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

Abstract

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.

Structural, mechanical, spectroscopic and thermodynamic characterization of the copper-uranyl tetrahydroxide mineral vandenbrandeite

“…The solution of this problem is currently being attempted by means of experimental X-ray diffraction techniques 72,[109][110][111][112] and rst-principles theoretical methods. [113][114][115][116][117][118][119] In the specic case of copper-containing uranyl minerals, the full-crystal structure of cuprosklodowkite was recently determined experimentally. 72 The full crystal structure of vandenbrandeite, including the positions of the hydrogen atoms within the corresponding unit cell, was rst obtained by using the rst principles methodology in 2016 (see the ESI of ref.…”

“…Similarly, the thermodynamic properties of reaction of the reactions of conversion of vandenbrandeite into schoepite, 114 [(UO 2 ) 8 O 2 (OH) 12 ]$12H 2 O(cr), and studtite, 121 (UO 2 )O 2 $4H 2 O, in the presence of water and hydrogen peroxide and in the presence of high concentrations of hydrogen peroxide, respectively, were obtained by considering the corresponding reactions:…”

“…The temperature dependent thermodynamic properties are key parameters for the performance assessment of radioactive waste repositories because the stability of the secondary phases of the spent nuclear fuel under nal geological disposal conditions is highly dependent of the temperature. 124,125 The computed thermodynamic parameters together with those derived in previous works 114,115,[121][122][123][124][125] permitted to determine the thermodynamic stability of vandenbrandeite with respect to a selected set of the most relevant secondary phases of SNF under nal geological disposal conditions.…”

“…One of the most important motivations of this work is to contribute to the development of nuclear Raman 113,117-120 and thermodynamic databases. 114,115,[121][122][123][124][125][125][126][127][128][129] Raman spectroscopy is one of the main techniques used to identify uraniumcontaining materials because it does not need a special preparation of the samples, a very small amount of sample is required and it is non-destructive. Thus, this spectroscopic technique minimizes the difficulties in the handling of these materials associated to its radiotoxicity and the costs of the analyses because the samples may be reutilized.…”

Periodic density functional theory study of the Raman spectrum of the hydrated uranyl oxyhydroxide mineral becquerelite

“…The development of a new high-quality norm conserving relativistic pseudopotential specific for uranium atom [27][28], has allowed an accurate theoretical solid-state treatment of these materials [29][30]. These theoretical studies have been used not only as a complement of the experimental techniques allowing the determination of the complete crystal structures of many uranyl containing materials [31][32][33] and the precise assignment of their Raman spectra [26,28,31,[34][35], but also as a very powerful predictive tool of their thermodynamic, mechanical and optical properties [31][32][33][34][35][36][37][38][39][40][41][42][43][44].…”