Oxidation and passivating effect in tin(II) fluoride and chloride fluoride solid solutions: a ¹¹⁹Sn Mössbauer study

Divalent tin fluorides and chloride fluorides appear to be stable relative to oxidation to tetravalent tin at ambient temperature. X-ray diffraction shows only the line of the tin(II) compound, however the ¹¹⁹Sn Mössbauer spectrum of all tin(II) polycrystalline samples has a small broad peak at ca. 0 mm/s. This is the case of polycrystalline #alpha# −SnF₂, while the spectrum of a large single crystal polished sufficiently thin shows only the tin(II) doublet, with no SnO₂ peak at 0 mm/s. This shows that there is surface oxidation of each solid particle, to give a thin amorphous layer of SnO₂ stannic oxide. However, the Mössbauer peak of SnO₂ does not grow with prolonged exposure to air at ambient temperature, therefore it must be assumed that the layer of SnO₂ has a passivating effect, however oxidation increases at higher temperatures. We have investigated in this work the passivating effect of a layer of SnO₂ in two types of solid solutions: (i) in the fluorite type M_1−xSn_xF₂, where the amount of tin at low x values is not sufficient to provide full coverage of the surface of the particles, and (ii) in the PbClF type doubly disordered solid solution, Ba_1−xSn_xCl_1+yF_1−y. It was found that passivation works well in the M_1−xSn_xF₂ solid solution, however most of the time, it does not work so well for Ba_1−xSn_xCl_1+yF_1−y where it is strongly dependent on the method of preparation and the bonding strength, as shown by the variation versus the tin(II) recoil-free fraction.