Thermal stability of hexagonal OsB2



Z. L. Xie; R. G. Blair; N. Orlovskaya; D. A. Cullen;E. A. Payzant


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Abbreviated Journal Title

J. Solid State Chem.


Osmium; Boron; Stability; Coefficient of thermochemical expansion; OSMIUM DIBORIDE; HARD MATERIAL; TRANSITION; BORIDES; Chemistry, Inorganic & Nuclear; Chemistry, Physical


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 B-10 and B-11 isotopes with a fine particle size, while another B powder was a purely crystalline (rhombohedral) material consisting of enriched B-11 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 Os2B3 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 degrees C and cooling in vacuo down to -225 degrees C. During the heating, the sacrificial reaction 2OsB(2) + 3O(2) - > 2Os + 2B(2)O(3) took place due to presence of O-2/water vapor molecules in the heating chamber, resulting in the oxidation of B atoms and formation of B2O3 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 degrees 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 O-2, 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. (C) 2014 Elsevier Inc. All rights reserved.

Journal Title

Journal of Solid State Chemistry



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