Cold melting and solid structures of dense lithium

Abstract

Recent theoretical and experimental studies have produced several unusual and interesting results on dense lithium, the first metal in the periodic table. These include the deviation from simple metal behaviour, superconductivity at 17 K, and a metal to semiconductor transition 1-5 . Despite these efforts, at present there is no agreement on the location of the highpressure solid phases and melting curve of Li, and there is no clear picture of its phase diagram above 50 GPa (refs 4-7). Using powder and single-crystal high-pressure diffraction techniques, we have mapped out the lithium phase diagram up to 130 GPa over a wide temperature range between 77 and 300 K. Whereas the melting temperatures of materials usually rise under pressure, and even the lightest condensed gases, hydrogen and helium, melt at temperatures of the order of 10 3 K at 50 GPa (refs 8,9), we find that at these pressures lithium remains a liquid at temperatures as low as 190 K, by far the lowest melting temperature observed for any material at such pressure. We also find that in its solid state above 60 GPa, lithium adopts three novel and complex crystal structures not previously observed in any element. Estimates of the zeropoint energy suggest that quantum effects play a significant role in shaping the lithium phase diagram.The familiar properties and states of matter can be markedly modified by applying pressure and temperature. Besides those encountered in daily life (gas, liquid and solid), some exotic states, for example superfluids or superconductors, can be observed. Quantum effects, the energies of which are very small on an everyday scale, are responsible for the formation of these unusual forms of matter. To create any of these states, low temperatures are needed to decrease the energy of the system to the level where the quantum effects become dominant. Conversely, by applying pressure, and thereby bringing the atoms closer to each other, it is possible to increase the kinetic energy (that is, the zero-point energy) of the system. If the other energy terms that make up the total energy increase more slowly with pressure than the zero-point energy, it might be possible to reach a compression at which the quantum effects play the dominant role 10 . One of the obvious consequences of the zero-point energy being comparable to or in excess of differences in characteristic structural energies per atom would be melting of the solid under compression (cold melting) 10,11 . For light elements, such as hydrogen, melting influenced by the zero-point energy is expected to happen even at T = 0 (at compressions which are at present beyond the capabilities of experimental techniques), leading to a metallic liquid ground state with exotic properties 12 .Is it then possible to create a metallic liquid ground state in systems other than dense hydrogen? Most metallic elements with strong interatomic interactions are solids under normal conditions,

High-pressure studies with x-rays using diamond anvil cells

“…An example is the complexity of structure revealed at HP. Contrary to the conventional understanding that the crystal structures at HP becomes increasingly simple like arrays of closely packed balls, HP increases the interaction of the inner electrons, and atoms may become irregularly-shaped and non-spherical, resulting in very complicated crystal structures and very large unit cells [345,346] (Fig. 19).…”

“…When they eventually solidify at lower temperatures, the simple metals adopt a range of highly complex structures previously unobserved in any element. Na remains a liquid at room temperature at 110 GPa, while Li has a melting point of 190 K at 45 GPa, by far the lowest melting point among the elemental metals [345].…”

Bonding, structures, and band gap closure of hydrogen at high pressures

“…Light elements in this regime tend to show structural diversity, [29][30][31] low-frequency vibrational modes 22,32 , and low melting temperatures. 30,31 This is the consequence of proton zero point energy becoming the important energy scale because of a decline of the intramolecular bonding (e.g., Refs. 22,31 ).…”

Room-Temperature Lithium Phases from Density Functional Theory

“…Based on the experimental phase diagram, 13 the γ phase becomes more stable than β at high pressure (above 6.6 GPa at 300K) and less stable than the α phase at low temperature (below 70 K). Experimentally some or all of these phases may be found in any particular sample depending on the history of the sample, indicating that these three forms all have similar cohesive energies.…”