Stability field of the high-(P, T) Re2C phase and properties of an analogous osmium carbide phase

The formation of a hexagonal rhenium carbide phase, Re2C, from the elements has been studied in a laser heated diamond anvil cell in a P, V range of 20-40 GPa and 1000-2000 K. No indication for the existence of cubic rhenium carbide, as suggested in the literature, or any other phase was found and Re2C is the only phase formed in the Re-C system up to around 70 GPa and 4000 K. A fit of a 3rd-order Birch-Murnaghan equation of state to the Re2C P, V-data results in a bulk modulus of B-0 = 405(30)GPa (B' = 4.6). The linear compressibility of Re2C along [0 0 1] (similar to 500 GPa) is significantly larger than the compressibility in the (0 0 1) plane (similar to 360 GPa parallel to [1 0 0]). Based on the observations for Re2C, we predict the structure and elastic properties of an analogous Os2C phase, which is, at least in the athermal limit, more stable than any other osmium carbide studied previously by density functional theory based calculations. (C) 2009 Elsevier B.V. All rights reserved.