Nature of the hyper-Raman active vibrations of lithium borate glasses

Hyper-Raman spectra of two lithium borate glasses, 4B2O3:Li2O and 2B2O3:Li2O, are compared to those of pure boron oxide glass, v-B2O3. A mode analysis is performed using a structural model based on the symmetry of the elementary structural units (ESUs) constituting the glasses. Most spectral components arise from internal vibrations of BO3 triangles, B3O3 boroxol rings, and BO4 tetrahedra. In particular, a mode associated with stretching motions of BO4 units can be assigned to a vibration of F2 symmetry in the Td tetrahedral point group. The hyper-Raman scattering intensity of its transverse optic component appears to be proportional to the number of BO4 units in the glass. The boson peak observed in hyper-Raman scattering arises from external modes of rigid ESUs which correspond to librational motions coupled to their translations. The scattering strength of these modes strongly decreases with increasing Li concentration. In pure v-B2O3, external modes of boroxols presumably dominate this scattering. The decrease in the boroxol concentration in lithium borate glasses correlates with the apparent hardening of the boson peak.