DEUTERATION EFFECT IN THE PHASE-TRANSITION OF (NH4)(2)PBCL6

Heat capacities of ammonium hexachloroplumbate (ND4)(2)PbCl6 and its d euterated analogue (ND4)(2)PbCl6 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 un derwent another first order phase transition at 38.4 K. The entropy ch ange of this first order phase transition was 6.5 J K-1 mol(-1). The h eat capacities at the lowest temperatures (13-22 K) were larger for th e hydrogenous crystal than for the deuterated analogue.