Low Dimensionality Materials: Origin of the Reduced Dimensonality in Tin(II) Fluoride- Containing Compounds and Its Study by X-Ray Diffraction and Mössbauer Spectroscopy

Isotropic materials have the same properties in all directions of space, with the same magnitude. Strict isotropy requires a spherical symmetry, hence a cubic unit-cell. All other crystal systems give rise to property anisotropy, i.e. direction dependence of properties and of their magnitude, although the anisotropy may often be weak enough to be quite insignificant. However, some materials show very strong anisotropy, owing to their layered structure, which is the result of unequal bond strength versus direction in space. Property anisotropy is usually the consequence of bonding anisotropy that gives anisotropic crystal growth, i.e. the crystals grow faster in some directions and slower in others, resulting in a crystallite shape that is often sheet-like (two-dimensional) or needle-like (one-dimensional). Many tin(II)-containing materials are found to have very strong low dimensionality: (1) SnF2/MCl (M = alkali metals and NH4) give needle shaped crystals even long hair-shaped. For example, in M3Sn5Cl3F10, the intersection of planes of lone pairs creates cleavage planes in two directions, giving needle shaped crystals. Extreme cases of two-dimensionality were observed in MSnF4, particularly in α-PbSnF4. Bonding anisotropy in tin(II)-containing materials is due to the tin stereoactive lone pair, when the lone pairs cluster in sheets, since no bonding to tin can take place in the lone pair direction. This gives rise to high preferred orientation of polycrystalline samples. The presentation will show how the anisotropy of the tin(II) quadrupole doublet, measured on polycrystalline samples subjected to an extremely enhanced preferred orientation, can be used to predict the direction of the lone pairs in the unit-cell and this, in turn, explain the direction of the cleavage planes. The presentation will focus on the use of X-ray diffraction and Mössbauer spectroscopy to characterize highly anisotropic phases and understand their structure-textural properties.