Thermal stability of hexagonal OsB₂

The synthesis of novel hexagonal ReB₂-type OsB₂ ceramic powder was performed by high energy ball milling of elemental Os and B powders. Two different sources of B powder have been used for this mechanochemical synthesis. One B powder consisted of a mixture of amorphous and crystalline phases and a mixture of ¹⁰B and ¹¹B isotopes with a fine particle size, while another B powder was a purely crystalline (rhombohedral) material consisting of enriched ¹¹B isotope with coarse particle size. The same Os powder was used for the synthesis in both cases. It was established that, in the first case, the hexagonal OsB₂ phase was the main product of synthesis with a small quantity of Os ₂B₃ phase present after synthesis as an intermediate product. In the second case, where coarse crystalline ¹¹B powder was used as a raw material, only Os₂B₃ boride was synthesized mechanochemically. The thermal stability of hexagonal OsB₂ powder was studied by heating under argon up to 876 °C and cooling in vacuo down to -225 °C. During the heating, the sacrificial reaction 2OsB ₂+3O₂→2Os+2B₂O₃ took place due to presence of O₂/water vapor molecules in the heating chamber, resulting in the oxidation of B atoms and formation of B₂O ₃ and precipitation of Os metal out of the OsB₂ lattice. As a result of such phase changes during heating, the lattice parameters of hexagonal OsB₂ changed significantly. The shrinkage of the a lattice parameter was recorded in 276-426 °C temperature range upon heating, which was attributed to the removal of B atoms from the OsB₂ lattice due to oxidation followed by the precipitation of Os atoms and formation of Os metal. While significant structural changes occurred upon heating due to presence of O₂, the hexagonal OsB₂ ceramic demonstrated good phase stability upon cooling in vacuo with linear shrinkage of the lattice parameters and no phase changes detected during cooling.