The Raman spectrum of tin tetraiodide: temperature and pressure measurements and lattice dynamical calculations

Tin tetraiodide was chosen for a temperature and pressure dependent lattice dynamical study because of its pronounced anharmonic properties. The variation of the Raman spectrum with temperature from 100K to 298K has been measured using a single-crystal specimen. The temperature dependence of the mode frequencies and linewidths are given together with symmetry assignments. Further measurements were made at room temperature using a powdered sample under pressure, showing considerable variation in internal as well as external modes. Isochoric and isobaric temperature shifts, relative pressure shifts and Gruneisen parameters are obtained from these data. The group theory for this system is presented. The authors' analysis was confirmed by extensive lattice dynamical calculations, first using a model in which the molecules were assumed to be rigid units, and then using a model which gave the internal modes as well as the lattice modes. The pressure variation was also calculated using both models.