Theoretical prediction of some layered Pa(2)O(5) phases: structure and properties

Density functional theory (DFT) was used to predict and study protactinium pentoxide (Pa(2)O(5)), 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 Pa(2)O(5) are not clear and understandable. Our theoretical prediction explored some possible stable structures of the Pa(2)O(5) stoichiometry according to the existing M(2)O(5) 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. Pa(2)O(5) in the β-V(2)O(5) structure was found to be a competitive structure in terms of stability, whereas Pa(2)O(5) in the ζ-Nb(2)O(5) structure was found to be the most stable overall. This is consistent with Sellers's experimental observations. In particular, Pa(2)O(5) in the ζ-Nb(2)O(5) structure is predicted to be charge-transfer insulators. Furthermore, we predict that ζ-Nb(2)O(5)-structured Pa(2)O(5) is the most thermodynamically stable under ambient conditions and pressure.