Interrelationship between Bond Ionicity and Lattice Instability of III-VI Layer Compounds

Spectroscopic ionicities have been evaluated for GaS, GaSe and InSe from experimental values of the dielectric constant in terms of the Penn-Phillips model. The transverse effective charge and the ratio of bond bending to stretching force constants have been compared with the spectroscopic ionicity. These isomorphic III-VI layer compounds are found to have characteristic values of ionicity which lie in the region intermediate between those of group III-V and II-VI compounds, showing that the nature of their valence bonds are quite similar to the tetrahedral bond in such cubic compounds. InS should have its ionicity comparable to that of GaSe and InSe if it crystallizes in the layer structure. A survey of experimental informations related to the phase transformation of III-VI compounds suggests that shear mode phonons play a particular role to determine the crystal energy of InS, leading to the instability of the layer structure.