Density functional investigation of fluorite-based Pa2O5 phases: Structure and properties

摘要

Protactinium pentoxide (Pa2O5) represents the fluorite and the layered structures of the protactinium-oxygen system and the least studied oxide in the group. Our DFT work explores some possible structures of Pa2O5 stoichiometry by starting from the fluorite PaO2 structure and adding oxygen in octahedral position of protactinium. For all simulated structures, the structural, thermodynamic and electronic properties including lattice parameters, bulk moduli, entropy, zero point energy (ZPE) and band gaps were predicted. The fluorite Pa2O5 in the b- and g-Pa2O5 structures were found to be the most stable overall. Indeed, the doping of oxygen atoms in the octahedral position of Pa is significantly better than other doping positions for the fluorite PaO2 structure. In particular, we found that the PaO2.5 structure is the most thermodynamically stable under ambient conditions. Furthermore, while x from 0 increases to 0.5 for protactinium oxides (PaO2+(sic) x = 0, 0.25, 0.5, 0.75, 1), the decreasing trend with increasing x of the optimized volume is in reasonable agreement with experimental findings.