Heat-capacity measurements and absolute entropy of ɛ-Mg2PO4OH

Abstract

The low-temperature heat capacity of epsilon-Mg2PO4OH was measured between 10 and 400 K by adiabatic calorimetry. No phase transition was observed over this temperature range. A relative enthalpy increment of 22,119 J mol(-1) and an absolute entropy value of 127.13 +/- 0.25 J mol(-1) K-1 at 298.15 K are derived from the results. The low-temperature heat-capacity data are compared with the DSC data obtained from 143 K to 775 K and show marginal differences in the common temperature range. The latter data are fitted by the polynomial C-p = {316.62 - 2957 T-0.5 - 563350 T-2 + 955.55 x 10(5) T-3} J K-1 mol(-1), which allows extrapolation to high temperatures.