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

U₂O₅ is the boundary composition between the fluorite and the layered structures of the UO_2→3 system and the least studied oxide in the group. δ-U₂O₅ 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 U₂O₅ stoichiometry by starting from existing M₂O₅ 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-U₂O₅. U₂O₅ in the R-Nb₂O₅ structure was found to be a competitive structure in terms of stability, whereas U₂O₅ in the Np₂O₅ 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 Np₂O₅ structured U₂O₅ is the most thermodynamically stable under ambient conditions. δ-U₂O₅ 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.