Coupled Phonon Modes and Infrared Band Shapes in Crystal SiF

The absorption intensities of combination bands in the infrared spectrum of crystal SiF₄ have been measured. One-and two-phonon energies and densities of states have been calculated with a dipole-dipole interaction potential. The contribution to absorption intensities due to non linear electric moments have been calculated. It is shown that most of the two-phonon infrared absorption is due to phonon-phonon coupling.