Thermodynamic data for hydrous and anhydrous PuO2+x(s)

Mixed valent PuO2+x(s, hyd) = (PuV)2x(PuIV)1−2xO2+x(s, hyd) is formed by the reaction of hydrous Pu(IV) oxyhydroxide precipitates, PuO2(s, hyd), with oxygen. The standard molar Gibbs energy of formation is calculated from the Pu(IV) and Pu(V) concentrations at 20–25 °C in equilibrium with PuO2+x(s, hyd) which may be written formally as (PuO2.5)2x(PuO2)1−2x(s, hyd) where x < 0.1:ΔfGm°(PuO2+x(s,hyd))={2x(−971.2±5.4)+(1−2x)(−965.5±4.0)} (kJ/mol).This value is only slightly more negative than ΔfGm°(PuO2(s, hyd)) = −965.5 ± 4.0 kJ/mol. The formation of PuO2+x(s, hyd) with x > 0.25, e.g. pure Pu(V) oxide, PuO2.5(s, hyd), is rather questionable.Anhydrous crystalline Pu(IV) dioxide (ΔfGm°(PuO2(cr)) = −998.1 ± 1.0 kJ/mol) is not oxidised by O2(g) unless the presence of water creates a surface layer of hydrous oxide. (The standard molar Gibbs energy for surface hydration is negative while ΔrGm° for the hydration of the bulk crystalline phase is positive.) Assuming analogous differences between the ΔfGm° values of AnO3(cr) and An(VI) oxyhydroxides or similar stabilization energies for analogous mixed valent compounds of U, Np and Pu, the known thermodynamic data for An(IV, V, VI) oxides and oxyhydroxides are used to estimate the following standard molar Gibbs energies of formation at 25 °C: ΔfGm°(PuO2.25(cr)) = −995 ± 3 kJ/mol, ΔfGm°(PuO2.5(cr)) = −987 ± 10 kJ/mol, and ΔfGm°(PuO3(cr)) = −952 ± 10 kJ/mol. These compounds are less stable than PuO2(cr).