Calorimetric study of transitions in hexahydrated fluorosilicates MSiF6·6H2O

Abstract

Heat-capacity results are reported from 8 to 310 K for nickel fluorosilicate and from 7 to 320 K for the magnesium salt. These results, together with similar information on the manganese, iron, cobalt and zinc salts which will be published elsewhere, confirm that the nickel and zinc salts have no transition in the temperature region covered by the C_p measurements. Each of the other four salts has a transition, probably largely though not wholly isothermal. The entropies of transition are all rather small, being 2.35, 1.615, 3.76 and 5.29 J K^–1 mol^–1 for the magnesium, manganese, iron and cobalt salts, respectively. The trend of the molar entropies at 298.15 K of the salts from manganese to zinc is the same as that of the molar volumes, both quantities reaching a minimum at the nickel compound.

There seems no reason to doubt the conclusion previously reached largely on the basis of the diffraction evidence that the anions in these salts at room temperature are orientationally disordered, and possibly the cations as well. However, the available evidence suggests that the transitions are not primarily of an order–disorder kind, but are rather associated with changes in the hydrogen bonding between the water molecules and the fluorine atoms, such that the orientational disorder is relatively little affected by cooling through the transition.