Thermodynamic study of disorder in zinc fluoride tetrahydrate

Abstract

The heat capacity of zinc fluoride tetrahydrate has been measured from 10 K to 300 K, giving a value for S_cal, the calorimetric entropy, at 25°C. The entropy S_eq of this salt has been determined by studying the solution thermodynamics of zinc fluoride at 25°C. S_eq exceeds S_cal by 8.9 ± 1.5 J K^–1 mol^–1, so that the crystalline tetrahydrate is disordered at 0 K, with a residual entropy of approximately R ln 3. The reason for this disorder is believed to be the same as that for iron(II) fluoride tetrahydrate at ordinary temperatures, namely that the metal atom is octahedrally surrounded by four water molecules and two fluorine atoms, and that the crystal does not distinguish between the oxygen and fluorine atoms. If the fluorine atoms in an octahedron are in a trans position to each other, each octahedron would have three possible orientations.

To obtain an improved estimate of the standard entropy of the fluoride ion (needed in the evaluation of S_eq), the heat of solution of sodium fluoride in water at 25°C has been redetermined. This value, together with existing information on the salt and its saturated solution, gives –13.1 ± 0.4 J K^–1 mol^–1 for the standard entropy of the fluoride ion at 25°C.