Rhenium diboride (ReB₂) is a synthetic superhard material. It was first synthesized in 1962 and re-emerged recently due to hopes of achieving high hardness comparable to that of diamond. The reported ultrahigh hardness has been questioned, although this is a matter of definition as in the initial test rhenium diboride was able to scratch diamond. The production method of this material does not involve high pressures as with other hard synthetic materials, such as cubic boron nitride, which makes production cheap. However, rhenium itself is an expensive metal. The compound is formed from a mixture of rhenium, noted for its resistance to high pressure, and

boron Boron is a chemical element with the symbol B and atomic number 5. In its crystalline form it is a brittle, dark, lustrous metalloid; in its amorphous form it is a brown powder. As the lightest element of the ''boron group'' it has th ...

, which forms short, strong

covalent bond A covalent bond is a chemical bond that involves the sharing of electrons to form electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs. The stable balance of attractive and repulsive forces between atoms ...

s with rhenium.

Synthesis

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. In the metathesis reaction,

rhenium trichloride Trirhenium nonachloride is a compound with the formula ReCl3, sometimes also written Re3Cl9. It is a dark red hygroscopic solid that is insoluble in ordinary solvents. The compound is important in the history of inorganic chemistry as an early e ...

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.

Properties

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) is much lower than that of diamond and is comparable to that of tungsten carbide, silicon carbide, titanium diboride or zirconium diboride. Two factors contribute to the high hardness of ReB₂: a high density of valence electrons, and an abundance of short

s. Rhenium has one of the highest valence electron densities of any transition metal (476 electrons/nm³, compare to 572 electrons/nm³ for

osmium Osmium (from Greek grc, ὀσμή, osme, smell, label=none) is a chemical element with the symbol Os and atomic number 76. It is a hard, brittle, bluish-white transition metal in the platinum group that is found as a trace element in alloys, mos ...

and 705 electrons/nm³ for diamond). 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 Electronegativity, symbolized as , is the tendency for an atom of a given chemical element to attract shared electrons (or electron density) when forming a chemical bond. An atom's electronegativity is affected by both its atomic number and ...

) that they form covalent bonds in which the electrons are shared almost equally.

References

{{Rhenium compounds Borides Rhenium compounds Superhard materials Substances discovered in the 1960s

Synthesis

Properties

See also

References