Rhenium diboride (ReB₂) is a synthetic

superhard material A superhard material is a material with a hardness value exceeding 40 gigapascals (GPa) when measured by the Vickers hardness test. They are virtually incompressible solids with high electron density and high bond covalency. As a result of their ...

. It was first synthesized in 1962 and re-emerged recently due to hopes of achieving high hardness comparable to that of

diamond Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, ...

. 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 Boron nitride is a thermally and chemically resistant refractory compound of boron and nitrogen with the chemical formula BN. It exists in various crystalline forms that are isoelectronic to a similarly structured carbon lattice. The hexagonal ...

, which makes production cheap. However, rhenium itself is an expensive metal. The compound is formed from a mixture of

rhenium Rhenium is a chemical element with the symbol Re and atomic number 75. It is a silvery-gray, heavy, third-row transition metal in group 7 of the periodic table. With an estimated average concentration of 1 part per billion (ppb), rhenium is one ...

, noted for its resistance to high pressure, and boron, which forms short, strong covalent bonds 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 An electric arc, or arc discharge, is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current through a normally nonconductive medium such as air produces a plasma; the plasma may produce visible light. An ...

, and direct heating of the elements. In the metathesis reaction, rhenium trichloride and magnesium diboride are mixed and heated in an inert atmosphere and the

magnesium chloride Magnesium chloride is the family of inorganic compounds with the formula , where x can range from 0 to 12. These salts are colorless or white solids that are highly soluble in water. These compounds and their solutions, both of which occur in natu ...

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 X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...

Properties

The hardiness of ReB₂ exhibits considerable

anisotropy Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physic ...

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 Indentation hardness tests are used in mechanical engineering to determine the hardness of a material to deformation. Several such tests exist, wherein the examined material is indented until an impression is formed; these tests can be performed on ...

(H_V ~ 22 GPa) is much lower than that of diamond and is comparable to that of

tungsten carbide Tungsten carbide (chemical formula: WC) is a chemical compound (specifically, a carbide) containing equal parts of tungsten and carbon atoms. In its most basic form, tungsten carbide is a fine gray powder, but it can be pressed and formed into ...

silicon carbide Silicon carbide (SiC), also known as carborundum (), is a hard chemical compound containing silicon and carbon. A semiconductor, it occurs in nature as the extremely rare mineral moissanite, but has been mass-produced as a powder and crystal s ...

titanium diboride Titanium diboride (TiB2) is an extremely hard ceramic which has excellent heat conductivity, oxidation stability and wear resistance. TiB2 is also a reasonable electrical conductor,J. Schmidt et al. "Preparation of titanium diboride TiB2 by spark ...

zirconium diboride Zirconium diboride (ZrB2) is a highly covalent refractory ceramic material with a hexagonal crystal structure. ZrB2 is an ultra high temperature ceramic (UHTC) with a melting point of 3246 °C. This along with its relatively low density of ...

. Two factors contribute to the high hardness of ReB₂: a high density of

valence electron In chemistry and physics, a valence electron is an electron in the outer shell associated with an atom, and that can participate in the formation of a chemical bond if the outer shell is not closed. In a single covalent bond, a shared pair form ...

s, and an abundance of short covalent bonds. 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). 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.

References

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

Synthesis

Properties

See also

References