Metallic Icosahedron Phase of Sodium at Terapascal Pressures

Abstract

Alkali metals exhibit unexpected structures and electronic behavior at high pressures. Compression of metallic sodium (Na) to 200 GPa leads to the stability of a wide-band-gap insulator with the double hexagonal hP4 structure. Post-hP4 structures remain unexplored, but they are important for addressing the question of the pressure at which Na reverts to a metal. Here we report the reentrant metallicity of Na at the very high pressure of 15.5 terapascal (TPa), predicted using first-principles structure searching simulations. Na is therefore insulating over the large pressure range of 0.2-15.5 TPa. Unusually, Na adopts an oP8 structure at pressures of 117-125 GPa, and the same oP8 structure at 1.75-15.5 TPa. Metallization of Na occurs on formation of a stable and striking body-centered cubic cI24 electride structure consisting of Na12 icosahedra, each housing at its center about one electron which is not associated with any Na ions.PACS numbers: 61.50. Ks, Alkali metals have long been known to possess simple electronic structures at ambient pressure that are well explained by a nearly-free-electron model. However, the simple low-pressure structures of alkali metals do not remain simple upon compression. Rich and complex phases and remarkable physical phenomena have been observed, such as greatly increased electrical resistivity[1], enhanced superconductivity [2,3], unusually low melting temperatures [4,5] and metalinsulator/semiconductor transitions [6,7].The metal-insulator transitions in Na and lithium (Li) [6,7] are among the most fascinating observations at high pressure. Neaton and Ashcroft predicted that upon compression Li[8] and Na[9] might transform into atomically-paired structures. If a paired structure was to be adopted, both Na and Li would have the potential to be semiconducting. These predictions have stimulated numerous studies of the high-pressure structures of Li and Na (e.g., Refs. [6,7,[10][11][12][13][14]).The metal-insulator transition in Na was established by a joint theoretical and experimental effort [7] and confirmed by further experiments [15]. Na is predicted and observed to transform into an optically transparent phase at above 200 GPa [7]. This phase was predicted and experimentally confirmed to have a double hexagonal hP4 structure [7], with a remarkably large bandgap reaching 6.5 eV at 600 GPa. A metalsemiconductor transition was observed in Li at 80 GPa. The semiconducting structure of dense Li remained unsolved despite considerable attention (e.g., Refs. [11][12][13][14]). Blind crystal structure prediction calculations [12] on dense Li predicted a complex semiconducting oC40 structure. Independent experimental and theoretical studies also found that semiconducting Li adopts the oC40 structure [16].Analysis of the charge density [7,11,12] shows that the emergence of an insulating state in dense Na/Li is, however, not attributable to atomic pairing, but rather to strong localization of valence electrons within lattice voids. Notably, the insulating phases of Na and Li can be regar...

High-Energy Density and Superhard Nitrogen-Rich B-N Compounds

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Pressure-induced metallization and reentrant insulativity in elemental crystal of phosphorus: a prediction by ab initio calculations

“…Elemental materials may exhibit peculiar properties under high pressure, e.g. the alkali metals are reported to possess unexpected electronic behavior at high pressures [52][53][54]. Here, we predict the tendency of pressureinduced reentrant insulativity character of phosphorus, while it is opposite to the reentrant metallicity of lithium and sodium at high pressures.…”

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] .…”