Calculated structural stabilities of U, Np, Pu and Am;: new high-pressure phases for Am and Pu

The high-pressure phase of americium Am IV recently found experimentally, which is a primitive-orthorhombic structure with four atoms in the unit cell, is also found to be stable theoretically at high pressure in Am by a calculation using the full-potential linearized augmented-plane-wave (FPLAPW) method. The possibility of the Am IV structure being a stable high-pressure phase of U, Np or Pu, like in the case of Am, is examined using the FPLAPW method by comparing the total energy as a function of volume with ten other crystal structures. The results indicate that the Am IV structure is not a high-pressure phase for U and Np. but it is for Pu, for which it is proposed that the sequence under pressure is alpha-Pu --> Am IV --> body-centred cubic. Our calculations are reliable, because they give theoretically the stability of the crystal a-phases of U, Np, Pu under ambient conditions. The normal-pressure phase of Am is modelled by our fully relativistic muffin-tin orbital method with unhybridized 5f electron states.