Heat capacity and phase behavior in (NH4)2HPO4 and (NH4)2HAsO2
Abstract High-resolution ac-calorimetric and low-frequency dielectric measurements have been used to study the energy changes and phase behavior of (NH4)2HPO4 and (NH4)2HAsO4 in the 80–350 K temperature range. For (NH4)2HPO4, 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 transition region, it is suggestive of a second-order phase transition. However, the sudden change of the dielectric constant near 246 K phase transition seems to indicate of being nearly first-order. For (NH4)2HAsO4, only one transition is observed near 210 K, for a freshly prepared sample, as revealed by both the heat capacity and dielectric constant data. But, when the sample were annealed above 350 K and then cooled, two transitions were observed at 210 K and 260 K, respectively, which is suggestive of an induced metastable phase in the 210–260 K intermediate region.