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

The heat capacity CP of powdered Cu(NO3)2·2.5H2O has been measured between 0.5° and 4.2°K. In zero applied field, CP exhibits a Schottky anomaly having its maximum value of 0.51R at 1.82°K. The data are quantitatively described by assuming that Cu2+ 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 Cu2+ ions in this material are coupled in extended linear chains. CP 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.