New high-pressure phases of lithium

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Abstract

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 predicted1 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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Figure 1: Synchrotron X-ray diffraction diagrams of high-pressure phases of lithium.
Figure 2: Structural parameters of compressed lithium.
Figure 3: Schematic representation of the cubic crystal structure of Li near 45 GPa.
Figure 4: Calculated charge density distribution and electronic density of states of Li at a relative volume V/V0 = 0.4 (theoretical pressure 48.8 GPa).
Figure 5: Calculated enthalpy differences (relative to f.c.c.) for Li in various crystal structures as a function of pressure.

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Correspondence to K. Syassen.

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