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

We present first-principles calculations of the equilibrium volumes an d crystal structures of the light actinides (Th-Pu). The calculated eq uilibrium volumes for f.c.c. Th, b.c.t. Pu, alpha-U, and beta-Np are f ound to agree reasonably well with the experimental data, and when com paring 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 st udied systems. Also, the calculated equilibrium volumes for Th-Pu, usi ng a hypothetical f.c.c. structure, have been calculated and it is dem onstrated that although spin-orbit coupling is included in these calcu lations 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 sug gest 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 w e are led to the conclusion that temperature effects must be of crucia l importance for stabilizing cubic structures in U, Np, and Pu. Furthe r, as a function of decreasing volume we predict a crystal structure s equence 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 N p. Also, a sequence of transitions in Sc as a function of decreasing v olume have been calculated, namely h.c.p. --> f.c.c. --> omega --> bet a-Np --> b.c.c.