CRYSTAL-STRUCTURE STABILITIES AND ELECTRONIC-STRUCTURE FOR THE LIGHT ACTINIDES TH, PA, AND U

The crystal-structure stabilities, equilibrium volumes, and bulk moduli (at T=O) of the light actinides Th, Pa, and U, have been calculated by means of full-potential, total-energy band-structure calculations. The total energies of the three elements were calculated as a function of volume in the three experimentally observed crystal structures: fcc, bct, and orthorhombic (alpha-U). Our calculations reproduce the experimentally observed crystal structures, as well as the equilibrium volumes and bulk moduli (the bulk modulus of Pa being an exception). Other calculated ground-state properties are also in good agreement with experiment, e.g., crystal-structure parameters (c/a ratio and positional parameters). On the basis of our results, we argue that the 5f electrons are participating in the chemical bonds, and that they have a large influence on the crystal structure. The equilibrium volumes of hypothetical fcc structures are found to show increasing deviations from the volumes obtained in the true crystal structures, as the 5f band becomes filled. Also, these fully relativistic calculations (assuming a fcc structure) show a smaller volume for Pu than for Np, in contrast to the experimental finding. We therefore propose that the anomalous volume of alpha-Pu is associated with its very unusual crystal structure, rather than with relativistic effects. Detailed information from the calculations is presented, such as the density of states, charge-density contour plots, and orbital occupation numbers.