Normal vibrational modes resulting from the lifting of degeneracies in anisotropic crystals: The internal optic modes of K2SO4

The vibrational normal coordinates of internal optic modes originating in molecular normal modes whose degeneracies have been lifted through site group effects are discussed. The normal mode frequencies are calculated in a molecular dipole formalism. The coupling between different Cartesian components of a phonon mode is described with particular attention to the symmetry based sufficiency condition for that coupling to occur.