The search for superhard materials: Doped boron

Laboratoire Matériaux et Phénomènes Quantiques, Université Paris 7 - CNRS, UMR 7162 Bâtiment Condorcet, 75205 Paris Cedex 13, France, EU

Corresponding author: mhe@univ-paris-diderot.fr

Received: 2 June 2009
Accepted: 22 June 2009

Abstract

Superhard materials have numerous industrial applications and are then the subject of intense investigations. The -rhombohedral polymorph of boron is the second hardest elemental crystal (H_V ~ 34 GPa). It is also very light and a p-type semiconductor. In the early seventies, it has been shown that the doping of boron with transition elements enhances its hardness by about 25%. We predict that, in general, heavily doped samples MB_x, with x 31 or equivalently a dopant concentration larger than 3.2 at.%, should be ultrahard, i.e., H_V > 43 GPa. The relevant dopants M are Al, Cu, Sc, Mn, Mg and Li. In addition to these properties, boron-rich materials have a very low volatility, a high chemical inertness and high melting point ( > 2400 °C). They are suitable for applications under extreme conditions and thermoelectric equipment.