Nitrogen–Oxygen Phase Diagram

We have determined the phase diagram for the notrogen–oxygen system by examining the x-ray diffraction patterns of polycrystalline samples of the solidified mixtures over the temperature range 21°–50°K. The diagram exhibits a surprising complexity: a eutectic line divides the liquid and the two-phase region of (γ-O2 + β-N2), and a peritectoid line separates this two-phase region and a new phase, designated “x”, between 18% and 37% N2 and 34° and 45°K. The “x” region is flanked by a two-phase region with β-O2 on one side, and a wide two-phase region with β-N2 on the other side, and is terminated at its low–temperature end by a two-phase region (β-O2 + β-N2) between 32.5° and 34°K; this region extends over nearly the whole concentration range. Oxygen has a striking range of solubility in β-N2 of 53% at 50°K and only 15% at 40°K. The lattice constants of β-N2 at 47.5°K decrease essentially linearly on addition of up to 40% O2, then decrease very sharply between 40% and 50% O2; the volume of the β-N2 unit decreases by 10% on addition of 50% O2.