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Lithium is considered a 'simple' metal because, under ordinary conditions of pressure and temperature, the motion of conduction electrons is only weakly perturbed by interactions with the cubic lattice of atomic cores. It was recently predicted that at pressures below 100 GPa, dense Li may undergo several structural transitions, possibly leading to a 'paired-atom' phase with low symmetry and near-insulating properties. Here we report synchrotron X-ray diffraction measurements that confirm that Li undergoes pronounced structural changes under pressure. Near 39 GPa, the element transforms from a high-pressure face-centred-cubic phase, through an intermediate rhombohedral modification, to a cubic polymorph with 16 atoms per unit cell. This cubic phase has not been observed previously in any element; unusually, its calculated electronic density of states exhibits a pronounced semimetal-like minimum near the Fermi energy. We present total-energy calculations that provide theoretical support for the observed phase transition sequence. Our calculations indicate a large stability range of the 16-atom cubic phase relative to various other crystal structures tested here.

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Wurtzite has the space-group symmetry P6 3 mc. The absence of inversion symmetry allows linear-k terms in the electronic band structure when the spin-orbit interaction is included. Their existence has been confirmed in a number of experiments, but no microscopic calculations have been published. In the present paper, we discuss the origin of these linear-k terms using group theory and k•p arguments. The various contributions to these terms are identified through band-structure models. We present an ab initio calculation, performed with the linear-muffin-tin-orbital method, of these spin splittings in CdS, CdSe, and ZnO. A renormalization of the valence-band spin-splitting coefficients obtained in the linear-muffin-tin-orbital calculations was found necessary to correct for errors in the relative energies of the uppermost valence bands as compared with the experimental values. We point out that a similar procedure should be used when evaluating masses and other band parameters from calculated local-density-approximation band structures.

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