Prediction of structural properties of protactinium pentoxide ${Pa}_2{O}_5$ by first-principlescalculations

For the first time we report on electronic structure calculations for different possible phases of protactinium pentoxide using density functional theory. Among these phases orthorhombic $\lambda -{Pa}_2{O}_5$ appears the stablest. We also evidenced a phase transition toward a monoclinic $Z-{Pa}_2{O}_5$ structure at 8 GPa. The electronic, mechanical and optical properties are calculated. Both phases are large gap charge-transfer insulators with dielectric gaps respectively 5.35 eV for $\lambda -{Pa}_2{O}_5$ and 4.11 eV for $Z-{Pa}_2{O}_5$. With its three phases ($PaO,{PaO}_2$ and ${Pa}_2{O}_5$) the protactinium oxide family shares a lot of physical properties with the niobium oxide family. (paper)