55 research outputs found
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 (~) 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
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