Thermodynamics of ammonium haloplatinates V. Heat capacity, phase transitions and thermodynamic properties of(ND4)2PtBr6fromT = 5 K toT = 350 K

Abstract

The heat capacity Cp, mof deuterated ammonium hexabromoplatinate, (ND4)2PtBr6, was measured from T= 5 K to T= 350 K by adiabatic calorimetry. One λ -shaped anomaly was found in the curve for heat capacity against temperature. This λ -shaped transition reaches its maximum Cp, m= 71.5 ·R at T= (68.0 ± 0.1) K, with ΔtrsSmo= (2.684 ± 0.028) ·R characteristic of an order–disorder transition. The resulting standard molar entropy is found to be: Δ0TSmo{(ND4)2PtBr6, cr}/R= (59.92 ± 0.09), where T= 298.15 K and R= 8.35451 J · K−1· mol−1. On the basis of this result, the free energy function is Φmo{(ND4)2PtBr6, cr}/R= (34.54 ± 0.05). Smoothed values of the standard thermodynamic quantities for pure (ND4)2PtBr6are tabulated at temperatures up to 350 K.