Raman polarizability tensors for layered crystals, ${\mathrm{CdI}}_2$, GaSe, and red ${\mathrm{HgI}}_2$ are calculated on the basis of the bond-polarizability concept. It is shown that out-of-plane folded modes are missing or very weak. Missing modes arise from the cancellation of the bond Raman polarizabilities within a unit cell. From a comparison with observed Raman spectra the nature of the atomic bonding is discussed.

• Received 19 March 1986

DOI:https://doi.org/10.1103/PhysRevB.34.5801