Metallic Icosahedron Phase of Sodium at Terapascal Pressures

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.Comment: 5 pages, 4 figures, PRL (2015

Spiral Chain O4 Form of Dense Oxygen

Oxygen is in many ways a unique element: the only known diatomic molecular magnet and the capability of stabilization of the hitherto unexpected O8 cluster structure in its solid form at high pressure. Molecular dissociations upon compression as one of the fundamental problems were reported for other diatomic solids (e.g., H2, I2, Br2, and N2), but it remains elusive for solid oxygen, making oxygen an intractable system. We here report the theoretical prediction on the dissociation of molecular oxygen into a polymeric spiral chain O4 structure (\theta-O4) by using first-principles calypso method on crystal structure prediction. The \theta-O4 stabilizes above 2 TPa and has been observed as the third high pressure phase of sulfur (S-III). We find that the molecular O8 phase remains extremely stable in a large pressure range of 0.008 - 2 TPa, whose breakdown is driven by the pressure-induced instability of a transverse acoustic phonon mode at zone boundary, leading to the ultimate formation of \theta-O4. Remarkably, stabilization of \theta-O4 turns oxygen from a superconductor into an insulator with a wide band gap (approximately 5.9 eV) originating from the sp3-like hybridized orbitals of oxygen and the localization of valence electrons. (This is a pre-print version of the following article: Li Zhu et al, Spiral chain O4 form of dense oxygen, Proc. Natl. Acad. Sci. U.S.A. (2011), doi: 10.1073/pnas.1119375109, which has been published online at http://www.pnas.org/content/early/2011/12/27/1119375109 .)Comment: 13 apages, 3 figure

CALYPSO: a method for crystal structure prediction

We have developed a software package CALYPSO (Crystal structure AnaLYsis by Particle Swarm Optimization) to predict the energetically stable/metastable crystal structures of materials at given chemical compositions and external conditions (e.g., pressure). The CALYPSO method is based on several major techniques (e.g. particle-swarm optimization algorithm, symmetry constraints on structural generation, bond characterization matrix on elimination of similar structures, partial random structures per generation on enhancing structural diversity, and penalty function, etc) for global structural minimization from scratch. All of these techniques have been demonstrated to be critical to the prediction of global stable structure. We have implemented these techniques into the CALYPSO code. Testing of the code on many known and unknown systems shows high efficiency and high successful rate of this CALYPSO method [Wang et al., Phys. Rev. B 82 (2010) 094116][1]. In this paper, we focus on descriptions of the implementation of CALYPSO code and why it works.Comment: accepted in Computer Physics Communication

Computational Discovery and Characterization of New B2O Phases

We present computational discoveries of new structural phases of B2O compound exhibiting novel bonding networks and electronic states at ambient and elevated pressures. Our advanced crystal structure searches in conjunction with density functional theory calculations have identified an orthorhombic phase of B2O that is energetically stable at ambient pressure and contains an intriguing bonding network of icosahedral B12 clusters bridged by oxygen atoms. As pressure increases above 1.9 GPa, a structural transformation takes the orthorhombic B2O into a pseudo- layered trigonal phase. We have performed extensive studies to investigate the evolution of chemical bonds and electronic states associated with the B12 icosahedral unit in the orthorhombic phase and the covalent B-O bonds in the trigonal phase. We also have examined the nature of the charge carriers and their coupling to the lattice vibrations in the newly identified B2O crystals. Interestingly, our results indicate that both B2O phases become superconducting at low temperatures, with transition temperatures of 6.4 K and 5.9 K, respectively, in the ambient and high-pressure phase. The present findings establish new B2O phases and characterize their structural and electronic properties, which offer insights and guidance for exploration toward further fundamental understanding and potential synthesis and application

On how Unsupervised Machine Learning Can Shape Minds: a Brief Overview

This paper briefly examines the relationship between unsupervised machine learning models, the learning affordances that such models offer, and the mental models of those who use them. We consider the unsupervised models as learning affordances. We use a case study involving unsupervised modelling via commonly used methods such as clustering, to argue that unsupervised models can be used as learning affordances, bychanging participants’ mental models, precisely because the models are unsupervised, and thus potentially lead to learning from unexpected or inexplicit patterns

Carbon network evolution from dimers to sheets in superconducting ytrrium dicarbide under pressure

Carbon-bearing compounds display intriguing structural diversity, due to variations in hybrid bonding of carbon. Here, first-principles calculations and unbiased structure searches on yttrium dicarbide at pressure reveal four new structures with varying carbon polymerisation, in addition to the experimentally observed high-temperature low-pressure I4/mmm dimer phase. At low pressures, a metallic C2/m phase (four-member single-chain carbide) is stable, which transforms into a Pnma phase (single-chain carbide) upon increasing pressure, with further transformation to an Immm structure (double-chain carbide) at 54 GPa and then to a P6/mmm phase (sheet carbide) at 267 GPa. Yttrium dicarbide is structurally diverse, with carbon bonded as dimers (at lowest pressure), four-member single chains, infinite single chains, double chains and eventually sheet structures on compression. Electron–phonon coupling calculations indicate that the high-pressure phases are superconducting. Our results aid the understanding and design of new superconductors and illuminate pressure-induced carbon polymerisation in carbides

High-Pressure Phase Stability and Superconductivity of Pnictogen Hydrides and Chemical Trends for Compressed Hydrides

Binary hydrides formed by the pnictogens of phosphorus, arsenic and antimony are studied at high pressures using first principles methods. Stable structures are predicted and their electronic, vibrational and superconducting properties are investigated. We predict that SbH$_{4}$ and AsH$_{8}$ will be high-temperature superconductors at megabar pressures, with critical temperatures in excess of 100 K. The highly symmetric hexagonal SbH$_{4}$ phase is predicted to be stabilized above about 150 GPa, which is readily achievable in diamond anvil cell experiments. We find that all phosphorus hydrides are metastable with respect to decomposition into the elements within the pressure range studied. Trends based on our results and literature data reveal a connection between the high-pressure behaviors and ambient-pressure chemical quantities which provides insight into understanding which elements may form hydrogen-rich high-temperature superconducting phases at high pressures.The authors thank Eva Zurek for sharing structure data for iodine hydride. The work at Jilin Univ. is supported by the funding of National Natural Science Foundation of China under Grant Nos. 11274136 and 11534003, 2012 Changjiang Scholar of Ministry of Education and the Postdoctoral Science Foundation of China under grant 2013M541283. L.Z. acknowledges funding support from the Recruitment Program of Global Youth Experts in China. Part of calculations was performed in the high performance computing center of Jilin Univ. R.J.N. acknowledges financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the UK [EP/J017639/1]. R.J.N. and C.J.P. acknowledge use of the Archer facility of the U.K.’s national high-performance computing service (for which access was obtained via the UKCP consortium [EP/K013564/1]).This is the final version of the article. It first appeared from ACS via https://doi.org/10.1021/acs.chemmater.5b0463

High-pressure Phase Stability and Superconductivity of Pnictogen Hydrides and Chemical Trends for Compressed Hydrides

The recent breakthrough discovery of unprecedentedly high temperature superconductivity of 203 K in compressed sulfur hydrides has stimulated significant interest in finding new hydrogen-containing superconductors and elucidating the physical and chemical principles that govern these materials and their superconductivity. Here we report the prediction of high temperature superconductivity in the family of pnictogen hydrides using first principles calculations in combination with global optimization structure searching methods. The hitherto unknown high-pressure phase diagrams of binary hydrides formed by the pnictogens of phosphorus, arsenic and antimony are explored, stable structures are identified and their electronic, vibrational and superconducting properties are investigated. We predict that SbH_4 and AsH_8 are high-temperature superconductors at megabar pressures, with critical temperatures in excess of 100 K. The highly symmetrical hexagonal SbH_4 phase is predicted to be stabilized above about 150 GPa, which is readily achievable in diamond anvil cell experiments. We find that all phosphorus hydrides are metastable with respect to decomposition into the elements within the pressure range studied. Trends based on our results and data in the literature reveal a connection between the high-pressure behaviors and ambient-pressure chemical quantities which provides insight into understanding which elements may form hydrogen-rich high-temperature superconducting phases at high pressures.The authors thank Eva Zurek for sharing structure data for iodine hydride. The work at Jilin Univ. is supported by the funding of National Natural Science Foundation of China under Grant Nos. 11274136 and 11534003, 2012 Changjiang Scholar of Ministry of Education and the Postdoctoral Science Foundation of China under grant 2013M541283. L.Z. acknowledges funding support from the Recruitment Program of Global Youth Experts in China. Part of calculations was performed in the high performance computing center of Jilin Univ. R.J.N. acknowledges financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the UK [EP/J017639/1]. R.J.N. and C.J.P. acknowledge use of the Archer facility of the U.K.’s national high-performance computing service (for which access was obtained via the UKCP consortium [EP/K013564/1]).This is the final version of the article. It first appeared from ACS via https://doi.org/10.1021/acs.chemmater.5b0463