Heat capacity of molecular solids: The special case of cryocrystals

The cryocrystals have been ideal systems for the development of basic ideas concerning solid state physics (thermodynamics, kinetics, etc.). The analysis suggested in this article is a continuation of the above trend, namely, we reconsider the old heat capacity results using the modern level of understanding. Namely, we analyze the available temperature dependence C(T) data, looking for the two “extra” term, the one proportional to T5 and, especially, the linear term. The cryocrystals considered are four classical atomic cryocrystals (Ar, Ne, Kr, and Xe) as well as four molecular cryocrystals (N2, CO, CO2, and N2O). We did not include the He isotopes because of their extreme quantum properties. We also do not consider the solid hydrogens for the same reason. Detailed analysis of the available experimental heat capacities of the solids enumerated above shows that one could hardly expect to detect a linear term. Yet, the situation is not that hopeless: the heat capacity C(T) of cryocrystals with properly chosen impurities can contain a linear term.