The search for superhard materials: Doped boron

Superhard materials have numerous industrial applications and are then the subject of intense investigations. The $\beta $-rhombohedral polymorph of boron is the second hardest elemental crystal (HV ~ 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 $3d$ transition elements enhances its hardness by about 25%. We predict that, in general, heavily doped samples MBx, with x $\leqslant$ 31 or equivalently a dopant concentration larger than 3.2 at.%, should be ultrahard, i.e., HV > 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