Theory of the thermal expansion anomaly in solid nitrogen due to O₂impurity

Two mechanisms are considered which contribute to the observed thermal expansion of the N2-O2 solid solution. One of them, connected with the renormalisation of the effective spin-figure coupling constant by the spin-libron interaction, is linear in impurity O2 concentration. The second one is due to formation of clusters of exchange-bound molecules and in the case of pair clusters is quadratic in concentration. The contributions from both the mechanisms are equalised for the O2-molecule concentration of about 1%. The data on the O2-O2 exchange interaction is obtained from comparison between the theoretical and experimental results on the excess thermal expansion of the N2-O2 solid solution. The theoretical model permits the explanation of the main features of the observed anomalies, but only a more sophisticated theory will be able to give a close fit to the experiment.

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