Novel high pressure hexagonal OsB2 by mechanochemistry

Hexagonal OsB2, a theoretically predicted high-pressure phase, has been synthesized for the first time by a mechanochemical method, i.e., high energy ball milling. X-ray diffraction indicated that formation of hexagonal OsB2 begins after 2.5 h of milling, and the reaction reaches equilibrium after 18 h of milling. Rietveld refinement of the powder data indicated that hexagonal OsB2 crystallizes in the P63/mmc space group (No. 194) with lattice parameters of a=2.916 angstrom and c=7.376 angstrom. Transmission electron microscopy confirmed the appearance of the hexagonal OsB2 phase after high energy ball milling, in situ X-ray diffraction experiments showed that the phase is stable from 225 degrees C to 1050 degrees C. The hexagonal OsB2 powder was annealed at 1050 degrees C for 6 days in vacua to improve crystallinity and remove strain induced during the mechanochemical synthesis. The structure partially converted to the orthorhombic phase (20 wt%) after fast current assisted sintering of hexagonal OsB2 at 1500 degrees C for 5 min. Mechanochemical approaches to the synthesis of hard boride materials allow new phases to be produced that cannot be prepared using conventional methods. (C) 2014 Elsevier Inc. All rights reserved.