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

334 17. Plutonium oxygen and hydrogen compounds and complexesThese are based on measurements by Riley [70RIL] in 1 atm oxygen, Lyon andBailey [67LYO/BAI], Chikalla et al. [64CHI/MCN], and comparison with urania. Wehave also taken for the value of the heat capacity of liquid PuO 2Cp,m ◦ (PuO −12, l) = 131 J·K−1·molas suggested by Glushko et al. [82GLU/GUR]. It should be emphasised again thatPuO 2 will only melt congruently at very high pressures of oxygen.17.2.1.2.cEnthalpy and Gibbs energy of formationThere is very good agreement between the several values for the enthalpy of combustionof plutonium in oxygen by Popov and Ivanov [57POP/IVA] (−(1056.0 ±4.0) kJ·mol −1 ), Holley et al. [58HOL/MUL] (−(1058.0 ± 1.6) kJ·mol −1 ) and Johnsonet al.[69JOH/DEV](−(1055.70 ± 0.71) kJ·mol −1 ). We select the value: f H ◦ m (PuO 2, cr, 298.15 K) = −(1055.8 ± 1.0) kJ·mol −1givenbyGlushkoet al. [82GLU/GUR], which is biased towards the appreciably moreprecise value by Johnson et al.[69JOH/DEV]. The derived Gibbs energy of formationof PuO 2 (cr) is thus f G ◦ m (PuO 2, cr, 298.15 K) =−(998.113 ± 1.031) kJ·mol −117.2.1.2.dSolubility measurementsThe ratios of the solubility products of PuO 2 (cr) and hydrated PuO 2 (cr) found in thestudies of Kim and Kanellakopulos [89KIM/KAN2]andRaiet al.[80RAI/SER] appearto be similar. The value from Rai et al.[80RAI/SER] is subject to the same problems asthe value for hydrated PuO 2 from the same study [84RAI] (cf. Section 17.2.2.3). Thesolubility product reported by Kim and Kanellakopulos [89KIM/KAN2] forPuO 2 (cr)appears to be a good measurement for a batch experiment on a radioactive solid. However,it is not surprising that the value of K s,0 is greater than the thermodynamic constantbased on bulk (rather than surface) properties of the crystalline material (see Section17.2.2.3).17.2.1.3 Pu 2 O 3 (cr)17.2.1.3.aCrystal structurePu 2 O 3 (cr) has the hexagonal type-A rare earth sesquioxide structure (space groupP3m1) with a = 3.841 × 10 −10 m, c = 5.958 × 10 −10 m and a calculated densityof 11.47 g·cm −3 . As discussed in [67IAE], there is some evidence for the occurrenceof a disordered variant of this structure in some samples of Pu 2 O 3 (cr), but it is notknown whether this is due to the presence of impurities or a change in composition.