130 research outputs found
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
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
Growth, structure analysis and anisotropic superconducting properties of MgB2 single crystals
Here we report the growth of sub-millimeter MgB2 single crystals of various
shapes under high pressure in Mg-B-N system. Structure refinement using a
single-crystal X-ray diffraction analysis gives lattice parameters a=3.0851(5)
A and c=3.5201(5) A with small reliability factors (Rw =0.025, R=0.018), which
enables us to analyze the Fourier and Fourier difference maps. The maps clearly
show the B sp2 orbitals and covalency of the B-B bonds. The sharp
superconducting transitions at Tc =38.1-38.3K were obtained in both
magnetization (DTc =0.6K) and resistivity (DTc <0.3K) measurements. Resistivity
measurements with magnetic fields applied parallel and perpendicular to the Mg
and B sheets reveal the anisotropic nature of this compound, with upper
critical field anisotropy ratio of about 2.7.Comment: 9 pages, 5 figures and 1 tabl
Phase Transition Lowering in Dynamically Compressed Silicon
Silicon, being one of the most abundant elements in nature, attracts wide-ranging scientific and technological interest. Specifically, in its elemental form, crystals of remarkable purity can be produced. One may assume that this would lead to silicon being well understood, and indeed, this is the case for many ambient properties, as well as for higher-pressure behaviour under quasi-static loading. However, despite many decades of study, a detailed understanding of the response of silicon to rapid compressionâsuch as that experienced under shock impactâremains elusive. Here, we combine a novel free-electron laser-based X-ray diffraction geometry with laser-driven compression to elucidate the importance of shear generated during shock compression on the occurrence of phase transitions. We observe lowering of the hydrostatic phase boundary in elemental silicon, an ideal model system for investigating high-strength materials, analogous to planetary constituents. Moreover, we unambiguously determine the onset of melting above 14 GPa, previously ascribed to a solidâsolid phase transition, undetectable in the now conventional shocked diffraction geometry; transitions to the liquid state are expected to be ubiquitous in all systems at sufficiently high pressures and temperatures
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