Rhenium diboride

Rhenium diboride (ReB₂) is a synthetic superhard material of hardness comparable to diamond. It was discovered by a team at UCLA.^[1]

Additional recommended knowledge

Guide to balance cleaning: 8 simple steps

Safe Weighing Range Ensures Accurate Results

Essential Laboratory Skills Guide

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.

The compound is formed from a mixture of rhenium, noted for its resistance to high pressure, and boron, which forms short, strong covalent bonds with rhenium. The resulting material is hard enough to scratch diamond.

Synthesis

ReB₂ can be synthesized by at least three different methods at atmospheric pressure: solid-state metathesis, melting in an electric arc, and direct heating of the elements.^[1]

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 (80 amperes) 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 for a long time (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

Two factors contribute to ReB₂'s great hardness: a high density of valence electrons and an abundance of short covalent bonds.^[1] Rhenium has one of the highest valence electron densities of any transition metal (0.476 electrons/ų, compare to 0.572 electrons/ų for osmium and 0.705 electrons/ų for diamond^[2]). 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.

Although ReB₂ is harder than diamond along certain directions, it exhibits considerable anisotropy because of its layered structure.

References

1. ^ ^{a b c} Chung, Hsiu-Ying; et al. (April 20, 2007). "Synthesis of Ultra-Incompressible Superhard Rhenium Diboride at Ambient Pressure". Science 316 (5823): 436. doi:10.1126/science.1139322. Lay summary – New Scientist.
2. ^ Cumberland, Robert W.; et al. (April 27, 2005). "Osmium Diboride, An Ultra-Incompressible, Hard Material". Journal of the American Chemical Society 127 (20): 7264. doi:10.1021/ja043806y.