Periodic DFT Study of the Thermodynamic Properties and Stability of Schoepite and Metaschoepite Mineral Phases

The thermodynamic properties of schoepite and metaschoepite were obtained by means of theoretical solid-state methods as a function of temperature. Since the values of these properties for schoepite have not been measured experimentally, they were predicted. The computed thermodynamic functions of metaschoepite were in excellent agreement with the experimental information. These functions were used to obtain the thermodynamic properties of formation of these materials from the corresponding elements. The calculated Gibbs free energy of formation of metaschoepite was shown to be very reliable and differ from the experimental value at 800 K by only 2.0%. Besides, it extends the range of temperature in which this property is known to 0-1000 K. Then, these properties were combined with those of other important uranyl-containing materials to study the reactions of formation of schoepite and metaschoepite from uranium trioxide and the reactions of transformation of these materials into dehydrated schoepite, rutherfordine, and soddyite. Schoepite becomes unstable with respect to uranium trioxide for temperatures higher than 110 degrees C (383 +/- 27 K) and its dehydration occurs at 64 degrees C (337 +/- 44 K). The corresponding values of these temperatures for metaschoepite are 82 degrees C (355 +/- 6 K) and 5 degrees C (278 +/- 9 K), respectively. Under hydrogen peroxide free conditions, schoepite and metaschoepite were found to be less stable than rutherfordine and soddyite. The thermodynamic stability of schoepite with respect to metastudtite and studtite was then studied under different conditions of temperature and concentrations of hydrogen peroxide. Schoepite and metaschoepite have very similar thermodynamic stabilities, the first being slightly more stable than the second one. The availability of the thermodynamic properties of these minerals allowed to determine their relative thermodynamic stability with respect to a rich subset of the most relevant secondary phases resulting from corrosion of spent nuclear fuel. Schoepite and metaschoepite were found to be the first and second most stable phases under intermediate hydrogen peroxide concentrations and the second and third most stable phases under high concentrations of hydrogen peroxide, respectively.