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

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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−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.