Anisotropy of lattice vibrations of layered compounds

The polarizable ion model is used to explain the observed anisotropy of the lattice vibrations of layered compounds with the Cd(OH) 2 or CdCl 2 structure. The electronic polarizabilities of the ions, the Szigeti effective charge, and the contribution of the short‐range forces to the frequencies of the optical modes are deduced from optical data for the lattice vibrations of PbI 2 , CdI 2 , SnS 2 , and SnSe 2 . From the analysis it is found that the contribution of the short‐range forces is rather isotropic. The large anisotropy of the optical modes is caused mainly by static dipoles of the anions induced by local electric fields. For the semiconductors SnS 2 and SnSe 2 the reduction of the ionic charges and static dipoles is larger than for the more ionic compounds PbI 2 and CdI 2 .