Pressure-induced changes in protactinium metal: Importance to actinide-metal bonding concepts

Protactinium occupies an important position in the actinide series of elements, as it represents the first of four elements (Pa-Pu) having 5f-electron character in their bonding at atmospheric pressure. We have determined in experimental studies with synchrotron radiation to 130 GPa, that the tetragonal structure of protactinium (space group I4/mmm) converts to an orthorhombic, alpha-uranium structure (space group Cmcm) at 77(5) GPa, where the atomic volume has been reduced by similar to30%. This structural change is interpreted as reflecting an increase in 5f-electron contribution to the bonding in protactinium over that initially present, becoming more similar to that present in alpha-uranium metal at atmospheric pressure. We determined experimentally that this structural transformation occurred at significantly higher pressures and at a smaller atomic volume than predicted by theory. The experimental results reported here represent the highest pressures under which protactinium metal has been studied.