Pressure is expected to drive the atoms in the crystal lattice closer to each other, forcing the electrons to

~ 30 T_

О

20 E

10 0

participate in the binding (delocalization),6 which particularly affects the heavy actinides with loca­lized f-electron behavior at ambient pressure. Recent studies using diamond anvil cells coupled to synchrotron radiation have provided strong evi­dence for that. As discussed by Heathman et a/.,7 americium shows a remarkable decrease in volume with increasing pressure (at ambient temperature) with three transitions up to 100 GPa (Figure 3). Its structure changes from hcp (Am-I) through fcc (Am-II) to orthorhombic (Am-III and Am-IV), indi­cating the appearance of the itinerant character 5f electrons. This behavior is also observed in curium, with a puzzling supplementary magnetically stabi­lized Cm-III structure at 40-60 GPa.8 Uranium shows a comparatively straightforward behavior and the a-structure is stable up to 100 GPa, with a much smaller volume decrease.6 A similar behavior has been found for protactinium, its a-form being stable up to
80 GPa. This is clearly reflected in the isothermal bulk modulus (Table 2), which is around 100 GPa for the elements Pa to Np but around 30-40 GPa for Am and Cm. The Am-IV phase shows a large bulk modulus (more similar to that of uranium), as expected for a metal with appreciable 5f-electron character in its bonding. This is also evident from the comparison of the actinide and lanthanide metals (Figure 4).

Uncertainty still exists about the bulk modulus of a-plutonium. As discussed by Ledbetter et a/.,12 the published B0 values at ambient range show a large variation, as do the theoretical calculations. The most accurate results for the isothermal bulk modulus vary between 51(2) GPa13 and 43(2) GPa.14