Temperature-Dependent Gibbs Free Energies of Reaction of Uranyl-Containing Materials Based on Density Functional Theory

The thermodynamic properties of uranyl-containing materials, including dehydrated schoepite, metastudtite, studtite, soddyite, rutherfordine, and gamma-UO3, determined by means of density functional theory using a new norm-conserving pseudopotential for uranium atom in previous works, were used to obtain the enthalpies and Gibbs free energies of eight reactions involving these materials and its variation with the temperature. The first five reactions represent the formation of the first five considered materials in terms of the corresponding oxides, and the remaining reactions are the transformations of rutherfordine into dehydrated schoepite, studtite into metastudtite, and uranium trioxide into triuranium octoxide, respectively. The experimental values of the enthalpies of these reactions, which are only known at the standard state (temperature of 298.15 K and pressure of 1 bar), were reproduced accurately by these calculations, the errors being 2.5, 2.5, 0.2, 0.0, 12.3, -1.1, 0.9, and 4.0 kj.mol(-1), respectively. These calculations predict that soddyite is stable with respect to the corresponding oxides at all the temperatures considered (280-500 K), and that studtite and metastudtite are unstable. However, dehydrated schoepite and rutherfordine are stable for temperatures lower than 462.5 +/- 16.7 and 513.7 +/- 35.8 K, respectively, and become unstable for higher temperatures. The relative stability of the uranyl peroxide hydrates, studtite and metastudtite, with respect to gamma-UO3, dehydrated schoepite, rutherfordine and soddyite in the presence of water and hydrogen peroxide was also studied by considering the corresponding reactions. The experimental values of the enthalpies of two of these reactions, those relating studtite and metastudtite with gamma-UO3 in the presence of water and hydrogen peroxide, which are only known at the standard state, were theoretically very well reproduced (within 0.1 kj.mol(-1)). From the results, we conclude that metastudtite and studtite become stable phases at low temperatures in the presence of water and small amounts of hydrogen peroxide. However, the stability of the studtite phase decreases rapidly when the temperature increases. Finally, the relative stability of these phases in the presence of very high concentrations of hydrogen peroxide was evaluated. The results show that in this case, occurring under very intense radiation fields causing the radiolysis of most of the water present, studtite is by far the most stable phase within the full range of temperature studied.