Deuteration effect in the phase transition of (NH4)2PbCl6

Abstract Heat capacities of ammonium hexachloroplumbate (ND 4 ) 2 PbCl 6 and its deuterated analogue (ND 4 ) 2 PbCl 6 were measured at temperatures between 13 and 300K. These compounds underwent lambda phase transitions at T c (H) = 78.1 K and T c (D) = 80.7 K respectively. The deuterated salt underwent another first order phase transition at 38.4K. The entropy change of this first order phase transition was 6.5 J K −1 mol −1 . The heat capacities at the lowest temperatures (13–22 22K) were larger for the hydrogenous crystal than for the deuterated analogue.