HEAT-CAPACITY MEASUREMENT AND THERMODYNAMIC STUDY OF AG8GETE6

Heat capacity of the high ionic conductor Ag8GeTe6 was measured betwee n 10 and 320 K by using a high precision adiabatic calorimeter. Heat c apacity anomalies were observed at 156.1, 169.9, 222.9 and 244.7 K, al l of which were considered to be of the first-order phase transitions. The corresponding transition enthalpies are 90, 194, 2560 and 1910 J mol-1, and the transition entropies, 0.58, 1.14, 11.5 and 7.83 JK-1 mo l-1, respectively. A large heat capacity jump of about 50 JK-1 mol-1 i s observed at the transition of 244.7 K, above which the heat capacity decreases with increasing temperature. The negative temperature depen dence of the heat capacity is analyzed on the basis of a Schottky-type mechanism.