Infrared and Raman spectra and phase transition of the SF6 crystal. Anharmonic interactions and two-phonon infrared absorption

The infrared and Raman spectra of the two crystalline forms of SF₆ have been investigated at 97 and 9 K, respectively. The experimental data on the high-temperature form are fully consistent with the known crystal structure while inferences on the unknown structure of the other form have been made and an orthorombic D_2h structure isomorphous with that of other hexafluorides is proposed. In the infrared-active fundamental region large LO-TO splittings due to long-range electrostatic interactions have been observed. The combination regions of the infrared spectrum show broad two-phonon absorption bands in both forms. The energy and density of two-phonon states have been obtained on the basis of an intermolecular harmonic dipole-dipole interaction. The intensity and line profile of two-phonon bands has been studied with the help of a recently developed Green function treatment. A renormalized two-phonon density of states has been calculated. It is shown that the indirect intramolecular mechanism through third-order terms in the intramolecular potential gives the main contribution to two-phonon band intensity. A good agreement with experiments is obtained.