Low-temperature heat capacity of alkali metal borate glass

Abstract

As a universal feature of glass, its low-energy inelastic scattering spectra show a broad response known as the boson peak (BP). Since the BP is the peak of g(E)/E2, in which g(E) is the vibrational density of states (VDoS), it originates from the low-energy excess of VDoS over the Debye prediction. As another feature of the BP, namely, the excess part of the heat capacity, Cp, at low temperatures shows a broad peak in the Cp/T3 versus T plot, which is higher than the Debye level, and is also related to the excess VDoS. The heat capacity of rapidly quenched alkali metal borate glass, i.e., xM2O (1 − x)B2O3 of x = 0.22 and M = Na, K, Rb, and Cs, was measured between 2 and 150 K. Upon increasing the alkali cation size, the peak temperature of Cp/T3 decreases, while the peak value of Cp/T3 increases. The peak temperature of Cp/T3 shows a good correlation with the shear modulus. This correlation can be related to the variation of the contraction of boron–oxygen network as the alkali charge density increases with the decrease in the alkali cation size. The composition dependence of Cp/T3 was also studied for a cesium borate glass. The universal nature of Cp/T3 curves among alkali borate glass indicates that the distribution of the low-energy excess VDoS does not depend on the alkali metal and its content below x = 0.22.