Rhenium diboride

Rhenium diboride, ReB₂

ReB₂ can be synthesized by at least three different methods at standard atmospheric pressure: solid-state metathesis, melting in an electric arc, and direct heating of the elements.[4]

In the metathesis reaction, rhenium trichloride and magnesium diboride are mixed and heated in an inert atmosphere and the magnesium chloride byproduct is washed away. Excess boron is needed to prevent formation of other phases such as Re₇B₃ and Re₃B.

In the arc-melting method, rhenium and boron powders are mixed and a large electric current is passed through the mixture, also in an inert atmosphere.

In the direct reaction method, the rhenium-boron mixture is sealed in a vacuum and held at a high temperature over a longer period (1000 °C for five days).

At least the last two methods are capable of producing pure ReB₂ without any other phases, as confirmed by X-ray crystallography.

The hardiness of ReB₂ exhibits considerable anisotropy because of its hexagonal layered structure (see structure model), being greatest along the c axis. In contrast to the scratch hardness test, its indentation hardness (H_V ~ 22 GPa)[5] is much lower than that of diamond and is comparable to that of tungsten carbide, silicon carbide, titanium diboride or zirconium diboride.[5]

Two factors contribute to the high hardness of ReB₂: a high density of valence electrons, and an abundance of short covalent bonds.[4][6] Rhenium has one of the highest valence electron densities of any transition metal (476 electrons/nm³, compare to 572 electrons/nm³ for osmium and 705 electrons/nm³ for diamond[7]). The addition of boron requires only a 5% expansion of the rhenium lattice, because the small boron atoms fill the existing spaces between the rhenium atoms. Furthermore, the electronegativities of rhenium and boron are close enough (1.9 and 2.04 on the Pauling scale) that they form covalent bonds in which the electrons are shared almost equally.

• Network covalent bonding
• Superhard material