Local-density-functional approach to the isostructualγ−αtransition in cerium using the self-consistent linearized-augmented-plane-wave method

Abstract

The isostructural $\gamma -\alpha$ phase transition of Ce which occurs at 8 kbar has been studied by means of fully self-consistent (non-muffin-tin potential) linearized-augmented-plane-wave energy band calculations carried out for five different values of the lattice constant. In contradiction to the $4f$ electron promotional model of the transition, the results yield essentially one $4f$ electron to be occupied in each phase but with the $4f$ wave function somewhat less localized, and therefore more bandlike, in the "collapsed" $\alpha$ phase. A singly occupied $4f$ state is shown to be consistent with the available experimental data. These results strongly support the picture of a $4f$ localized ↔ itinerant transition at the $\gamma -\alpha$ transition and conflict with the promotional model in which some fraction of $4f$ electrons are transferred to the $\mathrm{sd}$ conduction bands. The weaker bonding of the $4f$ electrons, compared to that of the $6s-5d$ valence electrons, accounts for $\alpha$-Ce appearing to have 3.5-3.7 bonding electrons in some respects. Calculation of the superconducting transition temperature $T_c$ suggests that a small spin-fluctuation contribution detrimental to superconductivity is necessary to account for the very low value of $T_c$ in $\alpha$-Ce. Comparison with specific-heat data also suggests a spin-fluctuation contribution to the effective mass; susceptibility data point to a moderate exchange enhancement in $\alpha$-Ce. We calculate a spin-susceptibility (Śtoner) enhancement that increases with temperature, in agreement with experiment up to 150 K, and which tends to diverge at higher temperatures. Certain experiments are suggested which could help greatly in understanding further some important characteristics of the $\alpha$ phase.