Firing of Clays Studied by X-ray Diffraction and Mössbauer Spectroscopy

Covalently bonded BaSnF₄, BaSn₂F₆ and new barium tin(II) chloride fluorides BaSn₂Cl₂F₄, BaSnClF₃?0.8H₂O, and non-stoichiometric Ba_1?x Sn _x Cl_1+y F_1?y were prepared and their stability upon long time storage in air was studied, with respect to tin(II) oxidation to tin(IV). The materials containing covalently bonded tin(II), i.e., all the stoichiometric phases, were found to passivate well due to particle coverage by a thin layer of SnO₂ and/or kinetic stabilization of hybridized tin(II), and they show a very minimal increase of tin(IV) signal with increasing storage time. At high temperature, passivation breaks down, then self restores. In contrast, the Ba_1?x Sn _x Cl_1+y F_1?y solid solution is not as well passivated. The rate of increase of tin(IV) Mössbauer signal is significantly higher for the precipitated solid solution, and was found to be inversely related to the amount of tin(IV) already present in the young sample. For the solid solution prepared in dry conditions, the ratio of tin oxidation was found to be higher at positive y values (Cl:F>1) and to decrease with increasing x (tin rich solid solution). In the solid solution, the bond type and strength vary widely with x and y, and so does the rate of oxidation. “Unprotected” Sn²⁺, weakly bound to the lattice, oxidizes much faster than covalently bonded tin(II).