Investigation of Hard Boron Rich Solids: Osmium Diboride and β-Rhombohedral Boron

Abstract

Recently, we succeeded in synthesizing three osmium borides, i.e., OsB_1.1, Os₂B₃ and OsB₂. Up to date, almost nothing is known about the physical properties of these materials. Microhardness measurements show that OsB₂ is extremely hard. Ab initio calculations show that it is due to formation of covalent bonds between boron atoms. OsB₂ is also a low compressibility material. It can be used for hard coatings. The β-rhombohedral polymorph of boron is the second hardest elemental crystal (H ≈ 33 GPa). It is also very light and a p-type semiconductor. In early 1970s, 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 MB_x, with x ≤ 31 or equivalently a dopant concentration larger than 3.2 at.%, should be ultrahard, i.e., H > 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. They are suitable for applications under extreme conditions and thermoelectric equipment.