Neutron, X-ray and lattice dynamical studies of paraelectric Sb<SUB>2</SUB>S<SUB>3</SUB>

Neutron and x-ray diffraction studies of Sb<SUB>2</SUB>S<SUB>3</SUB> indicate extensive diffuse scattering in the plane perpendicular to the chain axis of polymer-like (Sb<SUB>4</SUB>S<SUB>6</SUB>)<SUB>n</SUB> molecules. The crystal structure of the paraelectric phase is said to be orthorhombic with space group D<SUP>16</SUP><SUB>2h</SUB> with four molecules per unit cell. The observed diffuse scattering may be due to static disorder or some dynamical effects. In this paper the authors have examined the possible dynamical origin by recourse to lattice dynamical studies. Dispersion relation of phonons along the three symmetry directions a*,b* and c* is evaluated based on a lattice dynamical model incorporating Coulomb, covalent and a Born-Mayer-like short range interactions. Group theoretical analysis based on the group of neutral elements of crystal sites (GNES) was essential in order to examine and aid in the numerical computations. The group theoretical technique involving GNES extended to 'pseudo-molecular' systems is also discussed in this context. The phonon dispersion relation shows that there are rather flat TA-TO branches of very low frequency in the a and c directions which may give rise to diffuse scattering. The branches along the b-axis are quite dissimilar to those along a and c axes because of anisotropy. Variation of the potential parameters leads to instability of the lowest TA-TO branch. This is suggestive of a temperatures or pressure-dependent phase transition. However since these modes are optically 'silent' one needs to carry out either high resolution neutron scattering or ultrasonic studies to confirm various aspects of the theoretical studies