Structure and Properties of Some Layered U₂O₅ Phases: A Density Functional Theory Study

Abstract

UO is the boundary composition between the fluorite and the layered structures of the UO system and the least studied oxide in the group. δ-UO is the only layered structure proposed so far experimentally, although evidence of fluorite-based phases has also been reported. Our DFT work explores possible structures of UO stoichiometry by starting from existing MO structures (where M is an actinide or transition metal) and replacing the M ions with uranium ions. For all structures, we predicted structural and electronic properties including bulk moduli and band gaps. The majority of structures were found to be less stable than δ-UO. UO in the R-NbO structure was found to be a competitive structure in terms of stability, whereas UO in the NpO structure was found to be the most stable overall. Indeed, by including the vibrational contribution to the free energy using the frequencies obtained from the optimized unit cells we predict that NpO structured UO is the most thermodynamically stable under ambient conditions. δ-UO only becomes more stable at high temperatures and/or pressures. This suggests that a low-temperature synthesis route should be tested and so potentially opens a new avenue of research for pentavalent uranium oxides.

Nanoscale oxygen defect gradients in UO 2+ x surfaces

“…The latter study showed that it is possible to stabilize fluorite-like normalU3normalO8 at high temperature and pressure, which is retained upon quenching to ambient conditions. This experimental work is further supported by theory calculations (16, 17), which indicate that stoichiometries up to normalUnormalO2.5 should be possible within the fluorite-like structure. Together, these studies suggest that the primary impact of excess oxygen will be on the type and distribution of O anion defects, rather than a sizable distortion of the U cation sublattice.…”

Direct observation of pure pentavalent uranium in U2O5 thin films by high resolution photoemission spectroscopy

“…A similar approach has been used by Molinari et al . 47 to compare the relative stability of various candidate structures for U 2 O 5 . These authors conclude that the most stable U 2 O 5 structure is the Np 2 O 5 -type monoclinic one (containing uranyl square and pentagonal bipyramids linked by edge-sharing into sheets), whereas δ-U 2 O 5 would become energetically favoured only at high temperatures or pressure.…”

Theoretical prediction of some layered Pa2O5 phases: structure and properties

“…The DFT work from Molinari et al 25 predicted that the Np 2 O 5 -structured U 2 O 5 is the most stable among those considered under ambient conditions. However, the observed d-U 2 O 5 structure is found to be the second most stable.…”

“…In the actinide elements, uranium (U) exhibits rich oxide phases, such as UO 2 , U 2 O 3 , U 2 O 5 , and U 3 O 8 , [20][21][22][23][24][25][26][27] and its two prominent oxidation states are U 4+ and U 6+ . Plutonium (Pu) has stable Pu 4+ , Pu 5+ , and Pu 6+ oxidation states in aqueous solutions.…”