Osmium borides are compounds of osmium

Osmium

Osmium is a chemical element with the symbol Os and atomic number 76. Osmium is a hard, brittle, blue-gray or blue-blacktransition metal in the platinum family, and is the densest natural element. Osmium is twice as dense as lead. The density of osmium is , slightly greater than that of iridium,...

 and boron

Boron

Boron is the chemical element with atomic number 5 and the chemical symbol B. Boron is a metalloid. Because boron is not produced by stellar nucleosynthesis, it is a low-abundance element in both the solar system and the Earth's crust. However, boron is concentrated on Earth by the...

. 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

Covalent bond

A covalent bond is a form of chemical bonding that is characterized by the sharing of pairs of electrons between atoms. The stable balance of attractive and repulsive forces between atoms when they share electrons is known as covalent bonding....

s will make osmium borides superhard materials

Superhard materials

A superhard material is a material with a hardness value exceeding 40 GPa when measured by the Vickers hardness test. They are highly incompressible solids with high electron density and high bond covalency...

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

Sapphire

Sapphire is a gemstone variety of the mineral corundum, an aluminium oxide , when it is a color other than red or dark pink; in which case the gem would instead be called a ruby, considered to be a different gemstone. Trace amounts of other elements such as iron, titanium, or chromium can give...

), but not superhard.

Synthesis

Osmium borides are produced in vacuum or inert atmosphere to prevent formation
of osmium tetroxide, which is a hazardous compound. Synthesis occurs at high temperatures (~1000 °C) from a mixture of MgB₂

Magnesium diboride

Magnesium diboride is a simple ionic binary compound that has proven to be an inexpensive and useful superconducting material.Its superconductivity was announced in the journal Nature in March 2001. Its critical temperature of is the highest amongst conventional superconductors...

 and OsCl₃.

Structure

Three osmium borides are known: OsB_1.1, Os₂B₃ and OsB₂. The first two have hexagonal structure, similar to that of rhenium diboride

Rhenium diboride

Rhenium diboride 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...

. Osmium diboride was first also sought as hexagonal, but its structure was later reassigned to orthorhombic.