Density Functional Theory Study of the Thermodynamic and Raman Vibrational Properties of γ-UO₃ Polymorph

Abstract

Gamma uranium trioxide, γ-UO3, is one of the most important polymorphs in uranium trioxide system which is common throughout the nuclear fuel cycle and used industrially in the reprocessing of nuclear fuel and uranium enrichment. In this work, a detailed theoretical solid-state density functional theory study of this material was carried out. The computed lattice parameters, bond lengths, bond angles and X-Ray powder pattern were found in very good agreement with their experimental counterparts determined by X-Ray diffraction. The equation of state of γ-UO3 was obtained and, therefore, the values of the bulk modulus and its derivatives, for which there are not experimental data to compare with, were predicted. The computed bulk modulus differs from that of a previous density functional theory calculation by only 4.4%. The thermodynamic properties of this material, including heat capacity, entropy, enthalpy, free energy and Debye temperature were also determined as a function of temperature in the range 0-1000 K. The computed low-and high-temperature thermodynamic functions are in excellent agreement with the experimental ones determined from calorimetric measurements. At ambient temperature, the computed values of heat capacity, entropy, enthalpy and free energy differ from the experimental values by 5.3, 3.3, 3.9 and 2.6%, respectively. Finally, the Raman spectrum was determined and compared with the experimental one and was found to be in good agreement. A normal mode analysis of the theoretical spectra was carried out and used in order to resolve the uncertainty of the assignment in the observed Raman bands. The assignment permits to attribute the different bands to vibrations localized in the different distorted octahedra associated to the two non-equivalent uranium atom types present in the structure of γ-UO3.

Infrared and Raman spectroscopy of α‐ZrW2O8: A comprehensive density functional perturbation theory and experimental study

“…[38][39][40][46][47][48] Properties such as the thermodynamic ones previously calculated with PBEsol for non-NTE materials and oxides showed overall improvement compared with PBE results. [47,49,50] PBEsol elasticity and thermodynamics calculations for α-ZrW 2 O 8 also exhibit better agreement with experiments than other functionals. [48] The interaction between valence electrons and ionic cores was described by the projector-augmented wave method.…”

Becquerelite mineral phase: crystal structure and thermodynamic and mechanical stability by using periodic DFT

“…111,112 This pseudopotential includes scalar relativistic effects and has been used extensively in the research of uranyl containing materials. 88,[90][91][92][111][112][113][114][115][116][117] A single unit cell was used in the calculations. The initial guest to the unit cell of becquerelite was taken from Pagoaga et al 48 However, since the hydrogen atom positions were not determined in any of the previous experimental works, [47][48][49][50] the initial unit cell was supplemented by initial values of these positions and fully optimized.…”

“…The thermodynamic properties of becquerelite were obtained by performing phonon calculations at the optimized structure, by using the same methodology as in previous studies. [90][91][92]115,116,125 The phonon spectra at the different points of Brillouin zone were calculated using Density Functional Perturbation Theory (DFPT), as second order derivatives of the total energy. 126 Several important thermodynamic quantities, such as Gibbs free energy, enthalpy, entropy and specic heat 127 can be evaluated in the quasi-harmonic approximation from the knowledge of the entire phonon spectrum and the corresponding phonon dispersion curves and density of states.…”

“…In this case, there are no electrons in valence f orbitals and the use of the standard GGA approximation supplemented with empirical dispersion corrections has produced highly accurate results for the structural, spectroscopic, mechanic and thermodynamic properties of a large series of uranyl containing minerals as rutherfordine, soddyite, uranophane-a, studtite, metastudtite, g-UO 3 , dehydrated schoepite, schoepite and metaschoepite. 88,[90][91][92][111][112][113][114][115][116][117][104][105][106] It must be emphasized that the calculated thermodynamic properties of these materials were very accurate in those cases in which experimental values of these properties were available for comparison even at very low and high temperatures. [90][91][92]115,116 As an example, the errors in the computed enthalpies of formation of rutherfordine, 90 g-UO 3 , 90 and metaschoepite 92 at 700, 900, and 800 K were 1.6%, 1.0% and 2.0%, respectively.…”

Can we predict the structure and stability of molecular crystals under increased pressure? First‐principles study of glycine phase transitions

“…During the geometry optimization all atoms positions and cell parameters were optimized with only the crystal space group being constrained. The linear response density functional perturbation theory (DFPT) implemented in the CASTEP code was used to obtain the phonon dispersion curves and phonon density of states from which thermodynamic quantities in the quasi‐harmonic approximation: free energies (Δ G ), enthalpies (Δ H ), and entropies (Δ S ), at 293 K were evaluated …”