ENTHALPY, HEAT-CAPACITY, 2ND-ORDER TRANSITIONS AND ENTHALPY OF FUSIONOF LI4SIO4 BY HIGH-TEMPERATURE CALORIMETRY

The enthalpy of Li4SiO4 was obtained between 363 and 1300 K using isot hermal drop calorimetry. Anisothermal calorimetry was used for the ide ntification of two second-order transitions and the measurement of the enthalpy of fusion. The smoothed enthalpy between 298 and 1300 K resu lts in H degrees(T) - H degrees(298) = -17156 + 73.694 T + 0.103210 T- 2 -4163115 T-1 J mol(-1). The heat capacity was derived by differentia tion. The value extrapolated to 298 K is C-p,C-298 = 182.1 J K-1 mol(- 1). The critical temperatures of the second-order transitions are T-c1 = 938 K and T-c2 = 996 K. The enthalpy of fusion is Delta(fus)H = 53 kJ mol(-1) at the melting temperature T-m = 1531 K. An isotope effect of the Li-6 enrichment on the melting temperature and enthalpy of fusi on could not be discerned.