Knowledge Bank

The sulfate ion internal optic modes in monoclinic $Li2SO4\cdot H2O$ are studied using the molecular dipole model. The polarization direction of the B symmetry phonons lies in the a-c plane; however a unique direction is not given by symmetry considerations. An analysis of polarized near-normal incidence infrared reflection data complemented by Raman data suffices to uniquely determine the polarization of the phonon. In these systems a given A symmetry phonon is shown to be ""paired"" with a particular B symmetry phonon, that is, both phonons originate in the same site group internal mode. All site group modes here are of A symmetry. The correct ""pairing"" assignments for several internal modes are discussed.