Crystal Structures of Dense Lithium: A Metal-Semiconductor-Metal Transition

Abstract

Ab initio random structure searching and single-crystal x-ray diffraction have been used to determine the full structures of three phases of lithium, recently discovered at low temperature above 60 GPa. A structure with C2mb symmetry, calculated to be a poor metal, is proposed for the oC88 phase (60-65 GPa). The oC40 phase (65-95 GPa) is found to have a lowest-enthalpy structure with C2cb symmetry, in excellent agreement with the x-ray data. It is calculated to be a semiconductor with a band gap of ∼1 eV at 90 GPa. oC24, stable above 95 GPa, has the space group Cmca, and refined atomic coordinates are in excellent agreement with previous calculations.

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

“…The hypothetic high-pressure structure of H 2 (Cmca-4) demonstrates the presence of interstitial space "pockets" with an increased electronic density. Such phenomena have been previously reported under pressure in other light elements, for example in Li 29 , but this is the first indication of that kind of behavior in hydrogen. Similar to the light alkalis, the interatomic potentials in hydrogen evolve with density to become very flat prior to this change in chemical bonding scheme (Fig.…”

“…6). 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.…”

Structural and electronic properties of the alkali metal incommensurate phases

“…This is advantageous at high pressure because, while spherical objects have an optimal packing fraction of 0.74, better packing can be achieved with electrons and ions behaving as different objects. Due to the electron localization, electrides are often insulators or bad metals with large pseudogaps in the density of states (DOS) [13,16]. Typical metals are generally characterized by homogeneous profiles of valence electron density and ELF.…”

“…Indeed, all alkalis exhibit a phase transition to a close-packed face-centered cubic (fcc) phase under modest compression, see Table I. But compressing further results in a marked departure from this simple picture, and all alkali elements exhibit complex crystal structures demonstrated in a succession of experiments [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. These experiments were accompanied with calculations showing intriguing electronic features including electride behavior (the localization of electrons in interstitial lattice sites) and reconstructions to match the Brillouin zone to the Fermi surface [17][18][19][20][21].…”

Predicted novel insulating electride compound between alkali metals lithium and sodium under high pressure

“…At ambient conditions, all the alkali metals crystallize in the bcc structure [1,2] and display a free-electron like metallic character [3,4] . Application of pressure on these systems results in more complex structures [5,6] and remarkable physical phenomena such as unusual melting behavior [7,8] , Fermi-surface nesting [9] , phonon instabilities [10] , and superconductivities [11][12][13][14] , and transformations into poor metals or even insulators [15][16][17][18][19] . At ambient conditions, the ionic radius of Li has large disparity with respect to the other alkali metals [20] .…”