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

Abstract

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 D2h 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.