Vladimir L. Solozhenko

Ionic high-pressure form of elemental boron

Boron is an element of fascinating chemical complexity. Controversies have shrouded this element since its discovery was announced in 1808: the new ‘element’ turned out to be a compound containing less than 60–70% of boron, and it was not until 1909 that 99% pure boron was obtained. And although we now know of at least 16 polymorphs, the stable phase of boron is not yet experimentally established even at ambient conditions. Boron’s complexities arise from frustration: situated between metals and insulators in the periodic table, boron has only three valence electrons, which would favour metallicity, but they are sufficiently localized that insulating states emerge. However, this subtle balance between metallic and insulating states is easily shifted by pressure, temperature and impurities. Here we report the results of high-pressure experiments and ab initio evolutionary crystal structure predictions that explore the structural stability of boron under pressure and, strikingly, reveal a partially ionic high-pressure boron phase. This new phase is stable between 19 and 89 GPa, can be quenched to ambient conditions, and has a hitherto unknown structure (space group Pnnm, 28 atoms in the unit cell) consisting of icosahedral B12 clusters and B2 pairs in a NaCl-type arrangement. We find that the ionicity of the phase affects its electronic bandgap, infrared adsorption and dielectric constants, and that it arises from the different electronic properties of the B2 pairs and B12 clusters and the resultant charge transfer between them.

Synthesis of superhard cubic BC2N

Cubic BC2N was synthesized from graphite-like BC2N at pressures above 18 GPa and temperatures higher than 2200 K. The lattice parameter of c-BC2N at ambient conditions is 3.642(2) A, which is larger by 1.48% than would be expected based on ideal mixing between diamond and cubic boron nitride. The bulk modulus of c-BC2N is 282 GPa which is one of the highest bulk moduli known for any solid, and is exceeded only by the bulk moduli of diamond and c-BN. The hardness of c-BC2N is higher than that of c-BN single crystals which indicates that the synthesized phase is only slightly less hard than diamond.

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...

Mechanical properties of cubic BC2N, a new superhard phase

Abstract A new superhard phase, cubic BC 2 N, has very recently been synthesized by direct conversion of graphite-like BN–C solid solutions at 25 GPa and 2100 K. The hardness, Youngs modulus, fracture toughness and structure of this phase have been examined using micro- and nanoindentation and transmission electron microscopy. The hardness and elastic modulus values ( E , G ) of the c-BC 2 N are intermediate between diamond and cubic boron nitride, which makes this new phase the hardest known solid after diamond.

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 β-boron, and establishment of a new high-pressure polymorph, γ-boron is reviewed.

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.

Compression and thermal expansion of hexagonal graphite-like boron nitride up to 7 GPa and 1800 K

Abstract The lattice parameters of hexagonal graphite-like boron nitride (hBN) have been measured in the temperature range from 300 to 1800 K up to 7 GPa using energy-dispersive powder diffraction of synchrotron radiation. From the obtained p-V-T relation for hBN, the temperature dependence of isothermal bulk modulus ( B 0 [ GPa ] = 32.06(4) + 4.47(9)·e − (T[K] − 298) 298 ) and pressure dependence of thermal expansion coefficient (β × 106[K−] = 40.9(8) − 1.6(2)·p[GPa]) have been derived.

Comment on “Synthesis of Ultra-Incompressible Superhard Rhenium Diboride at Ambient Pressure”

Chung et al. (Reports, 20 April 2007, p. 436) reported the synthesis of superhard rhenium diboride (ReB2) at ambient pressure. We show that ReB2, first synthesized at ambient pressure 45 years ago, is not a superhard material. Together with the high cost of Re, this makes the prospect for large-scale industrial applications of ReB2 doubtful.

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.

Rhombohedral boron subnitride, B13N2, by X-ray powder diffraction.

The structure of the title compound consists of distorted B12 icosahedra linked by N-B-N chains. The compound crystallizes in the rhombohedral space group R3m (No. 166). The unit cell contains four symmetry-independent atom sites, three of which are occupied by boron [in the 18h, 18h (site symmetry m) and 3b (site symmetry 3m) Wyckoff positions] and one by nitrogen (in the 6c Wyckoff position, site symmetry 3m). Two of the B atoms form the icosahedra, while N atoms link the icosahedra together. The main feature of the structure is that the 3b position is occupied by the B atom, which makes the structure different from those of B(6)O, for which these atom sites are vacant, and B(4+x)C(1-x), for which this position is randomly occupied by both B and C atoms.