High-pressure crystal structures of actinide elements to 100 GPa

The structural behaviour of the actinide metals with pressure is being studied using diamond-anvil cells and the high-brilliance and angle-dispersive techniques now available at the ID30 beamline of the European Synchrotron Radiation Facility. A review is given of recent work at room temperature and up to 100 GPa. The results illustrate clearly the difference between the earlier metals (Pa, U) and those further across the actinide series (Am, Cm, and Cf). The complex structures at atmospheric pressure of the lighter actinides are a consequence of itinerant 5f electrons, and these metals show less compressibility (i.e. exhibit larger bulk moduli) and few (if any) phase transitions under pressure. In contrast, the transplutonium metals do not have 5f electrons contributing to their cohesive energies at atmospheric pressure, are therefore `soft' (have lower bulk moduli), and show multiple phase transitions before their 5f electrons become itinerant. After delocalization of their 5f electrons due to pressure, they acquire structures displayed by the light actinides.