New high-pressure phases of lithium

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 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.

High-temperature high-pressure phases of lithium from electron force field (eFF) quantum electron dynamics simulations

“…For the metallic fcc phase, we find IEs located at octahedral interstices between Li þ ions (six coordinate), whereas in the semimetallic cI16, half of the IEs are located inside trapezoidal zigzag chains in the (010) direction, with the other half sitting in between ions bridging the chains (eight coordinate but with four shorter bonds). These locations are identical to the electron density maxima observed in DFT calculations (10).…”

“…Such predictions stimulated X-ray diffraction experiments of compressed Li (10), that found a transition from fcc (CN 12) to the cI16 (CN 11) structure in which each electron is close to eight nuclei. This was followed by further DFT studies showing the cI16 (9, 21-24) structure to be stable at high P and low T Author contributions: H.K., J.T.S., and W.A.G.…”

Dynamical stability of face centered cubic lithium at 25 GPa

“…Although it could be expected to evolve to an even more free-electron like system with increasing pressure, it has been shown that pressure not only induces several structural transformations [2,3,4], but also gives rise to a plethora of fascinating physical properties [5] normally induced by nonlocality in the core pseudopotential with pressure [6,7]. For instance, lithium becomes a semiconductor near 80 GPa [8], it melts below ambient temperature (190 K) at around 50 GPa [2] and displays a periodic undamped plasmon according to theoretical calculations [9].…”

Diamonds on Diamond: structural studies at extreme conditions on the Diamond Light Source

“…On further compression, there is a transition to a body-centred cubic structure with 16 atoms per unit cell (cI16) at 103 GPa [30], followed by transitions to a number of very complex phases above that pressure [31,32]. The cI16 structure had already been determined in lithium, where it exists above 40 GPa at 180 K [33] and is shown in figure 3. It has space group I43d with 16 atoms on a 16c site, (x, x, x), and refinements of powder diffraction data for Li gave a value of 0.055(1) for x at 46 GPa and 180 K. The cI16 crystal of Na was prepared by compressing a powdered sample of Na to 108 GPa, followed by careful annealing just below the melting curve at 313 K. The diffraction data were collected on an image-plate detector in a sequence of contiguous ±0.25 • oscillations over a total scan range of 14 • around the vertical axis using an X-ray wavelength of 0.3444 Å.…”