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 B₀ = 405 (30) GPa (B^′ = 4.6). The linear compressibility of Re₂C along [0 0 1] (∼ 500 GPa) is significantly larger than the compressibility in the (0 0 1) plane (∼ 360 GPa {norm of matrix} [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.