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

Hyper-Raman spectra of two lithium borate glasses, 4B 2 O 3 :Li 2 O and 2B 2 O 3 :Li 2 O, are compared to those of pure boron oxide glass, v -B 2 O 3 . 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 BO 3 triangles, B 3 O 3 boroxol rings, and BO 4 tetrahedra. In particular, a mode associated with stretching motions of BO 4 units can be assigned to a vibration of F 2 symmetry in the T d tetrahedral point group. The hyper-Raman scattering intensity of its transverse optic component appears to be proportional to the number of BO 4 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 -B 2 O 3 , 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.