Rhenium Diboride

For the first time, this compound (in the form of a powder) was synthesized and its crystal structure (in the form of alternating layers of rhenium and boron atoms) was studied, in 1961, by researchers from the Brooklyn Polytechnic Institute ^[3] .

But its extreme hardness, comparable to the hardness of diamond, was discovered by scientists from the United States ^[4] , who published their results in April 2007 . Some researchers questioned the claimed hardness of this compound ^[5] .

The synthesis of this compound is relatively simple, since it does not require the use of ultrahigh pressures, in contrast, for example, to the synthesis of another superhard material - boron nitride with a crystal lattice such as diamond, which reduces the cost of the process, but, on the other hand, rhenium is very rare and very expensive metal, therefore, the gain in the price of the compared materials is unclear.

In this compound, rhenium provides a very high concentration of valence electrons, and boron forms short, very strong covalent bonds .

The synthesis of rhenium diboride is produced in at least three different ways:

1. solid phase metabolic reaction;
2. fusion of rhenium and boron powders in an electric arc ;
3. strong heating of a mixture of rhenium and boron powders.

In a solid-phase exchange reaction, a mixture of powders of rhenium and boron compounds, for example, rhenium trichloride and magnesium diboride, is heated to high temperature in a vacuum or inert gas (for example, argon ). The by-product resulting from the reaction, soluble magnesium chloride , is then washed from rhenium diboride with a solvent, for example, ethanol . To avoid the formation of other rhenium borides that reduce hardness (Re ₇ B ₃ and Re ₃ B), magnesium diboride is taken in excess.

When melting by an electric arc, a current of 80 A is passed through a mixture of rhenium and boron dust. The process is also conducted in an inert atmosphere.

In the direct synthesis method from elements, the pressed mixture of powders was kept at a temperature of about 1300 K. for several days. It was by this method that rhenium diboride was first synthesized.

The last two methods are suitable for obtaining practically pure rhenium diboride from rhenium borides; the purity of the product is confirmed by X-ray diffraction analysis .

The hardness of a rhenium diboride crystal is very different along different crystallographic planes, since layers of rhenium and boron atoms alternating in hexagonal lattices alternate in its crystal structure (see Fig.). In hardness (~ 22 GPa, according to Vickers ), it is noticeably inferior to diamond (~ 70–80 GPa) and is comparable with such solid substances as tungsten carbide , titanium diboride or zirconium diboride .

The hardness of the compound is due to two factors: a very high concentration of valence electrons (rhenium has one of the highest concentrations of valence electrons among transition metals, 476 nm ⁻³ , the record for this parameter among metals - osmium - 572 nm ⁻³ , among non-metals - diamond - 705 nm ^–3 ) and the strength of Re – B covalent bonds.

The incorporation of boron atoms into the rhenium crystal lattice slightly (~ 5%) increases the lattice constant of the rhenium crystal , since small boron atoms penetrate into the voids between the rhenium atoms. The bonds in the rhenium diboride crystal are almost covalent, since the electronegativity of these elements on the Polling scale is almost equal (1.9 for rhenium and 2.04 for boron).

• Superhard