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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.

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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.

The high-pressure structural behavior of lanthanum monochalcogenides is investigated by theory and experiment. Theory comprises density functional calculations of LaS, LaSe and LaTe with the general gradient approximation for exchange and correlation effects, as implemented within the full-potential linear muffin-tin orbital method. The experimental studies consist of high-pressure angle dispersive x-ray diffraction investigations of LaS and LaSe up to a maximum pressure of 41 GPa. A structural phase transition from the NaCl type to CsCl type crystal structure is found to occur in all cases. The experimental transition pressures are 27-28 GPa and 19 GPa, for LaS and LaSe, respectively, while the calculated transition pressures are 29 GPa, 21 GPa and 10 GPa for LaS, LaSe and LaTe, respectively. The calculated ground state properties such as equilibrium lattice constant, bulk modulus and its pressure derivative, and Debye temperatures are in good agreement with experimental results. Elastic constants are predicted from the calculations.