DYNAMICS OF PARA-ELECTRIC AND FERROELECTRIC SBSI

The lattice dynamics of paraelectric and ferroelectric phases of SbSI is studied using a realistic interatomic force model comprising Coulombic and short range interactions, with covalency included between Sb-S pairs. Characteristic of highly anisotropic systems, the transverse acoustic modes along VM LS TH 100 VM GT TH and VM LS TH 001 VM GT TH directions exhibit anisotropy with respect to the TA modes along VM LS TH 010 VM GT TH . This feature explains the nature of diffuse scattering seen earlier in X-ray measurements. Comparison of optical data made with the results of this study indicates that experimental phonon frequencies and the LO-TO splittings are in reasonable agreement with calculated results. One can refine the calculations by suitable choice of covalent potential parameters and least-squares-fitting procedure; however, this has to await further optical and neutron experimental work.