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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. This has not been demonstrated yet. For example, OsB2 is hard (hardness comparable to that of sapphire), but not superhard.
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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.
Structure
Three osmium borides are known: OsB, Os2B3 and OsB2. The first two have hexagonal structure, similar to that of rhenium diboride. Osmium diboride was first also sought as hexagonal, but one of its phase's 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.