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A specific-heat anomaly of Schottky type has been observed in the heat capacities of both solid CO and N2, which is due to O2 impurity. For both host lattices the peak of the anomaly is at T ∼ 1.95°K, and the shape corresponds to a two-level system with a splitting of 5.14°K and a weight of 2 for the upper level. It is believed to be due to the spin splitting of the molecular oxygen ground state.
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Burford et al. (1968) studied this question.
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