Low frequency Raman spectra of v-B2O3 as a function of temperature and pressure

Abstract

Raman studies of vitreous B2O3 have been conducted between 8 and 700 K, and as a function of hydrostatic pressure (∼8 kbar) at room temperature. The low temperature Raman spectra exhibit two broad bands at 50 and 137 cm−1. From the temperature dependence of the 50 cm−1 band, it has been concluded that the vibrational density of states in the low frequency region (30<ω<300 cm−1) is better described when the Raman coupling coefficient Cb varies as ω. A shift in frequency of the Raman spectra as a function of temperature is attributed to a structural change near the glass transition. It is postulated that at room temperature, the structure of v-B2O3 is comprised of an equal proportion of boroxol (B3O6) rings and BO3 triangles. The low frequency vibrational band at 50 cm−1 arises from a cooperative motion of random distributions of mass of various shapes and sizes in the B2O3 glass. Near the glass transition temperature, the boroxol rings break up thereby leading to a more open structure. Above the melting point, i.e., when the viscosity is low, a regrouping of atoms occurs which results in a low density random network structure of BO3 triangles. The mode Gruneisen constants obtained from the high pressure Raman data are found to be +3.42 and −2.05 for the two bands. The observed positive and negative values of γ lend support to our proposed model in which we assign the 137 cm−1 band to the librational motion of boroxol rings.