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 °C (383 ± 27 K) and its dehydration occurs
at 64 °C (337 ± 44 K). The corresponding values of these
temperatures for metaschoepite are 82 °C (355 ± 6 K) and
5 °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.