HEAT-CAPACITY AND PHASE-BEHAVIOR IN (NH4)(2)HPO4 AND (NH4)(2)HASO4

High-resolution ac-calorimetric and low-frequency dielectric measureme nts have been used to study the energy changes and phase behavior of ( NH4)(2)HPO4 and (NH4)(2)HAsO4 in the 80-350 K temperature range. For ( NH4)(2)HPO4, two distinct and reversible heat capacity and dielectric constant anomalies are identified, one at 174 K and the other at 246 K . Due to the gradual change of these quantities in the 174 K transitio n region, it is suggestive of a second-order phase transition. However , the sudden change of the dielectric constant near 246 K phase transi tion seems to indicate of being nearly first-order. For (NH4)(2)HAsO4, only one transition is observed near 210 K, for a freshly prepared sa mple, as revealed by both the heat capacity and dielectric constant da ta. But, when the sample were annealed above 350 K and then cooled, tw o transitions were observed at 210 K and 260 K, respectively, which is suggestive of an induced metastable phase in the 210-260 K intermedia te region.