Osmium borides are compounds of

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

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 ...

. Their most remarkable property is potentially high hardness. It is thought that a combination of high electron density of osmium with the strength of boron-osmium

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 will make osmium borides

superhard materials 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 u ...

, however this has not been demonstrated yet. For example, OsB₂ is hard (hardness comparable to that of

sapphire Sapphire is a precious gemstone, a variety of the mineral corundum, consisting of aluminium oxide () with trace amounts of elements such as iron, titanium, chromium, vanadium, or magnesium. The name sapphire is derived via the Latin "sapphir ...

), but not superhard.

Synthesis

Osmium borides are produced in vacuum or inert atmosphere to prevent formation of

osmium tetroxide Osmium tetroxide (also osmium(VIII) oxide) is the chemical compound with the formula OsO4. The compound is noteworthy for its many uses, despite its toxicity and the rarity of osmium. It also has a number of unusual properties, one being that the ...

, which is a hazardous compound. Synthesis occurs at high temperatures (~1000 °C) from a mixture of MgB₂ and OsCl₃.

Structure

Three osmium borides are known: OsB, Os₂B₃ and OsB₂. The first two have hexagonal structure, similar to that of

rhenium diboride 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, althou ...

. Osmium diboride was first also sought as hexagonal, but one of its phases was later reassigned to orthorhombic. In recent methods of synthesis, it has also been found that a hexagonal phase of OsB₂ exists with a similar structure to ReB₂.

References

{{DEFAULTSORT:Osmium Borides Osmium compounds Borides Superhard materials