Boroxol rings in SiO2–B2O3 glasses: Influence on low-temperature thermal properties

The heat capacity of a rapidly quenched B₂O₃–SiO₂ glass has been measured by adiabatic calorimetry from 10 to 350 K. The vibrational entropy derived from the measurements, S₂₉₈–S₀ = 54.16 ± 0.10 J/mol K, is close to the linear trend defined by the entropies of pure SiO₂ and B₂O₃ glasses. Owing to the strong sensitivity of S₂₉₈ ? S₀ to the oxygen coordination of cations, this result is in agreement with the 3- and 4-fold coordination found for Si and B, respectively, in binary borosilicates. Because additivity of heat capacity extends below 90 K, where the low-frequency modes of boroxol rings dominate the vibrational density of states, the results indicate also that the fraction of boron atoms in these rings is proportional to the B₂O₃ content. According to recent NMR evidence, these conclusions apply to truly homogenous glasses at a microscopic scale.