Divalent metal halide double salts in equilibrium with their aqueous solutions II. Factors determining their crystal structures

The double salts of many divalent metal halides that exist in equilibrium with their saturated aqueous solutions are composed of discrete or polymerized complex ions in simple ratios (e.g., 1:1 or 1:2). By treating the complex ions as spheres, ellipsoids, or cylinders whose dimensions can be easily estimated, one can predict possible crystal structures for these compounds. The observed cell dimensions generally lie within 0.8 Å of those predicted by the model. We show that the existence of some compositions can only be understood when crystal packing is considered. In particular, we are able to predict why interstitial water appears in some compounds.