Argon—Oxygen Phase Diagram

The phase diagram of Ar–O2 was determined by x-ray diffraction. On the Ar-rich side, a phase having hexagonal-close-packed structure exists under the solidus. This phase undergoes a strain-induced transformation at low temperatures to face-centered cubic in mixtures from 1% to 20% O2. From 20% to 50% O2, the hcp persists to 0°K. On the O2-rich side, γ-O2 dissolves Ar to 12½% Ar, and β-O2 to 21% O2. Both structures coexist with the hcp in wide two-phase regions. The α phase can accept up to 20% Ar. A new phase, designated δ, was discovered, which can exist between 55% and 90% O2. Transformation characteristics suggest that there is a structural relationship between the δ and γ phases, and between the α and β phases. The lattice constants of 20% and 45% O2 hcp mixtures, determined at 23°K, are nearly identical.

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