Double Salts Formation in the Systems MX–MeX ₂–H₂O (M=K, NH₄, Rb, Cs; Me=Mg, Mn, Fe, Co, Ni, Cu; X=Cl, Br)

Summary.

 The data available on the solubility phase diagrams and on the crystallization of chloride and bromide double salts in the systems MX–MeX ₂–H₂O (M=K, NH₄, Rb, Cs; Me=Mg, Mn, Fe, Co, Ni, Cu; X=Cl, Br) over a wide temperature range are collected and systematized. Formation of mainly 6 formula types mMX·MeX ₂·nH₂O (m=1, 2, 3; n=0, 2, 6) double salts are established. Using crystal structure elucidations the effect of M ⁺, Me ²⁺, and X ⁻ ions on the solubility diagrams and on the compositions of the double salts formed in these systems are discussed. The conclusion is drawn that the appearance of double salts in the solubility phase diagrams is a result of the equilibrium between the different ionic forms in the solution. The availability of a sufficient concentration (activity) of such complexes, which can be incorporated in the crystal structure during the crystal growth process through appropriate sticking, almost unchanged or with small changes, as e.g. condensation with elimination of halogenide ions and by linking together with other ions or complexes (e.g. M ⁺, X ⁻ ions) and H₂O – molecules, determines the solubility and hence the crystallization field of the respective double salt.