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.