HEAT-CAPACITY AND PHASE-TRANSITIONS IN (NH4)(2)HPO4

Heat capacity measurements on (NH4)(2)HPO4 Single crystals in the 77-3 00 K temperature range are reported. Two distinct and reversible heat capacity anomalies at normal pressure are identified, one near 174 K a nd the other near 246 K. The amplitude of the heat capacity anomalies are relatively small, less than 5.0% of the total lattice contribution . No latent heat and no thermal hysteresis were detected in the transi tion regions, indicating that the corresponding phase transitions are characterized by gradual changes in the activation energy for ionic mi gration and the heat capacity.