chemical thermodynamics of neptunium and plutonium - U.S. ...

20.1 Plutonium nitrogen compounds and complexes 403This equation is derived using the recommended values for the enthalpy of formation[69JOH/DEV], the entropy [78HAL/LEE] at 298.15 K, and the Gibbs energyfunctions for PuN(cr) [78OET]. The Gibbs energy functions for elemental plutonium[76OET/RAN] andforN 2 (g) [89COX/WAG] were also used in the assessment. Therecommended value for the Gibbs energy of formation at 298.15 K is, f G ◦ m (PuN, cr, 298.15 K) =−(273.7 ± 2.6) kJ·mol−1There are several determinations of the Gibbs energy of formation of PuN(cr) frommeasurements of vapour pressure of congruently vaporizing PuN(cr). One set of data[69KEN/LEA] gives values for f G ◦ m (PuN, cr) at 1000 K within 1 kJ·mol−1 and at2000 K within 3 kJ·mol −1 of the recommended values. The vaporisation behaviour ofPuN(cr) has been discussed in detail [87MAT/OHS, 92SUZ/MAE].The decomposition pressures of N 2 gas for the reaction,PuN(cr) Å Pu(l) + 0.5N 2 (g)have been measured [64OLS/MUL] in the temperature range 2563-3043 K.20.1.1.1.fThe phase diagramDetailed investigations of the range of composition of the Pu mononitride phase havenot been undertaken, but it is probably quite small. There is no detailed informationabout the solubility of nitrogen in elemental plutonium, but in the temperature range1544-1635 K the solubility is low [69KEN/LEA]. PuN(cr) has been frequently preparedby the reaction of nitrogen with Pu hydride or with elemental plutonium in thepresence of hydrogen [64OLS/MUL] at elevated temperatures; typically at ca. 850K. During the preparation at relatively high temperatures, PuN(cr) can easily becomecontaminated with both oxygen and carbon. PuN(cr) can also be prepared by the carbothermicreduction of PuO 2 (cr) in the presence of N 2 gas [83SUZ/ARA]. Becauseof the possibility of incorporation of both carbon and oxygen in the mononitride lattice,care must be taken in the interpretation of measurements of the variation of latticeparameters.The lattice parameters of so-called “pure” PuN(cr), that is material with low levelsof impurities, vary in the range, a 0 = (4.9025 − 4.9075) × 10 −10 m[68SPE/LEI].The presence of the impurities carbon and oxygen increases the lattice parameter, forexample, up to 4.9117 × 10 −10 m[64ANS/DEA].Pu mononitride and Pu monocarbide form a complete range of solid solutions; thesecompounds are isomorphous. The solubility of oxygen expressed in terms of the hypotheticalPu monoxide is (13 ± 3mol%) [93JAI/GAN]. The relatively small variationsof the lattice parameter of PuN in the presence of oxygen would indicate this. Thelattice parameter and thermodynamic properties of “PuO” have been determined fromstudies of the Pu-C-O system, and this enabled meagre thermodynamic information tobe obtained for the Pu-N-O system. An assessment of the thermodynamic propertiesand phase equilibria of these systems has been published [80POT/SPE].Studies of the vaporisation of Pu mononitride in the temperature ranges, 1658-1976K[69KEN/LEA] and 1400-2400 K [76ALE/CLA], indicated that Pu mononitride had