Osmium borides

Osmium borides are compounds of osmium and boron. 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 bonds will make osmium borides superhard materials, however this has not been demonstrated yet. For example, OsB₂ is hard (hardness comparable to that of sapphire), but not superhard.^[1]

Structure of orthorhombic OsB₂. Green atoms are Os, pink – boron

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 and OsCl₃.^[1]

Structure

Three osmium borides are known: OsB, Os₂B₃ and OsB₂. The first two have hexagonal structure,^[2] similar to that of rhenium diboride. Osmium diboride was first also sought as hexagonal,^[3] but one of its phases was later reassigned to orthorhombic.^[1][4] In recent methods of synthesis, it has also been found that a hexagonal phase of OsB₂ exists with a similar structure to ReB₂.^[5]