RAMAN SPECTRUM OF CRYSTALLINE LITHIUM NITRATE

Polarized Raman spectra of an anhydrous single crystal of LiNO3 have been obtained under optimum experimental conditions. The internal vibrations of the nitrate ion are observed as follows: v4(E g) =735.4, v1 (A1g} = 1071.0, and v3 (Eg) = 1384.6 cm-1; these frequencies are essentially independent of the cation in the host lattice. The two Raman-active (Eg) lattice modes have been located and assigned to primarily translational (124.4-cm-1) and librational (237.2-cm-1) motions. No indication of coupling between these vibrations is observed. The frequencies of the external vibrations relative to those of NaNO3 demonstrate that they are dependent on the volume of the crystalline unit cell. Except for small intensity contributions in some orientations which are ascribable to depolarization effects in the birefringent crystal, the polarizability activities of both the internal and external modes follow the expected symmetry selection rules.