Calculations of crystal-structure stabilities of Ce under pressure

The total energies of the observed crystal structures of Ce [face-centered cubic (fcc), orthorhombic, and body-centered tetragonal (bct)] under pressure have been calculated, using the local-density approximation. The linear-muffin-tin-orbital calculations were full potential, all electron, and fully relativistic. The experimental data for the different crystallographic transitions are well reproduced by the calculations and we have extracted two terms that are mainly responsible for the {α}{→}{α}' transition: a one-electron term and a Madelung term. The {α}{→}{α}' transition is driven by the increasing importance of the 4f contribution with decreasing volume. This finding is also supported by a calculation without the 4f contribution to the cohesion which yields the {α}' phase unstable. The {α}'{→}bct transition is found to be somewhat more complex in nature since it is quite heavily influenced also by the 5d electrons. The calculated ground state is (correctly) found to be fcc and the equilibrium volume as well as the bulk modulus are in good agreement with experiment. The present ab initio calculation of a crystallographic phase diagram of an f electron system suggests delocalized 4f electrons exist in the high-pressure phases, including the {α} phase, of Ce.