Spectroscopic Studies of a Superionic Plastic Phase Crystal: Lithium Sulfate

Temperature dependent Raman spectroscopic studies of single crystal lithium sulfate are summarized. Coupling between sulfate ion bending modes and the lithium ion translatory modes becomes weaker with increasing temperature until the lithium modes can no longer be observed above 250° C. The temperature interval above 450° in the monoclinic phase is marked by the onset of significant sulfate ion reorientational motion, as evidenced by the bandshape studies of the sulfate ion v1, mode and the librational modes. Bandshape analysis of the v3mode in the plastic phase strongly suggests that the symmetry of the sulfate ion potential energy environment has planar anisotropy.The lithium ion-sulfate ion interactions are modeled for correlated sulfate ion configurations as a function of lithium ion position. The results support the role of the octahedral site in lithium ion transitions contributing to the ionic conductivity.