Heat capacity and thermodynamic functions of β-242Pu2O3 from 8 to 350 K. Contributions to the excess entropy

The heat capacity of β-242Pu2O3 was determined by a quasiadiabatic method from 8 to 350 K. The preparation of a single-phase hexagonal plutonium sesquioxide by hydrogen reduction of 242PuO2 at 2250 K is described. A λ-type heat capacity anomaly with its peak at 17.65 K was found and this anomaly is associated with an antiferromagnetic transition which is reported by McCart et al. in the preceding paper. An excess entropy of β-242Pu2O3 at 298.15 K is evaluated and shown to approach Sexcess = 2R ln6 = 29.79 J K−1 mol−1. In the same manner the previously published experimental entropy of 242PuF3 at 298.15 K was shown to contain an excess entropy contribution approaching Sexcess = R ln6 at 298.15 K. For each compound it was concluded that the excess entropy associated with antiferromagnetic ordering is R ln2 per mol of Pu+3 ions. For β-242Pu2O3 at 298.15 K the heat capacity C°p, entropy S°, enthalpy increment H°(T)−H°(0), and the Gibbs energy divided by temperature [G°(T)−H°(0)]/T are, respectively, (116.98±0.47) J K−1 mol−1, (163.02±0.65) J K−1 mol−1, (22 572±90) J mol−1, and (−87.31±0.35) J K−1 mol−1. The standard Gibbs free energy of formation of Pu2O3 at 298.15 K is calculated from available experimental data.