Low-frequency Raman scattering from vitreous and molten B2O3

Abstract

Integrated Raman intensities of bands between ∼0–900 cm−1 have been obtained for molten B2O3 from 288–906 °C. Emphasis was placed on the highly depolarized low-frequency contour (adjoining the exciting line) which is intense, broad, skewed to high frequencies, and displays a weak, broad, shoulder centered between ∼100–150 cm−1. The total integrated contour intensity from 0–300 cm−1, as well as the peak intensity, obey the one-phonon Bose–Einstein (BE) relation within 6%. The integrated intensity of the polarized 800 cm−1 boroxol ring (B3O6) line, however, decreases with temperature rise, in quantitative agreement with previous work [J. Chem. Phys. 72, 113 (1980)], and correction using neutron scattering results yields a ΔH° value of 5.0 kcal/mol ring rupture. The low-frequency contour probably involves nonsymmetric collective modes of very large assemblies, cages, or rings, as well as librations of B3O6 groups (∼100–150 cm−1), and the fact that the BE distribution is obeyed at temperatures for which B3O6 ring rupture is important indicates that such large temporal aggregates are involved that whether BO3 triangles or B3O6 rings are present may be nearly irrelevant. The peak frequency of the BE corrected low-frequency contour was also observed to be constant, ∼50 cm−1, from −265 to ∼200 °C, but it falls rapidly near TG≊280 °C, and then reaches a ∼27 cm−1 limit above 400 °C. This suggests that the force constants of the low-frequency modes decrease upon melting because the anharmonicity increases due to the increased melt free volume produced by boroxol ring rupture. However, most features of the low-frequency Raman data, including depolarization, may be explained in equivalent terms of ultrasonic modes and viscoelastic stress wave theory.