Infrared and raman spectrum of KNCS crystal. Mode analysis and the order-disorder phase transition

The laser excited polarized single crystal Raman spectrum between 30 and 2500 cm^-1, the polarized single crystal transmission infrared spectrum between 400 and 4000 cm^-1, and thin film transmission infrared spectrum between 70 and 250 cm^-1 of potassium thiocyanate (KNCS), have been recorded. The NCS^- ion internal mode frequencies were found to be in good agreement with previous observations and are collected together in this paper. The external motions of the KNCS lattice are assigned in terms of factor group selection rules. The infrared absorptions due to combinations of the internal N-C stretching (v₃) mode with the external modes, have been recorded at 90°K and the observed fine structure has been analyzed according to the selection rules for the entire Brillouin zone. The external modes, the infrared active bending mode and the v ₃±external modes, have been studied as a function of temperature in the range 90°-450°K. Above the transition temperature (T_c=413°K), the resonant Raman spectrum due to the external modes is replaced by a broad band spectrum with singularities which correspond to those observed in the v₃±external mode spectrum above T _c. This has been related to the loss of local order and the emergence of dynamic disorder near the transition temperature. The temperature dependence of the Raman active external mode frequencies is discussed in the light of the lattice dynamics viewpoint of the phase transition problem.