Heat capacities of crystalline and glassy lithium metaphosphate up to the transition region

Abstract

Heat capacity measurements have been conducted by means of DSC on both crystalline and glassy lithium metaphosphate, from room temperature up to the melting region. The heat capacity of the glass is slightly higher than that of the crystal. Contrary to the crystal, in the neighborhood of Tg, Cp increases rapidly by 10 J mol−1 K−1 conferring to this glass a “fragile character.” Nevertheless, the passage through Tm does not show any discontinuity and the values of the glass and of the crystal are identical. The Debye model appears to be realistic to describe the glass heat capacity to temperature dependence. The Debye temperature and frequency were determined by minimizing the Rp and χ2 parameters of the Cv fitting curve. From the calculation of the entropy of the liquid at T > Tm, the excess entropy of the glass at Tg was determined. Using the dependence of the glass transition on the heating rate, we calculated the values of the activation energy for structural relaxation (Erelax) and of the lower limit of the glass transition temperature (\( {T_{\text{g}}^{{^\circ }} } \)) which is a thermodynamic parameter, contrary to Tg which is a kinetic parameter.