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...

In this paper, we report on the hydrogen photochromism in Nb2O5 powders with different structures. Four different powder phases were prepared by calcining Nb2O5·nH2O powders at various temperatures, and their morphology, structure, and electronic band structure were characterized by scanning electron microscopy, structural analyses, thermogravimetric analysis, differential scanning calorimetry, and optical spectroscopy. Nb2O5 powders with different structures and very different properties were formed after different high-temperature treatments of the polymorphous oxide. A pronounced photochromic effect was observed in the M and H phases of Nb2O5, whereas the other phases exhibited poor photochromic responses. Because photochromism arises due to the detachment of hydrogen atoms under the action of light from hydrogen donor molecules previously adsorbed on the oxide surface, the electronic band structure and the morphology have strong influences on the photochromic properties of Nb2O5 powders. For these reasons, a pronounced photochromic effect was achieved in the H phase.

Bulk TMDCs are diamagnetic materials; however, two-dimensional TMDCs exhibit spin polarized edge states, which results in a coupling between the unsaturated transition metal and chalcogenide atoms at the edges. The magnetism in two-dimensional TMDCs broadens their applications in spintronic and multi-functional devices. Herein, by combining macro/micro-magnetic experimental measurements and density functional theory (DFT) calculations, we demonstrate that among five possible edge-terminated WSe nanosheets only two types have a magnetic ground state, corresponding to the 100% Se edge terminated and 50% Se edge terminated nanosheets, respectively. The calculation results on WSe clusters and WSe zig-zag nanoribbons with different terminations and Se coverage rate confirmed that the unpaired electrons of the edge atoms play a crucial role in the appearance of ferromagnetism in WSe nanosheets. Furthermore, due to the possible quantum confinement effect and surface effect, there exist thickness-dependent magnetic properties, and the magnitude of magnetism at the edge increases as the number of layers decreases.

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The structural properties, electronic band structure and Bader charge of Sb2Te3 under hydrostatic pressure were simulated using density functional theory in order to study isostructural phase transitions (IPT) in Sb2Te3. The theoretical results showed that the axial ratio c /a did not exhibit any anomaly below 6 GPa. The variations of bond lengths were discontinuous at 2.5 GPa, which suggested considerable changes in interatomic interactions and provided sound support to the IPT. The effective charges of Sb and Te atoms showed significant discontinuous variations at 2.5 GPa, which revealed a strong redistribution of the electronic charge density and considerably changed interactions among bonding atoms. Thus, the IPT is originated from the considerable variation in the electronic charge density. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)