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Though hydrogen and lithium have been assigned a common column of the periodic table, their crystalline states under common conditions are drastically different: the former at temperatures where it is crystalline is a molecular insulator, whereas the latter is a metal that takes on simple structures. On compression, however, the two come to share some structural and other similarities associated with the insulator-to-metal and metal-to-insulator transitions, respectively. To gain a deeper understanding of differences and parallels in the behaviors of compressed hydrogen and lithium, we performed an ab initio comparative study of these systems in selected identical structures. Both elements undergo a continuous pressure-induced s-p electronic transition, though this is at a much earlier stage of development for H. The valence charge density accumulates in interstitial regions in Li but not in H in structures examined over the same range of compression. Moreover, the valence charge density distributions or electron localization functions for the same arrangement of atoms mirror each other as one proceeds from one element to the other. Application of the virial theorem shows that the kinetic and potential energies jump across the first-order phase transitions in H and Li are opposite in sign because of non-local effects in the Li pseudopotential. Finally, the common tendency of compressed H and Li to adopt three-fold coordinated structures as found is explained by the fact that such structures are capable of yielding a profound pseudogap in the electronic densities of states at the Fermi level, thereby reducing the kinetic energy. These results have implications for the phase diagrams of these elements and also for the search for new structures with novel properties. [ABSTRACT FROM AUTHOR]

We discuss the high-pressure phases of crystalline lithium hydroxide, LiOH. Using first-principles calculations, and assisted by evolutionary structure searches, we reproduce the experimentally known phase transition under pressure, but we suggest that the high-pressure phase LiOH-III be assigned to a new hydrogen-bonded tetragonal structure type that is unique amongst alkali hydroxides. LiOH is at the intersection of both ionic and hydrogen bonding, and we examine the various ensuing structural features and their energetic driving mechanisms. At P = 17 GPa, we predict another phase transition to a new phase, Pbcm-LiOH-IV, which we find to be stable over a wide pressure range. Eventually, at extremely high pressures of 1100 GPa, the ground state of LiOH is predicted to become a polymeric structure with an unusual graphitic oxygen-hydrogen net. However, because of its ionic character, the anticipated metallization of LiOH is much delayed; in fact, its electronic band gap increases monotonically into the TPa pressure range. [ABSTRACT FROM AUTHOR]

In order to explain the intricate dance of intramolecular (intra-proton-pair) H-H separations observed in a numerical laboratory of calculationally preferred static hydrogen structures under pressure, we examine two effects through discrete molecular models. The first effect, we call it physical, is of simple confinement. We review a salient model already in the literature, that of LeSar and Herschbach, of a hydrogen molecule in a spheroidal cavity. As a complement, we also study a hydrogen molecule confined along a line between two helium atoms. As the size of the cavity/confining distance decreases (a surrogate for increasing pressure), in both models the equilibrium proton separation decreases and the force constant of the stretching vibration increases. The second effect, which is an orbital or chemical factor, emerges from the electronic structure of the known molecular transition metal complexes of dihydrogen. In these the H-H bond is significantly elongated (and the vibron much decreased in frequency) as a result of depopulation of the σg bonding molecular orbital of H2, and population of the antibonding σu* MO. The general phenomenon, long known in chemistry, is analyzed through a specific molecular model of three hydrogen molecules interacting in a ring, a motif found in some candidate structures for dense hydrogen. [ABSTRACT FROM AUTHOR]

A preliminary discussion of the general problem of localization of wave functions, and the way it is approached in theoretical condensed matter physics (Wannier functions) and theoretical chemistry (localized or fragment orbitals) is followed by an application of the ideas of Paper II in this series to the structures of hydrogen as they evolve under increasing pressure. The idea that emerges is that of simultaneously operative physical (reduction of available space by an increasingly stiff wall of neighboring molecules) and chemical (depopulation of the σg bonding molecular orbital of H2, and population of the antibonding σu* MO) factors. The two effects work in the same direction of reducing the intermolecular separation as the pressure increases, but compete, working in opposite directions, in their effect on the intramolecular (nearest neighbor, intra-pair) distance. We examine the population of σg and σu* MOs in our numerical laboratory, as well as the total electron transfer (small), and polarization (moderate, where allowed by symmetry) of the component H2 molecules. From a molecular model of two interacting H2 molecules we find a linear relationship between the electron transfer from σg to σu* of a hydrogen molecular fragment and the intramolecular H-H separation, and that, in turn, allows us to estimate the expected bond lengths in H2 under pressure if the first effect (that of simple confinement) was absent. In essence, the intramolecular H-H separations under pressure are much shorter than they would be, were there no physical/confinement effect. We then use this knowledge to understand how the separate E and PV terms contribute to hydrogen phase changes with increasing pressure. [ABSTRACT FROM AUTHOR]

In this paper, we examine the transition from a molecular to monatomic solid in hydrogen over a wide pressure range. This is achieved by setting up two models in which a single parameter δ allows the evolution from a molecular structure to a monatomic one of high coordination. Both models are based on a cubic Bravais lattice with eight atoms in the unit cell; one belongs to space group [formula], the other to space group [formula]. In [formula] one moves from effective 1-coordination, a molecule, to a simple cubic 6-coordinated structure but through a very special point (the golden mean is involved) of 7-coordination. In [formula], the evolution is from 1 to 4 and then to 3 to 6-coordinate. If one studies the enthalpy as a function of pressure as these two structures evolve (δ increases), one sees the expected stabilization of minima with increased coordination (moving from 1 to 6 to 7 in the [formula] structure, for instance). Interestingly, at some specific pressures, there are in both structures relatively large regions of phase space where the enthalpy remains roughly the same. Although the structures studied are always higher in enthalpy than the computationally best structures for solid hydrogen - those emerging from the Pickard and Needs or McMahon and Ceperley numerical laboratories - this result is suggestive of the possibility of a microscopically non-crystalline or 'soft' phase of hydrogen at elevated pressures, one in which there is a substantial range of roughly equi-enthalpic geometries available to the system. A scaling argument for potential dynamic stabilization of such a phase is presented. [ABSTRACT FROM AUTHOR]

In the first of a series of four papers on hydrogen under pressure, and its transitions from an initiating molecular state, we begin by defining carefully the problem, and setting the distance scale of interactions of protons and electrons in molecular aggregates of the first of the elements. Following a review of the experimental situation, in particular the phase diagram of hydrogen, in as much as it is known, and the behavior of its vibrons and rotons, we move onto the setting up of a numerical laboratory for probing the underlying physics and chemistry of interactions in hydrogen as the pressure increases. The laboratory consists of the preferred static structures emerging from calculations on the system in the range of 1 atm to 500 GPa, those of Pickard and Needs. The intermolecular (inter-pair) H···H separations naturally decrease with increasing pressure, first rapidly so, then more slowly. The intramolecular (intra-pair) H-H distances vary over a much smaller scale (0.05 Å) as the pressure increases, first decreasing, then increasing, and finally decreasing. We define an equalization function to gauge the approach to equality of the first neighbor and shortest next neighbor H (proton) separations in this numerical laboratory. And we find that metallization is likely to occur before bond equalization. [ABSTRACT FROM AUTHOR]

Under normal conditions, sodium forms a 1:1 stoichiometric compound with indium, and also with thallium, both in the double-diamond structure. But sodium does not combine with aluminum at all. Could NaAl exist? If so, under what conditions and in which structural types? Instead of beginning with a purely computational and first-principles structure search, we are led to apply the early Brillouin and higher (Jones) zone ideas of the physics determining structural selection. We begin with a brief recapitulation of the higher zone concept as applied to the stability of metals and intermetallic compounds. We then discuss the extension of this concept to problems where density becomes a primary variable, within the second-order band structure approximation. An analysis of the range of applicability of pressure-induced Jones zone activation is presented. The simple NaAl compound serves us as a numerical laboratory for the application of this concept. Higher zone arguments and chemical intuition lead quite naturally to the suggestion that 1:1 compound formation between sodium and aluminum should be favored under pressure and specifically in the double-diamond structure. This is confirmed computationally by density functional theoretic methods within the generalized gradient approximation. [ABSTRACT FROM AUTHOR]

An extension of the modified weighted-density approximation (MWDA) is presented which retains the key features of the original MWDA in that it continues to describe the nonuniform system through the use of low-order correlation functions of the uniform counterpart. However, the approximate free energy functional is now exact up to third order in the functional expansion of the free energy, and therefore requires as input both the second- and third-order direct correlation functions of the uniform liquid, as well as its excess free energy per particle. The theory has been applied previously to the problem of isochoric freezing of the classical one-component plasma, and is here applied to the well-known problem of isobaric freezing of hard spheres. We use two different approaches to describe the less well-known third-order direct correlation function of the uniform liquid. The first approach is representative of a class of models for this function that are derived through three functional density differentiations of an approximate free energy model. The second is a factorization ansatz based on liquid-state diagrammatic expansions. The results are quite sensitive to these choices: The first leads to an improvement over the already satisfactory results of the original MWDA for the hard-sphere system, whereas the second does not lead to freezing at all. These differences are traced to the ways in which the approximations treat long-range and short-range potentials. [ABSTRACT FROM AUTHOR]

A known identity relating the functional density derivative of the pair distribution function g(12) of nonuniform liquids to the triplet distribution function g(123) is obtained within the grand canonical ensemble, but then generalized to the case of nonuniform liquid mixtures with either short- or long-range forces. The resulting identities are easily applied to the case of uniform one- or multicomponent liquids, and when applied to the case of a binary Coulomb mixtures, they yield the canonical sum rules relating the n-point to the (n+1)-point distribution functions. Finally, when applied to the one-component plasma (OCP), taken as a particular limiting case, the expected result that the total compressibility of the OCP must vanish in the thermodynamic limit is obtained. [ABSTRACT FROM AUTHOR]

The crystalline phase of a system of classical rigid spheres is treated using an adaptation of the Kikuchi cluster-variation approximation in the variational formulation of Morita. The theory is developed for an arbitrary choice of basic clusters, but the numerical calculations that are presented are carried out for two sets of basic clusters, namely nearest neighbor pairs and a combination of isosceles and equilateral triangles. The integrations required in the evaluation of the clusters are performed using Monte Carlo techniques, which permit the use of moderately large clusters, and in consequence fairly general parametrizations of the density functions. No symmetry conditions need to be imposed beyond those already inherent in an assumed fcc crystal. Numerical results are given for the entropy and density functions, the latter being presented in a parametrized form suitable for use in a variational calculation which takes the hard sphere crystal as a reference system. [ABSTRACT FROM AUTHOR]

A systematic structure search in the La-H and Y-H systems under pressure reveals some hydrogen-rich structures with intriguing electronic properties. For example, LaH10 is found to adopt a sodalite-like face-centered cubic (fcc) structure, stable above 200 GPa and LaH8 a C2/m space group structure. Phonon calculations indicate both are dynamically stable; electron phonon calculations coupled to Bardeen-Cooper-Schrieffer (BCS) arguments indicate they might be high-Tc superconductors. In particular, the superconducting transition temperature Tc calculated for LaH10 is 274-286 K at 210 GPa. Similar calculations for the Y-H system predict stability of the sodalite-like fcc YH10 and a c above room temperature, reaching 305-326 K at 250 GPa. The study suggests that dense hydrides consisting of these and related hydrogen polyhedral networks may represent new classes of potential very hightemperature superconductors. [ABSTRACT FROM AUTHOR]

The low-temperature crystal structure of elemental lithium, the prototypical simple metal, is a several-decades-old problem. At 1 atm pressure and 298 K, Li forms a body-centered cubic lattice, which is common to all alkali metals. However, a low-temperature phase transition was experimentally detected to a structure initially identified as having the 9R stacking. This structure, proposed by Overhauser in 1984, has been questioned repeatedly but has not been confirmed. Here we present a theoretical analysis of the Fermi surface of lithium in several relevant structures. We demonstrate that experimental measurements of the Fermi surface based on the de Haas-van Alphen effect can be used as a diagnostic method to investigate the low-temperature phase diagram of lithium. This approach may overcome the limitations of X-ray and neutron diffraction techniques and makes possible, in principle, the determination of the lithium low-temperature structure (and that of other metals) at both ambient and high pressure. The theoretical results are compared with existing low-temperature ambient pressure experimental data, which are shown to be inconsistent with a 9R phase for the low-temperature structure of lithium. [ABSTRACT FROM AUTHOR]

Atomic and cationic radii have been calculated for the first 96 elements, together with selected anionic radii. The metric adopted is the average distance from the nucleus where the electron density falls to 0.001 electrons per bohr3, following earlier work by Boyd. Our radii are derived using relativistic all-electron density functional theory calculations, close to the basis set limit. They offer a systematic quantitative measure of the sizes of non-interacting atoms, commonly invoked in the rationalization of chemical bonding, structure, and different properties. Remarkably, the atomic radii as defined in this way correlate well with van der Waals radii derived from crystal structures. A rationalization for trends and exceptions in those correlations is provided. [ABSTRACT FROM AUTHOR]

We explore ground-state structures and stoichiometries of the BeB system in the static limit, with Be atom concentrations of 20 % or greater, and from P=1 atm up to 320 GPa. At P=1 atm, predictions are offered for several known compounds, the structures of which have not yet been determined experimentally. Specifically, at 1 atm, we predict a structure of R $\bar 3$ m symmetry for the compound Be2B3, seen experimentally at high temperatures, which contains interesting BeBBBBe rods; and for the compound BeB4 we calculate metastability with respect to the elements with a structure similar to MgB4, which is quickly replaced as the pressure is elevated by a Cmcm structure that features 6- and 4-membered rings in B cages, with Be interstitials. For another high-temperature compound, Be2B, we confirm the CaF2 structure, but find a competitive and actually slightly more stable ground-state structure of C2/ m symmetry that features B2 pairs. In the case of BeB2, a material for which the stoichiometry has been the subject of debate, we have a clear prediction of a stable F $\bar 4$3 m structure at P=1 atm. It has a diamondoid structure that is based on cubic (lower P) or hexagonal (higher P) diamond networks of B, but with Be in the interstices. This Zintl structure is a semiconductor at low and intermediate pressures. At higher pressures, BeB2 dominates the phase diagram. In general, the Zintl-Klemm concept of effective electron transfer from the more electropositive ion and bond formation among the resulting anions has proven useful in analyzing the structural preferences of many compositions in the BeB system at P=1 atm and at elevated pressures. An unusual feature of this binary system is that the 1:1 BeB stoichiometry never appears to reach stability in the static limit, although it comes close, as does Be17B12. Also stable at high pressures are stoichiometries BeB3, BeB4, and Be5B2. [ABSTRACT FROM AUTHOR]

H2O will be more resistant to metallization than previously thought. From computational evolutionary structure searches, we find a sequence of new stable and meta-stable structures for the ground state of ice in the 1-5 TPa (10 to 50 Mbar) regime, in the static approximation. The previously proposed Pbcm structure is superseded by a Pmc21 phase at p = 930 GPa, followed by a predicted transition to a P21 crystal structure at p = 1.3 TPa. This phase, featuring higher coordination at O and H, is stable over a wide pressure range, reaching 4.8 TPa. We analyze carefully the geometrical changes in the calculated structures, especially the buckling at the H in O-H-O motifs. All structures are insulating-chemistry burns a deep and (with pressure increase) lasting hole in the density of states near the highest occupied electronic levels of what might be component metallic lattices. Metallization of ice in our calculations occurs only near 4.8 TPa, where the metallic C2/m phase becomes most stable. In this regime, zero-point energies much larger than typical enthalpy differences suggest possible melting of the H sublattice, or even the entire crystal. [ABSTRACT FROM AUTHOR]

At atmospheric pressure, some tungsten hydrides exist as discrete molecular complexes in low temperature matrices, but are very reactive and cannot be isolated in bulk at room temperature. Under pressure, one observes WH in experiment at room temperature, and calculations indicate several other stoichiometries are stable, all extended and not molecular. This work aims at constructing a bridge between discrete molecules and the pressurized extended structures, with the emblematic example of WH6. The equilibrium structure, known as a distorted trigonal prismatic molecule, is likely to be a good acceptor. In numerical experiments, discrete WH6molecular complexes are allowed to interact with each other in their ground states at T= 0 K and P= 1 atm. They spontaneously rearrange to form 1D-chains, with the W centers adopting a tricapped trigonal prismatic environment. The electronic structure of these chains is examined and related to that of the monomer and extended solids. [ABSTRACT FROM AUTHOR]

Eight isomeric two-dimensional graphane sheets are found in a theoretical study. Four of these nets-two built on chair cyclohexanes, two on boat-are more stable thermodynamically than the isomeric benzene. or polyacetylene. Three-dimensional crystals are built up from the two-dimensional sheets, and their hypothetical behavior under pressure (up to 300 GPa) is explored. While the three-dimensional graphanes remain, as expected, insulating or semiconducting in this pressure range, there is a remarkable inversion in stability of the five crystals studied. Two stacking polytypes that are not the most stable at ambient pressure (one based on an unusual chair cyclohexane net, the other on a boat) are significantly stabilized with increasing pressure relative to stackings of simple chair sheets. The explanation may lie in the balance on intra and intersheet contacts in the extended arrays. [ABSTRACT FROM AUTHOR]

From detailed assessments of electronic structure, we find that a combination of significantly quantal elements, six of seven atoms being hydrogen, becomes a stable metal at a pressure approximately 1/4 of that required to metalize pure hydrogen itself. The system, LiH6 (and other LiHn), may well have extensions beyond the constituent lithium. These hypothetical materials demonstrate that nontraditional stoichiometries can considerably expand the view of chemical combination under moderate pressure. [ABSTRACT FROM AUTHOR]

High pressure is known to influence electronic structure and crystal packing, and can in some cases even induce compound formation between elements that do not bond under ambient conditions. Here we present a computational study showing that high pressure fundamentally alters the reactivity of the light elements lithium (Li) and beryllium (Be), which are the first of the metals in the condensed state and immiscible under normal conditions. We identify four stoichiometric LixBe1-x compounds that are stable over a range of pressures, and find that the electronic density of states of one of them displays a remarkable step-like feature near the bottom of the valence band and then remains almost constant with increasing energy. These characteristics are typical of a quasi-two-dimensional electronic structure, the emergence of which in a three-dimensional environment is rather unexpected. We attribute this observation to large size differences between the ionic cores of Li and Be: as the density increases, the Li cores start to overlap and thereby expel valence electrons into quasi-two-dimensional layers characterized by delocalized free-particle-like states in the vicinity of Be ions. [ABSTRACT FROM AUTHOR]

Non-classical response to rotation is a hallmark of quantum ordered states such as superconductors and superfluids. The rotational responses of all currently known single-component ‘super’ states of matter (superconductors, superfluids and supersolids) are largely described by two fundamental principles and fall into two categories according to whether the systems are composed of charged or neutral particles: the London law relating the angular velocity to a subsequently established magnetic field and the Onsager–Feynman quantization of superfluid velocity. These laws are theoretically shown to be violated in a two-component superconductor such as the projected liquid metallic states of hydrogen and deuterium at high pressures. The rotational responses of liquid metallic hydrogen or deuterium identify them as a new class of dissipationless states; they also directly point to a particular experimental route for verification of their existence. [ABSTRACT FROM AUTHOR]

By increase in density, impelled by pressure, the electronic energy bands in dense hydrogen attain significant widths. Nevertheless, arguments can be advanced suggesting that a physically consistent description of the general consequences of this electronic structure can still be constructed from interacting but state-dependent multipoles. These reflect, in fact self-consistently, a disorder-induced localization of electron states partially manifesting the effects of proton dynamics; they retain very considerable spatial inhomogeneity (as they certainly do in the molecular limit). This description, which is valid provided that an overall energy gap has not closed, leads at a mean-field level to the expected quadrupolar coupling, but also for certain structures to the eventual emergence of dipolar terms and their coupling when a state of broken charge symmetry is developed. A simple Hamiltonian incorporating these basic features then leads to a high-density, low-temperature phase diagram that appears to be in substantial agreement with experiment. In particular, it accounts for the fact that whereas the phase I-II phase boundary has a significant isotope dependence, the phase II-III boundary has very little. [ABSTRACT FROM AUTHOR]

Using density functional theory, the ground-state structural, electronic, and magnetic properties of monoatomic lithium and sodium chains with low average density are investigated. A metallic, zigzag ground-state structure is predicted, but, most interestingly, stable equilibria for chains under tension are predicted to be ferromagnetic, which can be traced to exchange effects arising from occupation of the second subband as a function of the interatomic distance. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2003 [ABSTRACT FROM AUTHOR]

The many-body problem associated with dense hydrogen can represented by the standard interacting electron gas problem (complete with compensating charge continuum) augmented by the proton equivalent and a coupling between the two. This viewpoint is useful as a starting point in the pursuit of ground-state instabilities (particularly the onset of a charge density wave) as a function of the continuum charge density, and even more generally in the identification of forms for the fields required to establish a thermodynamic treatment via coherent state functional integral representations of the partition function for dense hydrogen. [ABSTRACT FROM AUTHOR]

Presents articles on the topic of condensed matter physics in a special physics news supplement for 1984. Nuclear-magnetic-resonance technique used to prove the existence of spin waves in spin-polarized hydrogen gas; Application of fractal geometry to physics; Noise from a single electron trap in transistors; Heavy fermion metals and superconductivity; Dendrite crystal growth; Connection of wetting to epitaxial growth in semiconductor device fabrication; Growth in the field of free-electron lasers.

The electronic structure of wurtzite InN has been investigated by electron energy loss spectroscopy (EELS). Spectra of the nitrogen K edge and the indium M[sub 4,5] edge have been measured and were compared with calculated partial, N 2p and In 5p conduction band density of states in InN. Excellent agreement on the relative positions of the characteristic peaks were obtained. From low-loss EELS the bulk plasmon energy at 15.5 eV, location of the In 4d deep valence states at about 16.3 eV below the conduction band maximum and strong interband transitions with 6.2 eV excitation energy are also found. [ABSTRACT FROM AUTHOR]

A correction to the article about atomic and ionic radii of elements is presented.

The article discusses research on the behavior of the metals lithium and sodium when placed under high pressure. These simple metals provide an integral role in the study of metallic solids due to the fact that their electronic behavior is described using the nearly-free electron model. However, these studies show that they can take on semiconductor-like forms. In lithium, the electrical resistance of the element increases to more than 10,000 times its value at atmospheric pressure. Slices of sodium become transparent to visible light, suggesting that they have become insulators. Spectroscopic data obtained on chemical transformations is also described.

The article discusses the structure of the chemical element sodium. It first appears as a liquid metal or globules with high metallic lustre. It has malleable property and considered softer compared to other metallic substances. Sodium in room temperatures with increased pressure and density is passing from its familiar body-centered-cubic (BCC) structure to face-centered cubic (FCC).