Thermodynamic Investigation of Solid Argon–Nitrogen Mixtures

The heat capacity from 13° to 77°K, the enthalpies of transition and fusion have been measured for argon–nitrogen mixtures containing 70.193, 83.072, 90.033, and 93.702 mole % nitrogen. The entropy of vaporization could not be measured, and therefore its value was estimated by calculation from other data. The zero-point entropy of the mixtures with 90.033% and 93.702% nitrogen consists entirely of the entropy of ideal mixing, and of the 70.193% and 83.072% mixtures consists largely, or even entirely, of the entropy of mixing of the two compounds. In the case of the 70.193% mixture, the presence of excess heat capacity at 13°K indicates that the quadrupole ordering of nitrogen molecules in the solid is not complete at the lowest experimental temperature. However, for the other compositions which are richer in nitrogen, it appears quite likely that the ordering is complete at the lowest experimental temperatures. Correlation of the measured thermodynamic properties with the theoretical values predicted by the theory of Pople and Karasz has been attempted and the results appear reasonable for the temperatures and entropies of transition and fusion.