Heat Capacity of Zinc Fluoride from 11 to 300°K. Thermodynamic Functions of Zinc Fluoride. Entropy and Heat Capacity Associated with the Antiferromagnetic Ordering of Manganous Fluoride, Ferrous Fluoride, Cobaltous Fluoride, and Nickelous Fluoride

Abstract

The heat capacity of ZnF2 has been measured calorimetrically between 11 and 300°K. In addition to the direct experimental data, values of heat capacity, entropy, enthalpy, and free energy are tabulated at selected temperatures. The values of entropy, and enthalpy at 298.16°K are S0=17.61±0.03 cal deg—1 mole—1 and H0–H00=2827±5 cal mole—1. The heat capacity of ZnF2 has been used, with a corresponding states argument, to estimate the lattice contributions to the entropy and heat capacity of the isomorphous fluorides MnF2, FeF2, CoF2, and NiF2 which exhibit heat capacity maxima associated with antiferromagnetic ordering at 66.5, 78.35, 37.70, and 73.22°K, respectively. The values of electronic entropy at the maximum in heat capacity are: MnF2, 0.85R ln6; FeF2, 0.87R ln5; CoF2, 0.80R ln2; NiF2, 0.71R ln3. The electronic entropies and heat capacities are compared with molecular field and spin wave theories of antiferromagnetism. At very low temperatures the electronic heat capacity on NiF2 is varying approximately as T32; whereas that of the other antiferromagnetic fluorides depends much more strongly on temperature. This behavior of the electronic heat capacity of NiF2 is believed to be associated with the small ferromagnetic moment found in magnetic anisotropy measurements on this salt.