Phase Transition in Ammonium-d4 Sulfate as Studied by Heat Capacity Measurements between 3 and 300 K

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Abstract
Molar heat capacities were determined of (ND4)2SO4 between 2.8 and 301 K. The phase transition at 223.94±0.03 K is of the first-order with a long tail extending down to 160 K. The overall heat and entropy of transition are 4270 J mol−1 and 20.35 JK−1 mol−1, respectively, the first-order portion contributing only 40%. The contraction of the molar volume at the transition was evaluated as 0.33 cm3 mol−1. The enthaply to misorient ions in an otherwise perfectly ordered lattice was derived from the low-temperature tail of the anomalous heat capacity. It was 12.0 kJ/mol of (ND4)2SO4, in reasonable agreement with 16 kJ mol−1 derived from O’Reilly-Tsang’s mean field theory. The present results support the order-disorder mechanism of the transition.