Phase Diagram of Argon—Carbon Monoxide

The phase diagram Ar–CO has been determined by cooling curves and x-ray diffraction. Like the Ar–N2 system, a solid solution of hcp structure extends over essentially the entire range of concentration at temperatures just under the solidus, but unlike the Ar–N2 system, the hcp phase decomposes (eutectoidally) into a fcc Ar-rich phase and a primitive cubic CO-rich phase at 53°K; the eutectoid point is at 66% CO. The miscibility gap at 53°K extends from 5% to 85% CO and is probably complete at 0°K. There are quite extensive two-phase regions between the phases in the solid state, which were not found in the Ar–N2 system. There is again an indication that Ar would have the hcp structure just a few degrees above the melting point if melting did not intervene. Liquidus and solidus temperatures for Ar–CO reach a minimum (65°K) near 60%—70% CO. Lattice constants for the different phases are presented.

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