Heat capacities of molten salts with polyatomic anions

The molar heat capacities at constant pressure, CP, of molten salts with polyatomic anions, obtained from the literature, are examined. As a rule, the CP values are independent of the temperature T, but the molar heat capacities at constant volume, CV, derived from them, depend on T. The latter were obtained, as far as the required density, expansibility and compressibility data are available, for 1.1Tm, presumed to be the corresponding state, Tm being the melting temperature. Their ratio γ = CP/CV is linear with the cation–anion distance in the molten salt, dC–A. The communal, quasi-lattice, heat capacity ΔCP = CP − CP(i.g.) is obtained by subtraction of the sum of published ideal gas heat capacities of the constituent ions at 1.1Tm, CP(i.g.). This communal heat capacity ΔCP is proportional to the packing fraction of the ions in the melt, y = πNAνdC−A3/6V. Here NA is Avogadroʹs number, ν the number of ions per formula unit, and V the molar volume at 1.1Tm. Some models for the heat capacities of molten salts are shown not to be well applicable to the set of salts discussed here, but no alternative could be suggested.