THEORETICAL INVESTIGATION OF THE HIGH-PRESSURE PHASES OF CE

In order to shed light on the recent experimental controversy concerni ng the intermediate pressure phases of Ce we have made systematic elec tronic structure and total-energy studies on Ce in the experimentally reported low-pressure phase alpha-Ce (fcc), the intermediate-pressure alpha-U (alpha'), the body-centered monoclinic [alpha ''(I)], and C-fa ce-centered monoclinic [alpha ''(II)] phases, together with the stable high-pressure body-centered tetragonal phase. We also included the bo dy-centered cubic, hexagonal-close-packed, and omega (hP3) phases. In this study we used the accurate full-potential linear muffin-tin orbit al (FPLMTO) method. The optimized structural parameters obtained from our total-energy studies for the alpha' and alpha ''(II) phases are fo und to be in good agreement with corresponding experimental values. Th e structural optimization of the alpha ''(I) phase always yields the f ee or bet phase stable, depending upon the volume considered. Except f or an improvement in the equilibrium volume, the generalized gradient correction reproduces the calculated relative stability between differ ent phases of Ce at high pressure of the local-density approximation. Of the experimentally reported intermediate pressure structures [alpha ', alpha ''(I) and alpha ''(II)] we find that the alpha ''(II) phase i s the most stable. Among the contending phases, alpha' and alpha ''(I) , the latter is very close in energy to the alpha '' phase whereas the former is substantially higher in energy. We thus rule out the alpha- U structure as an intermediate pressure phase of Ce. Our work suggests that the most probable structural phase transition sequence of Ce met al is fcc(gamma)-->fcc(alpha)-->alpha ''(II)-->bct, which is consisten t with current experimental results.