New high-pressure phases of lithium

Lithium is considered a 'simple' metal because, under ordinary conditions o f pressure and temperature, the motion of conduction electrons is only weak ly perturbed by interactions with the cubic lattice of atomic cores. It was recently predicted(1) that at pressures below 100 GPa, dense Li may underg o several structural transitions, possibly leading to a 'paired-atom' phase with low symmetry and near-insulating properties. Here we report synchrotr on X-ray diffraction measurements that confirm that Li undergoes pronounced structural changes under pressure. Near 39 GPa, the element transforms fro m a high-pressure face-centred-cubic phase, through an intermediate rhomboh edral 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-li ke minimum near the Fermi energy. We present total-energy calculations that provide theoretical support for the observed phase transition sequence. Ou r calculations indicate a large stability range of the 16-atom cubic phase relative to various other crystal structures tested here.