Hard and superhard carbon phases synthesised from fullerites under pressure

A review has been presented on the structural and mechanical properties of hard carbon phases synthesized from fullerite C₆₀ under pressure. The density and nanostructure have been recognized as the key parameters defining the mechanical properties of hard carbon phases. By suggesting a version of the transitional high-pressure diagram of C₆₀ (developed up to 20 GPa), the three areas of the formation of hard carbon phases have been highlighted. The corresponding phases of superhard carbon are (1) disordered sp²-type atomic structures at moderate pressures and high temperatures (> 1100 K), (2) three-dimensionally polymerized C₆₀ structures at moderate temperatures and high pressures (> 8 GPa), and (3) sp³-based amorphous and nanocomposite phases at high pressures and temperatures. First region can be in turn separated into 2 subparts with different peculiarities of sp² structures and properties: low pressure part (0.1–2 GPa) and high-pressure part (2–8 GPa). Temperature can be recognized as a factor responsible for the formation of nanostructures by the partial destruction of molecular phases, whereas pressure is a factor responsible for stimulating the formation of rigid polymerized structures consisting of covalently bonded C₆₀ molecules, whereas the combination of both factors leads to the formation of atomic-based phases with dominating sp³ bonding.Представлено огляд структурних і механічних властивостей твердих вуглецевих фаз, синтезованих з фулерита C₆₀ під тиском. Щільність і наноструктура є ключовими параметрами, що визначають механічні властивості твердих фаз вуглецю. Пропонується версія діаграми перетворення при високому тиску C₆₀ (розроблена до 20 ГПа), виділено три області формування твердих фаз вуглецю, яким відповідають: 1) невпорядковані sp²-типу атомні структури при помірних тисках і високих (> 1100 K) температурах, 2) тривимірно полімеризовані C₆₀ структури при помірних температурах і високих (> 8 ГПа) тисках, 3) аморфні і нанокомпозитні фази на основі sp³ при високих тисках і температурах. Перша область може бути, в свою чергу, розділена на два підрозділи з різними особливостями sp²-структури і властивостей: область низького (0.1–2 ГПа) і високого (2–8 ГПа) тиску. Температура може бути визнана фактором, відповідальним за формування наноструктур завдяки частковому руйнуванню молекулярних фаз, тоді як тиск є чинником, що стимулює формування жорстких полімеризованих структур, які складаються з ковалентно зв’язаних молекул С₆₀, а поєднання обох факторів приводить до утворення фаз на основі атомів з домінуючим sp³-зв’язком.Представлен обзор структурных и механических свойств твердых углеродных фаз, синтезированных из фуллерита C₆₀ под давлением. Плотность и наноструктура являются ключевыми параметрами, определяющими механические свойства твердых фаз углерода. Предлагается версия диаграммы превращения при высоком давлении C₆₀ (разработана до 20 ГПа), выделены три области формирования твердых фаз углерода, которым соответствуют: 1) неупорядоченные sp²-типа атомные структуры при умеренных давлениях и высоких (> 1100 K) температурах, 2) трехмерно полимеризованные C₆₀ структуры при умеренных температурах и высоких (> 8 ГПа) давлениях, 3) аморфные и нанокомпозитные фазы на основе sp³ при высоких давлениях и температурах. Первая область может быть, в свою очередь, разделена на два подраздела с различными особенностями sp²-структуры и свойств: область низкого (0.1–2 ГПа) и высокого (2–8 ГПа) давления. Температура может быть признана фактором, ответственным за формирование наноструктур путем частичного разрушения молекулярных фаз, в то время как давление является фактором, стимулирующим формирование жестких полимеризованных структур, состоящих из ковалентно связанных молекул С₆₀, а сочетание обоих факторов приводит к образованию фаз на основе атомов с доминирующей sp³-связью

Simple Fluids with Complex Phase Behavior

We find that a system of particles interacting through a simple isotropic potential with a softened core is able to exhibit a rich phase behavior including: a liquid-liquid phase transition in the supercooled phase, as has been suggested for water; a gas-liquid-liquid triple point; a freezing line with anomalous reentrant behavior. The essential ingredient leading to these features resides in that the potential investigated gives origin to two effective core radii.Comment: 7 pages including 3 eps figures + 1 jpeg figur

Structural transitions and nonmonotonic relaxation processes in liquid metals

Structural transitions in melts as well as their dynamics are considered. It is supposed that liquid represents the solution of relatively stable solid-like locally favored structures (LFS) in the surrounding of disordered normal-liquid structures. Within the framework of this approach the step changes of liquid Co viscosity are considered as liquid-liquid transitions. It is supposed that this sort of transitions represents the cooperative medium-range bond ordering, and corresponds to the transition of the "Newtonian fluid" to the "structured fluid". It is shown that relaxation processes with oscillating-like time behavior ($\omega \sim 10^{-2}$~$s^{-1}$) of viscosity are possibly close to this point

Superhard Phases of Simple Substances and Binary Compounds of the B-C-N-O System: from Diamond to the Latest Results (a Review)

The basic known and hypothetic one- and two-element phases of the B-C-N-O system (both superhard phases having diamond and boron structures and precursors to synthesize them) are described. The attention has been given to the structure, basic mechanical properties, and methods to identify and characterize the materials. For some phases that have been recently described in the literature the synthesis conditions at high pressures and temperatures are indicated.Comment: Review on superhard B-C-N-O phase

First Principles Calculation of Elastic Properties of Solid Argon at High Pressures

The density and the elastic stiffness coefficients of fcc solid argon at high pressures from 1 GPa up to 80 GPa are computed by first-principles pseudopotential method with plane-wave basis set and the generalized gradient approximation (GGA). The result is in good agreement with the experimental result recently obtained with the Brillouin spectroscopy by Shimizu et al. [Phys. Rev. Lett. 86, 4568 (2001)]. The Cauchy condition was found to be strongly violated as in the experimental result, indicating large contribution from non-central many-body force. The present result has made it clear that the standard density functional method with periodic boundary conditions can be successfully applied for calculating elastic properties of rare gas solids at high pressures in contrast to those at low pressures where dispersion forces are important.Comment: 4 pages, 5 figures, submitted to PR

The high-pressure phase of boron, {\gamma}-B28: disputes and conclusions of 5 years after discovery

{\gamma}-B28 is a recently established high-pressure phase of boron. Its structure consists of icosahedral B12 clusters and B2 dumbbells in a NaCl-type arrangement (B2){\delta}+(B12){\delta}- and displays a significant charge transfer {\delta}~0.5- 0.6. The discovery of this phase proved essential for the understanding and construction of the phase diagram of boron. {\gamma}-B28 was first experimentally obtained as a pure boron allotrope in early 2004 and its structure was discovered in 2006. This paper reviews recent results and in particular deals with the contentious issues related to the equation of state, hardness, putative isostructural phase transformation at ~40 GPa, and debates on the nature of chemical bonding in this phase. Our analysis confirms that (a) calculations based on density functional theory give an accurate description of its equation of state, (b) the reported isostructural phase transformation in {\gamma}-B28 is an artifact rather than a fact, (c) the best estimate of hardness of this phase is 50 GPa, (d) chemical bonding in this phase has a significant degree of ionicity. Apart from presenting an overview of previous results within a consistent view grounded in experiment, thermodynamics and quantum mechanics, we present new results on Bader charges in {\gamma}-B28 using different levels of quantum-mechanical theory (GGA, exact exchange, and HSE06 hybrid functional), and show that the earlier conclusion about significant degree of partial ionicity in this phase is very robust

Structure and Dynamics of Liquid Iron under Earth's Core Conditions

First-principles molecular dynamics simulations based on density-functional theory and the projector augmented wave (PAW) technique have been used to study the structural and dynamical properties of liquid iron under Earth's core conditions. As evidence for the accuracy of the techniques, we present PAW results for a range of solid-state properties of low- and high-pressure iron, and compare them with experimental values and the results of other first-principles calculations. In the liquid-state simulations, we address particular effort to the study of finite-size effects, Brillouin-zone sampling and other sources of technical error. Results for the radial distribution function, the diffusion coefficient and the shear viscosity are presented for a wide range of thermodynamic states relevant to the Earth's core. Throughout this range, liquid iron is a close-packed simple liquid with a diffusion coefficient and viscosity similar to those of typical simple liquids under ambient conditions.Comment: 13 pages, 8 figure

Thermodynamic model of hardness: Particular case of boron-rich solids

A number of successful theoretical models of hardness have been developed recently. A thermodynamic model of hardness, which supposes the intrinsic character of correlation between hardness and thermodynamic properties of solids, allows one to predict hardness of known or even hypothetical solids from the data on Gibbs energy of atomization of the elements, which implicitly determine the energy density per chemical bonding. The only structural data needed is the coordination number of the atoms in a lattice. Using this approach, the hardness of known and hypothetical polymorphs of pure boron and a number of boron-rich solids has been calculated. The thermodynamic interpretation of the bonding energy allows one to predict the hardness as a function of thermodynamic parameters. In particular, the excellent agreement between experimental and calculated values has been observed not only for the room- temperature values of the Vickers hardness of stoichiometric compounds, but also for its temperature and concentration dependencies

Extended short-range order determines the overall structure of liquid gallium

This journal is © the Owner Societies. Polyvalent metal melts (gallium, tin, bismuth, etc.) have microscopic structural features, which are detected by neutron and X-ray diffraction and which are absent in simple liquids. Based on neutron and X-ray diffraction data and the results of ab initio molecular dynamics simulations for liquid gallium, we examine the structure of this liquid metal at the atomistic level. Time-resolved cluster analysis allows one to reveal that the short-range structural order in liquid gallium is determined by a range of the correlation lengths. This analysis, performed on a set of independent samples corresponding to equilibrium liquid phase, discloses that there are no stable crystalline domains and molecule-like Ga2 dimers typical for crystal phases of gallium. The structure of liquid gallium can be reproduced by the simplified model of the close-packed system of soft quasi-spheres. The results can be applied to analyze the fine structure of other polyvalent liquid metals