Joy and frustrations of growing crystals of divalent tin compounds: getting only powders or low-dimensionalcrystals

Abstract

This work shows that the stereoactivity of the tin(II) lone pair, and its orientation in space when it is stereoactive, determines the dimensionality of crystal growth. In addition to one problematic case when good single crystals were available, often the absence of large enough crystals for single crystal diffraction, or their insufficient quality due to their low-dimensionality, has prompted us to find other methods to help solving crystal structures. A striking example of this is the case of α − PbSnF₄. the structure of which could not be solved by X-ray single crystals or powder diffraction, due to the crystal shape. The two-dimensionality of the crystal structure of α − PbSnF₄ creates an enormous amount of preferred orientation that cannot be eliminated by milling because it results in an order-disorder phase transition with formation of a nanocrytalline material. This report shows that the combination of X-ray powder diffraction and tin-119 Mössbauer spectroscopy was very successful in many cases. X-ray diffraction shows the crystal lattice and expected atomic positions, either from the normal methods of solving crystal structures, or by comparison with isostructural compounds, or by partial substitution on some sites in the case of solid solutions. However, Mössbauer spectroscopy, that is a very convenient method for determining the oxidation number of tin and its bonding method, shows that the suggested tin(II) positions obtained by diffraction methods can be erroneous in some cases, when the symmetry elements of the site would repeat the tin(II) stereoactive lone pair to make several lone pairs on the same tin atom. In this work, the combination of the two methods has been used to show that crystal structures previously determined from a single crystal (α − SnF₂) or by the Rietveld method on hopelessly oriented samples (α − PbSnF₄) over many years of difficult/hopeless efforts would have been greatly simplified and done much faster, had anyone thought of combining the two techniques. In particular, the crystallographers did not think of using the Mösbauer results. In addition, we have prepared stoichiometric compounds and also solid solutions, where the cubic symmetry of the unit-cell requires that tin(II) be in Wyckoff sites that would repeat the tin(II) lone pair many times on the same tin(II) atom. In these cases too, the combination of the two methods made it possible to propose a highly reasonable model of the crystal structure.