Theoretical investigation of the high-pressure phases of Ce

摘要

In order to shed light on the recent experimental controversy concerning the intermediate pressure phases of Ce we have made systematic electronic 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-face-centered monoclinic [alpha "(II)] phases, together with the stable high-pressure body-centered tetragonal phase. We also included the body-centered cubic, hexagonal-close-packed, and omega (hP3) phases. In this study we used the accurate full-potential linear muffin-tin orbital (FPLMTO) method. The optimized structural parameters obtained from our total-energy studies for the alpha' and alpha "(II) phases are found to be in good agreement with corresponding experimental values. The structural optimization of the alpha "(I) phase always yields the fee or bet phase stable, depending upon the volume considered. Except for an improvement in the equilibrium volume, the generalized gradient correction reproduces the calculated relative stability between different 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 is 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 metal is fcc(gamma)-->fcc(alpha)-->alpha "(II)-->bct, which is consistent with current experimental results. [References: 63]