Calorimetric Study on Phase Transitions in (NH4)3H(SO4)2

Specific heat of (NH 4 ) 3 H(SO 4 ) 2 was measured in a temperature range \(90\,K T 440\) K covering the four phase transitions. The transition entropies were estimated to be Δ S I–II =2.5 ±0.2, Δ S II–III =2.10 ±0.08, Δ S III–IV =0.51 ±0.03 and Δ S IV–V =0.53 ±0.03 in the units of cal ·mol -1 ·deg -1 for the I–II, II–III, III–IV and IV–V phase transitions, respectively.