175 research outputs found
Boron: a Hunt for Superhard Polymorphs
Boron is a unique element, being the only element, all known polymorphs of
which are superhard, and all of its crystal structures are distinct from any
other element. The electron-deficient bonding in boron explains its remarkable
sensitivity to even small concentrations of impurity atoms and allows boron to
form peculiar chemical compounds with very different elements. These
complications made the study of boron a great challenge, creating also a unique
and instructive chapter in the history of science. Strange though it may sound,
the discovery of boron in 1808 was ambiguous, with pure boron polymorphs
established only starting from the 1950s-1970s, and only in 2007 was the stable
phase at ambient conditions determined. The history of boron research from its
discovery to the latest discoveries pertaining to the phase diagram of this
element, the structure and stability of beta-boron, and establishment of a new
high-pressure polymorph, gamma-boron, is reviewed
High pressure synthesis of FeO-ZnO solid solutions with rock salt structure: in situ X-ray diffraction studies
X-ray diffraction with synchrotron radiation has been used for the first time
to study chemical interaction in the FeO-ZnO system at 4.8 GPa and temperatures
up to 1300 K. Above 750 K, the chemical reaction between FeO and ZnO has been
observed that resulted in the formation of rock salt (rs) Fe1-xZnxO solid
solutions (0.3 \leq x \leq 0.85). The lattice parameters of these solid
solutions have been in situ measured as a function of temperature under
pressure, and corresponding thermal expansion coefficients have been
calculated.Comment: 9 pages, 2 figures, 1 tabl
On the Hardness of a New Boron Phase, Orthorhombic {\gamma}-B28
Measurements of the hardness of a new high-pressure boron phase, orthorhombic
{\gamma}-B28, are reported. According to the data obtained, {\gamma}-B28 has
the highest hardness (~50 GPa) of all known crystalline modifications of boron
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
On the crystal lattice parameters of graphite-like phases of the B-C system
The structure of graphite-like BCx phases (x = 1, 1.5, 3, 4, 32) has been
studied using conventional X-ray diffraction. The results have been obtained,
which unambiguously point to turbostratic (one- dimensionally disordered)
structure of all phases under study. The crystal lattice parameters, sizes of
coherent scattering domains, and microstrain values have been defined, which
have allowed us to find a correlation between the structure and stoichiometry
of the phases synthesized at the same temperature
High-pressure synthesis of rock salt LiMeO2-ZnO (Me = Fe3+, Ti3+) solid solutions
Metastable LiMeO2-ZnO (Me = Fe3+, Ti3+) solid solutions with rock salt
crystal structure have been synthesized by solid state reaction of ZnO with
LiMeO2 complex oxides at 7.7 GPa and 1350-1450 K. Structure, phase composition,
thermal stability and thermal expansion of the recovered samples have been
studied by X-ray diffraction with synchrotron radiation. At ambient pressure
rock salt LiMeO2-ZnO solid solutions are kinetically stable up to 670-800 K
depending on the composition.Comment: 11 pages, 3 figures, 1 tabl
Effect of nanostructuration on compressibility of cubic BN
Compressibility of high-purity nanostructured cBN has been studied under
quasi-hydrostatic conditions at 300 K up to 35 GPa using diamond anvil cell and
angle-dispersive synchrotron X-ray powder diffraction. A data fit to the Vinet
equation of state yields the values of the bulk modulus B0 of 375(4) GPa with
its first pressure derivative B0' of 2.3(3). The nanometer grain size (\sim20
nm) results in decrease of the bulk modulus by ~9%
Thermodynamic aspects of materials' hardness: prediction of novel superhard high-pressure phases
In the present work we have proposed the method that allows one to easily
estimate hardness and bulk modulus of known or hypothetical solid phases from
the data on Gibbs energy of atomization of the elements and corresponding
covalent radii. It has been shown that hardness and bulk moduli of compounds
strongly correlate with their thermodynamic and structural properties. The
proposed method may be used for a large number of compounds with various types
of chemical bonding and structures; moreover, the temperature dependence of
hardness may be calculated, that has been performed for diamond and cubic boron
nitride. The correctness of this approach has been shown for the recently
synthesized superhard diamond-like BC5. It has been predicted that the
hypothetical forms of B2O3, diamond-like boron, BCx and COx, which could be
synthesized at high pressures and temperatures, should have extreme hardness
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