in the far infrared at 4.2^∘{}K have been made on single crystals of cubic ZnS, ZnSe, ZnTe, CdTe, and GaAs. Absorption peaks are seen which correspond to the creation of 2TA(L) and 2TA(X) phonons, allowed two-phonon processes in the zinc-blende structure. These same processes are forbidden in the diamond, rock-salt, and cesium-chloride structures. The TA(L), TA(X), LA(L), and LA(X) phonon energies for 12 different adamantine compounds can readily be correlated with the measured elastic constants by assuming that the phonon-dispersion curves for all the crystals are similar. From this correlation, some predictions are made for acoustic-phonon energies in GaSb, InP, and InAs. This correlation does not work well for either SiC or diamond crystals.