Heat capacity and thermodynamic functions of bgr;hyphen;242Pu2O3from 8 to 350 K. Contributions to the excess entropy

摘要

The heat capacity of bgr;hyphen;242Pu2O3was determined by a quasiadiabatic method from 8 to 350 K. The preparation of a singlehyphen;phase hexagonal plutonium sesquioxide by hydrogen reduction of242PuO2at 2250 K is described. A lgr;hyphen;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 McCartetal. in the preceding paper. An excess entropy of bgr;hyphen;242Pu2O3at 298.15 K is evaluated and shown to approachSexcess= 2Rthinsp;ln6 = 29.79 Jthinsp;Kminus;1thinsp;molminus;1. In the same manner the previously published experimental entropy of242PuF3at 298.15 K was shown to contain an excess entropy contribution approachingSexcess=Rthinsp;ln6 at 298.15 K. For each compound it was concluded that the excess entropy associated with antiferromagnetic ordering isRthinsp;ln2 per mol of Pu+3ions. For bgr;hyphen;242Pu2O3at 298.15 K the heat capacityCdeg;p, entropySdeg;, enthalpy incrementHdeg;(T)minus;Hdeg;(0), and the Gibbs energy divided by temperature lsqb;Gdeg;(T)minus;Hdeg;(0)rsqb;/Tare, respectively, (116.98plusmn;0.47) Jthinsp;Kminus;1thinsp;molminus;1, (163.02plusmn;0.65) Jthinsp;Kminus;1thinsp;molminus;1, (22thinsp;572plusmn;90) Jthinsp;molminus;1, and (minus;87.31plusmn;0.35) Jthinsp;Kminus;1thinsp;molminus;1. The standard Gibbs free energy of formation of Pu2O3at 298.15 K is calculated from available experimental data.