Optically Active Lattice Vibrations in Wurtzite-Type Crystals of Zinc Oxide and Cadmium Sulfide

By the use of a set of equations previously derived by Tsuboi and a set of Raman data recently published by Damen, Porto, and Tell, a set of reasonable values of force constants in the wurtzite-type ZnO and CdS crystals has been obtained. It was shown that, in the infrared-active A1- and E1-type vibrations, an appreciable amount of electrostatic force is involved besides the short-range interatomic forces through the chemical bonds. Such an electrostatic force is considered to be caused by a macroscopic polarization due to an in-phase displacement of the ions in the crystal in question, and this should not be involved in the infrared-inactive E2-type vibrations. The amount of this particular electrostatic force has also been estimated.