CRYSTALLOGRAPHIC PHASE-TRANSITIONS IN ACTINIDE METALS AS A FUNCTION OF PRESSURE

We present first-principles calculations of the equilibrium volumes and crystal structures of the light actinides (Th-Pu). The calculated equilibrium volumes for f.c.c. Th, b.c.t. Pu, alpha-U, and beta-Np are found to agree reasonably well with the experimental data, and when comparing the total energies of the b.c.c., f.c.c., b.c.t., alpha-U, and beta-Np structures we obtain the correct crystal structures for all studied systems. Also, the calculated equilibrium volumes for Th-Pu, using a hypothetical f.c.c. structure, have been calculated and it is demonstrated that although spin-orbit coupling is included in these calculations the calculated equilibrium volume of Pu is smaller than for Np, in disagreement with experiment. Moreover, the calculated tetragonal elastic constant, C', is shown to be negative for b.c.c. U, b.c.c. Np, b.c.c. Pu and f.c.c. Pu. Thus, our zero temperature calculations suggest that the b.c.c. structure is unstable for these elements and that f.c.c. Pu is also unstable. This is in conflict with experiment and we are led to the conclusion that temperature effects must be of crucial importance for stabilizing cubic structures in U, Np, and Pu. Further, as a function of decreasing volume we predict a crystal structure sequence f.c.c. --> b.c.t. --> f.c.c. in Th, a sequence alpha-U --> b.c.t. --> b.c.c, in U, and a sequence beta-Np --> b.c.t. --> b.c.c. in Np. Also, a sequence of transitions in Sc as a function of decreasing volume have been calculated, namely h.c.p. --> f.c.c. --> omega --> beta-Np --> b.c.c.