ACTA

Materials Formed between Tin(II) Fluoride and Alkaline Earth Metal Fluorides and Chlorides

G. Dénès^a, A. Muntasar^a, M.C. Madamba^a, H. Merazig^b, Z. Zhu^a
^aLaboratory of Solid State Chemistry and Mössbauer spectroscopy, Department of Chemistry and Biochemistry, Concordia University, 7141 Sherbrooke St. West, Montreal, H4B 1R6, Qc, Canada
^bUnité de Recherche de Chimie de l'Environnement et Moléculaire Structurale CHEMS, Université des frères Mentouri de Constantine, Constantine, Algeria

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This report deals with compounds of tin(II) fluoride, also commonly called stannous fluoride, SnF₂, the structure of which is related to the fluorite (CaF₂) type. The fluorides are exceptionally high performance fluoride-ion conductors, particularly when SnF₂ is combined with PbF₂. This has been attributed to Pb²⁺ having a 6s² lone pair, making the ion softer than alkaline earth metal ions. The phases can be either ordered or disordered, or with gradual change from fully disordered to fully ordered. Some of the disordered phases are nanocrystalline. In MSnF₄ (M = Pb and Ba), a change from crystalline to nanocrystalline, simultaneous with the formation of two types of disorder, a positional disorder and an orientational disorder, is observed to take place upon an unusually short ball-milling time. One of the SnF₂/PbF₂ phases, PbSn₄F₁₀, deserves a special attention: with full Pb/Sn disorder and the undistorted fluorite-type β-PbF₂ structure, just its existence seems to be a miracle, and as a consequence it is metastable with a fairly short life time. Tin(II) alkaline earth metal chloride fluorides were obtained only with barium, and one has a disordered form of the BaClF structure, that is itself derived from the fluorite-type structure of BaF₂ by Cl/F ordering in the form of alternating planar sheets of fluoride ions and corrugated sheets of chloride ions perpendicularly to the c axis of the tetragonal unit-cell. The Ba/Sn(II)/Cl/F phase, doubly disordered Ba_1-xSn_xCl_1+yF_1-y solid solution was obtained, that has the BaClF structure, with two types of unique disorders: (i) a limited disorder between Cl and F, the limits of the solid solution being a function of the method of preparation (precipitation or solid state reactions), (ii) a very unique disorder between Ba²⁺ ions, Sn²⁺ ions and Sn(II) covalently bonded on the same site. Disorder between the two types of bonding of divalent tin on the same site had never been observed before and is in direct violation with the normal criteria for substitution. In addition, the presence of Sn²⁺ on the much larger Ba²⁺ site results in Sn²⁺ being very much loose in the much oversized site, resulting in considerable rattling, that is frozen at cryogenic temperatures. Furthermore, the Ba_1-xSn_xCl_1+yF_1-y solid solution obtained by precipitation undergoes a disorder-disorder phase transition on heating, i.e. from one form of disorder to another. The study of these phases and their properties was made possible by a combined use of X-ray powder diffraction and ¹¹⁹Sn Mössbauer spectroscopy.

DOI:10.12693/APhysPolA.134.1021
PACS numbers: 82.80.Ej, 76.80.+y, 61.05.Qr, 61.66.Fn