Boron is an unusual element. Its three valence electrons are too localised to make it metallic but insufficient in number to form a simple covalent structure. As a compromise, boron atoms form quasimolecular B12 icosahedra that become building blocks of boron and boron-rich crystalline phases. Only -rhombohedral and ß-rhombohedral boron, obtained at ambient pressure, are established as pure boron crystalline forms. The high-pressure behaviour of boron is of considerable interest in physics and materials science. Our investigations of superconducting polycrystalline boron-doped diamonds [1] revealed boron segregation in an intergranular space that may influence the mechanism of superconductivity.

Fig. 59 : Single crystals of the orthorhombic high-pressure boron phase.

Many of the fundamental questions regarding the solid-state chemistry of boron are still unanswered. Synthesis of single-phase products in macroscopic quantities and in the form of single crystals remains one of the great challenges of boron chemistry [2]. Using a large volume press, at 20 GPa and 1700 K, we have grown single crystals of a high-pressure high-temperature orthorhombic B28 boron phase for the first time. The crystals appeared to have a dark-red colour and an elongated prismatic shape (Figure 59). We confirmed the purity of the crystals by chemical analysis, collected single-crystal X-ray diffraction data (BM01), then solved and refined the crystal structure. There is remarkable agreement between structural parameters obtained from single crystal and powder X-ray diffraction data [3] and those from our ab initio calculations. The bc projection (Figure 60) shows that the structure is built of B12 icosahedra and B2 dumbbells linked together, thus forming a three-dimensional network. The distances B-B within a B12 icosahedron (1.766(3)-1.880(3) Å) are slightly longer than those within a dumbbell (1.721(4) Å). Chemical bonds within B2 dumbbells and B12 icosahedra, as well as between icosahedra and dumbbells, are strongly covalent, as evident from the presence of residual electron density in the difference Fourier maps.

Fig. 60: The structure of the HPHT boron phase shown in the bc projection. The insert shows difference electron density plots around the atoms B5 (dumbbell, upper right) and B2 and B5 (dumbbell-icosahedron contact) extracted from single-crystal X-ray diffraction data. The maximum electron density is centred on the middle of the bond, suggesting covalent bonding between the B2 dumbbell and the B12 icosahedron.

The high density and strong covalent bonding in this B28 phase suggest that it could be less compressible than the other known boron phases. This was confirmed in the compression experiment (ID09) up to ~30 GPa in a diamond anvil cell with Ne as a pressure transmitting medium and a powder of B28 that gave values for the bulk modulus of K300 = 227(2) GPa and its pressure derivative K´ = 2.2(2). Pure orthorhombic B28 is a poor electrical conductor with a resistivity of the order of 106 cm at ambient conditions. With increasing temperature, the resistivity decreases indicating semiconducting behaviour. The activation energy (1.9(2) eV) is in reasonable agreement with the value of the band-gap energy (2.1 eV) determined from optical spectroscopy measurements.

Boron is known as a hard material (with the Vickers hardness reported as high as 25-30 GPa for ß-boron). We found that samples of B28 with submicrometre grain sizes have a hardness of 58(5) GPa. This value is in the range of polycrystalline cBN which makes B28 the second (after diamond) elemental superhard material.

In summary, our study demonstrates that the orthorhombic high-pressure high-temperature boron phase, synthesised above 9 GPa, has a structure consisting of covalently bonded B12 icosahedra and B2 dumbbells, and combines unusual properties – it is a wide band gap semiconductor that is superhard, optically transparent, and thermally stable (above 1000 K in air). We have demonstrated the possibility to grow single crystals of this phase, which opens prospects for applications of this material in areas of electronics and optics.


[1] N. Dubrovinskaia, R. Wirth, J. Wosnitza, T. Papageorgiou, H.F. Braun, N. Miyajima and L. Dubrovinsky, Proc. Natl. Acad. Sci. USA 105, 11619 (2008).
[2] B. Albert and H. Hillebrecht, Angew. Chem. Int. Ed. 48, 2 (2009).
[3] E. Yu. Zarechnaya, L. Dubrovinsky, N. Dubrovinskaia, N. Miyajima, Y. Filinchuk, D. Chernyshov and V. Dmitriev, Sci. Tech. Adv. Mater. 9, 044209 (2008).


Principal publication and authors

Chernyshov (d), V. Dmitriev (d), N. Miyajima (a), A. El Goresy (a), H.F. Braun (e), S. Van Smaalen (c), I. Kantor (f), A. Kantor (f), V. Prakapenka (f), M. Hanfland (d), A.S. Mikhaylushkin (g), I.A. Abrikosov (g) and S.I. Simak (g), Phys. Rev. Lett. 102, 185501 (2009).
(a) Bayerisches Geoinstitut, University of Bayreuth (Germany)
(b) Institut für Geowissenschaften, University of Heidelberg (Germany)
(c) Lehrstuhl für Kristallographie, Physikalisches Institut, University of Bayreuth (Germany)
(d) ESRF
(e) Experimentalphysik V, Physikalisches Institut, University of Bayreuth (Germany)
(f) GeoSoilEnviroCARS, University of Chicago (USA)
(g) Department of Physics, Chemistry and Biology, Linköping University (Sweden)