Lattice vibrations and the crystal structure of GaS and GaSe

The infrared-active lattice vibrational modes of GaS and GaSe were studied in the range 50–500 cm −1 . The previously observed single reststrahlen band is found to be split into three peaks in the appropriate experimental configurations. The existence of two doubly degenerate infrared-active E modes in both crystals is explained by breaking of symmetry selection rules due to a high number of stacking faults. Based on this conclusion, a new assignment is given to some of the modes and correlation is drawn between the zone-center vibrational frequencies in the two crystals. A simple scaling factor is found to relate most of the modes.