Theoretical prediction of some layered Pa₂O₅ phases: structure and properties

Density functional theory (DFT) was used to predict and study protactinium pentoxide (Pa₂O₅), which presents a fluorite and layered protactinium oxide-type structure. Although the layered structure has been observed with the isostructural transition Nb and Ta metal pentoxides experimentally, the detailed structure and properties of the layered Pa₂O₅ are not clear and understandable. Our theoretical prediction explored some possible stable structures of the Pa₂O₅ stoichiometry according to the existing M₂O₅ structures (where M is an actinide Np or transition Nb, Ta, and V metal) and replacing the M ions with protactinium ions. The structural, mechanical, thermodynamic and electronic properties including lattice parameters, bulk moduli, elastic constants, entropy and band gaps were predicted for all the simulated structures. Pa₂O₅ in the β-V₂O₅ structure was found to be a competitive structure in terms of stability, whereas Pa₂O₅ in the ζ-Nb₂O₅ structure was found to be the most stable overall. This is consistent with Sellers's experimental observations. In particular, Pa₂O₅ in the ζ-Nb₂O₅ structure is predicted to be charge-transfer insulators. Furthermore, we predict that ζ-Nb₂O₅-structured Pa₂O₅ is the most thermodynamically stable under ambient conditions and pressure.