The Heat Capacity of Cu(NO3)2·2.5H2O at Low Temperatures

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Abstract
The heat capacity C P of powdered Cu(NO3)2·2.5H2O has been measured between 0.5° and 4.2°K. In zero applied field, C P exhibits a Schottky anomaly having its maximum value of 0.51R at 1.82°K. The data are quantitatively described by assuming that Cu 2+ ions exist in this salt in isolated pairs (binary clusters) coupled by isotropic antiferromagnetic exchange with J/k = −5.18°K. The same model was previously found to give a good fit of the observed magnetic susceptibility. The present results, however, clearly exclude the possibility that Cu 2+ ions in this material are coupled in extended linear chains. C P measured in an applied field of 8.7 kG was found to be as predicted by the pair model. As a result of the isotropy of the pair interaction and the small single‐triplet separation, it has been possible to cool this salt by adiabatic magnetization of the powder in relatively small fields.

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