Dense Low-Coordination Phases of Lithium

Abstract

Ab initio density-functional-theory calculations and a structure-searching technique are used to identify candidate high-pressure phases of lithium (Li). We predict threefold coordinated structures to be stable in the pressure range 40-450 GPa and fourfold structures at higher pressures. We describe these lowcoordination phases as elemental electrides. All of the stable phases are metallic but the Cmca-24 structure, and two distortions of it which are marginally the most stable in the pressure range 86-106 GPa, are nearly semiconducting with densities of electronic states at the Fermi energy of only a few percent of the free-electron value. DOI: 10.1103/PhysRevLett.102.146401 PACS numbers: 71.15.Nc, 61.66.Àf, 62.50.Àp In the close-packed structures adopted by Li at low pressures [1] the 2s electrons combine to form a half-filled nearly-free-electron band, consistent with the picture of a simple alkali metal. However, strong deviations from the canonical picture develop at higher pressures. Overlap of the Li 1s core electrons on neighboring atoms under strong compression has a profound effect upon the structures, which adopt lower coordination structures [2,3].The close-packed face-centered-cubic (fcc) phase of Li transforms to a structure of I 43d space group symmetry [4] at $40 GPa, via an intermediate phase of R 3m symmetry [3]. The R 3m phase has a distorted fcc structure, but the I 43d structure departs very substantially from close packing. I 43d has three short interatomic distances giving threefold coordination. Calculations show that the valence electronic density of states (e-DOS) dips around the Fermi energy (E F ) [3]. These features are consistent with the picture that reduced coordination numbers at high pressures arise from a Jahn-Teller-like distortion [2]. Calculations of the high-pressure phases also show an accumulation of electronic charge density within the interstitial region as the 2s electron density is pushed away from the atomic cores by Coulomb repulsion, Pauli exclusion, and the orthogonality of core and valence orbitals [2,3,5].Experiments suggest another phase transition at 60-70 GPa, although the new structure has not been determined [6,7]. A number of theoretical studies have sought to identify candidate phases at higher pressures [2,8,9]. A DFT study by Rousseau et al.[8] found a structure of Cmca symmetry to be more stable than I 43d above 88 GPa. This structure is named Cmca-24, appending the number of atoms in the conventional unit cell [8]. Cmca-24 consists of threefold coordinated atoms [8].Ma et al.[9] studied high-pressure phases of Li using a combination of ab initio DFT methods and an evolutionary search algorithm, finding a new structure of P4 1 32 symmetry which they calculated to be more stable than Cmca-24 above 300 GPa. This structure contains sixfold coordinated atoms. On the other hand, recent hightemperature DFT molecular dynamics simulations found fourfold coordinated Li atoms in the pressure range 150-810 GPa [10].Superconductivity has been observed in Li up...

Perspective: Crystal structure prediction at high pressures

“…4 Crystal structure of this semiconducting phase has attracted numerous attentions. 4,25,28,[127][128][129][130][131] The observed insulation seems in accordance with earlier prediction of pairing mechanism, 128 which however is unlikely in view of the proposed energetically much preferred metallic Cmca-24 (24 atoms/cell) structure. 127 Using CALYPSO method on structure prediction, a complex orthorhombic Aba2-40 structure (Fig.…”

“…127 Using CALYPSO method on structure prediction, a complex orthorhombic Aba2-40 structure (Fig. 3(b), 40 atom/cell, Pearson symbol, oC40) was predicted to be energetically more stable than all earlier proposed structures [127][128][129][130] in the pressure range 60-80 GPa. 25 Band structure calculation of Aba2-40 structure reveals indeed a semiconducting behavior with a band gap >0.8 eV at 70 GPa.…”

A little bit of lithium does a lot for hydrogen

“…3B reveals that at 100 GPa LiH 2 is just metallic, and the small density of states at the Fermi energy (g(E F )/valence electron ϭ 0.001 eV Ϫ1 ) is reminiscent of the DOS calculated for compressed Li (19)(20)(21), rising only a little to 0.007 eV Ϫ1 at 300 GPa. For elemental Li, the 1s cores start to overlap with increasing pressure, and the valence electrons are pushed into the interstitial regions (19).…”

Electride and superconductivity behaviors in Mn5Si3-type intermetallics

“…Conceptually, we describe these materials as electrides. 1,[7][8][9][10][11][12] A canonical example is the completely reduced mayenite, [Ca 12 Al 14 O 32 ] 2+ :2e -which exhibits remarkably rich chemistries. [13][14][15][16] Of particular note is the observation of a superconducting transition (T c~0 .4 K) intimately related to the anionic electrons in this electride, thereby opening a new frontier in the field of superconductors.…”