Oleksandr O. Kurakevych

Superhard nanocomposite of dense polymorphs of boron nitride: Noncarbon material has reached diamond hardness

The authors report a synthesis of unique superhard aggregated boron nitride nanocomposites (ABNNCs) showing the enhancement of hardness up to 100% in comparison with single crystal c-BN. Such a great hardness increase is due to the combination of the Hall-Petch and the quantum confinement effects. The decrease of the grain size down to 14nm and the simultaneous formation of the two dense BN phases with hexagonal and cubic structures within the grains at nano- and subnanolevel result in enormous mechanical property enhancement with maximum hardness of 85(5)GPa. Thus, ABNNC is the first non-carbon-based bulk material with the value of hard-ness approaching that of single crystal and polycrystalline diamond and aggregated diamond nanorods. ABNNC also has an unusually high fracture toughness for superhard materials (K1C=15MPam0.5) and wear resistance (WH=11; compare, for industrial polycrystalline diamond, WH=3–4), in combination with high thermal stability (above 1600K in air), making it an exceptional super...

On the hardness of a new boron phase, orthorhombic γ-B28

Measurements of the hardness of a new high-pressure boron phase, orthorhombic γ-B28, are reported. According to the data obtained, γ-B28 has the highest hardness (∼ 50 GPa) of all known crystal-line modifications of boron.

The interrelation between hardness and compressibility of substances and their structure and thermodynamic properties

A strong correlation relationship has been established between the structure and specific Gibbs free energy of the substance atomization on the one hand, and the substance hardness and volume compressibility on the other. In the framework of the model proposed hardness is directly proportional to the specific Gibbs free energy per bond in isodesmic crystals. An application of a correction coefficient to the ionic component of chemical bonds allows one to evaluate the hardness of compounds having both the covalent (polar and nonpolar) and ion bonds. In the framework of the suggested approach we have been the first to correctly calculate the temperature dependence of the hardness by the example for diamond and cubic boron nitride.

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.

Thermodynamic aspects of materials’ hardness: prediction of novel superhard high-pressure phases

In the present work, we have proposed a method that allows one to easily estimate the 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, which 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, BC x and CO x , which could be synthesized at high pressures and temperatures, should have extreme hardness.

Experimental study and critical review of structural, thermodynamic and mechanical properties of superhard refractory boron suboxide B6O

In the present study the analysis of available data on structural, thermodynamic and mechanical properties of B6O has been performed. Although the compound is known for half a century and has been extensively studied, many properties of this boron-rich solid remain unknown or doubtful. A semi-empirical analysis of our experimental and literature data has allowed us to choose the best values of main thermodynamic and mechanical characteristics among previously reported data, to predict the thermo-elastic equation of state of B6O, and dependence of its hardness on non-stoichiometry and temperature.

Synthesis of rock-salt MeO–ZnO solid solutions (Me=Ni 2+, Co2+, Fe2+, Mn2+) at high pressure and high temperature

A series of metastable Me1−x Zn x O solid solutions (Me = Ni2+, Co2+, Fe2+, Mn2+) with the rock-salt (rs) crystal structure have been synthesized from binary oxides by quenching at 7.7 GPa and 1450–1650 K. Phase composition of the samples, as well as structural properties and stoichiometry of synthesized solid solutions, have been studied by X-ray powder diffraction, both conventional and with synchrotron radiation. The widest (0.3 ≤x≤0.8) composition range of the existence of individual rs solid solution has been established for the NiO–ZnO system. The bulk rs-Co1−x Zn x O, rs-Fe1−x Zn x O and rs-Mn1−x Zn x O solid solutions may be quenched down to ambient conditions with only twice as low ZnO content, i.e. x≤0.5, 0.5 and 0.4, respectively; while formation of rs solid solutions in the CdO–ZnO system has not been observed in the entire concentration range.

The high-pressure phase of boron, γ-B28: Disputes and conclusions of 5 years after discovery

Abstractγ-B28 is a recently established high-pressure phase of boron. Its structure consists of icosa-hedral B12 clusters and B2 dumbbells in a NaCl-type arrangement (B2)δ+(B12)δ− and displays a significant charge transfer δ ∼ 0.5–0.6. The discovery of this phase proved to be essential for the understanding and construction of the phase diagram of boron. It 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 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 γ-B28 is an artifact, (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 the previous results within a consistent view grounded in experiment, thermodynamics and quantum mechanics, we present new results on Bader charges in γ-B28 using different levels of quantum-mechanical theory (GGA, exact exchange, and HSE06 hybrid functional), and show that the earlier conclusion about a significant degree of a partial ionicity in this phase is very robust. An additional insight into the nature of the partial ionicity is obtained from a number of boron structures theoretically constructed in this work.

Erratum: Ionic high-pressure form of elemental boron

This corrects the article DOI: 10.1038/nature07736

Phase Diagram of the B−BN System at 5 GPa

The chemical interaction and phase relations in the B-BN system have been in situ studied at 5 GPa and temperatures up to 2800 K using X-ray diffraction with synchrotron radiation. The thermodynamic analysis of the B-BN system based on experimental data allowed us to construct equilibrium and metastable phase diagrams of the system at 5 GPa. The only thermodynamically stable boron subnitride, B(13)N(2), melts incongruently at 2600 K and forms eutectic equilibrium with boron at 2300 K and 4 at. % of nitrogen.