Americium occupies a pivotal position in the actinide series with regard to the behavior of 5f electrons. High-pressure techniques together with synchrotron radiation have been used to determine the structural behavior up to 100 GPa. We have resolved earlier controversial findings regarding americium and find that our experimental results are in discord with recent theoretical predictions. We have two new findings: (1) that there exists a critical, new structural link between americium under pressure and its near neighbor, plutonium; and (2) that the 5f electron delocalization in americium occurs in two rather than one step.

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Curium lies at the center of the actinide series and has a half-filled shell with seven 5f electrons spatially residing inside its radon core. As a function of pressure, curium exhibits five different crystallographic phases up to 100 gigapascals, of which all but one are also found in the preceding element, americium. We describe here a structure in curium, Cm III, with monoclinic symmetry, space group C2/c, found at intermediate pressures (between 37 and 56 gigapascals). Ab initio electronic structure calculations agree with the observed sequence of structures and establish that it is the spin polarization of curium's 5f electrons that stabilizes Cm III. The results reveal that curium is one of a few elements that has a lattice structure stabilized by magnetism.

The thermal expansion and anisotropic magnetomiction of the R C q Laves phases were studied in the temperature range 4-500 K using the x-ray powder diffraction methcd.In the heavy RCo2 the magnetic moment of the itinerant d electron subsystem derived imm the magnetovolume effect was found to fit well with the magnetization curve of YCo2. A pronounced paraprocess above the metamagnetic m i t i o n has been observed when increasing the f-d exchange field. "%e type and temperature variation of the distortion of the njbic unit cell of the t h ~ compounds PrCol, NdCo2 and SmCoz have been studied in detail. The corresponding magnetostriction ~nslants 1111 or AIM were calculated. At 4 K the following values have been obtained: R c 0 2 . temgonai distortion. easy axes (100). AIM = -3.4 x SmCq, rhombohedral distortion, easy axes (Ill), A l l , r -4.6 x NdC4, teuagonal distortion and eaxy axes (100) for 42 K c I e T,. orthorhombic distortion and easy axes (110) for T c 42 K with 111111 = 1.9 x aid AIM = -4.0 x IO-' .

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Protactinium occupies an important position in the actinide series of elements, as it represents the first of four elements ͑Pa-Pu͒ having 5 f -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 Cmcm͒ at 77͑5͒ GPa, where the atomic volume has been reduced by ϳ30%. This structural change is interpreted as reflecting an increase in 5 f -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.