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

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.