Abdualhafeed Muntasar

Stabilization of the unhybridized Sn2+ stannous ion in the BaClF structure and its characterization by119Sn Mössbauer spectroscopy

In divalent tin halides, when the halogen is small and highly electronegative (F, Cl), the tin valence orbitals are hybridized, the tin(II) non-bonded electron pair is located on one of the hybrid orbitals, and the resulting large electric field gradient gives a large quadrupole splitting. The reaction of barium chloride and tin difluoride in aqueous solutions, for large BaCl2.2H2O/SnF2 ratios (>10) results in the precipitation of a white powdered material, which is identified by X-ray diffraction to be BaCIF. However, Tin-119 Mossbauer spectroscopy shows the material contains a fairly large amount of divalent tin in the Sn2+ ionic form, with unhybridized orbitals, like in SnCl2. Using X-ray diffraction, we have established that Sn2+ ions substitute 15% of the Ba2+ ions at random, and chemical analysis shows the material has the formula Ba5.66SnCl7.30F6.04 and thus is enriched in chlorine.

Fluoride-Ion Conductors Derived from the Fluorite Type

Mossbauer spectroscopy has seldom been used for the study of ionic conductors. When tin(II) is incorporated in the best binary fluoride ion conductors, namely in fluorite type MF2, various new materials are formed, which have a structure related to the fluorite type, and their fluoride ion conductivity is enhanced by up to 103. Most of these new conducting materials show order/disorder phenomena. PbSnF4, which is the highest performance fluoride ion conductor, can exist in several polymorphic forms, and has a very complex system of phase transitions. SnF2, can be incorporated in BaCIF to form a wide Ba1-xSnxCl1+yF1-y solid solution, which is fully disordered in terms of metals and of excess anion of one kind relative to the other. Tin-119 Mossbauer spectroscopy can be an invaluable technique for probing the tin sites. It has made possible the interpretation of the structure of the disordered phases. In addition, the information obtained about the valence electronic structure of tin(II) makes it very easy to distinguish between: (i) a stereoactive lone pair that is located on a tin hybrid orbital, and therefore cannot be a charge carrier, and (ii) a potentially mobile non-stereoactive lone pair located on the unhybridized 5s orbital of tin.

Variations of BaSnF4 fast ion conductor with the method of preparation and temperature

Ionic conductors are solids that have a large number of defects and easy pathways that make it possible for ions to move over long distances in an electric field. In order to be mobile an ion must be small and have a low charge. The fluoride ion is the most mobile anion. The highest performance fluoride ion conductors contain divalent tin, and have a highly layered crystal structure related to the CaF2 fluorite type. BaSnF4 has the α-PbSnF4 structure, which is a √2/2 × √2/2 × 2 superstructure of the fluorite type, where the tetragonal unit-cell and the value of the a and b parameters being equal to half the diagonals of the (a,b) face of fluorite are due to the loss of the F Bravais lattice, and the Sn Sn Ba Ba order along the c parameter is at the origin of the doubling of the c parameter. The BaSnF4 material was prepared first by Dénès et al. (C. R. Acad. Paris C, 280: 831, 1975), and its superionic properties were characterized by Dénès et al. (Solid State Ion., 13: 213, 1984). It was found to have a conductivity three orders of magnitude higher than that of BaF2, with an ionic conduction rate τi > 0.99. No BaSnF4 was obtained by the aqueous medium, when aqueous solutions of SnF2 and Ba(NO3)2 are mixed together; BaSn2F6 was obtained instead. In a new development of this work, BaSnF4 has been obtained by the wet method for the first time. X-ray powder diffraction showed that the BaSnF4 phase obtained by the wet method varies substantially from one sample to another: (a) signification variations of the c parameter of the tetragonal unit-cell reveals that the interlayer distance is sensitive to the leaching conditions, possibly because some of the leached ions remain in the interlayer spacing; (b) large variations of the crystallite dimensions and, as a result of the two-dimensionality of the structure, a strong crystallite dimension anisotropy are observed, with d∥ < d⊥, where d∥ and d⊥ are the crystallite dimensions parallel to the four-fold main axis, and perpendicular to it, respectively, showing that the layers are very thin and the interlayer interactions are very weak. Variable temperature Mössbauer spectroscopy showed an unusual large variation of the quadrupole splitting with temperature. A tentative explanation based on unusually large bond angles has been proposed.

Mössbauer Studies of Bonding and Tin(II) Sublattice Softening in the Highly Unusual Doubly Disordered Ba1−xSnxCl1+yF1−y Solid Solution

Two methods (wet and dry) have been designed to prepare a wide Ba1−xSnxCl1+yF1−y solid solution. Mossbauer spectroscopy has shown that the tin(II) recoil-free fraction becomes very small at high x and highly positive y, revealing considerable lattice softening.

Mössbauer Studies of Stannous Fluoride Reactivity with Synthetic Tooth Enamel – A Model for the Tooth Cavity Protection Actions of Novel Dentifrices

SnF2 is an important toothpaste ingredient, added for the provision of clinical efficacy for hard and soft tissue diseases and in breath protection. Synthetic calcium hydroxyapatite powders were exposed to liquid supernates (25 w/w% toothpaste water slurries, centrifuged) of Crest Gum Care® (SnF2) dentifrice. One-minute treatments were followed by 3x water washing, centrifugation and lyophilization. Post treatment, powders were analyzed by Mössbauer spectroscopy with 0.5–1 gram of treated apatite powder. Results show that tooth mineral stannous fluoride interactions include: (1) formation of surface reaction products with both Sn(II) and Sn(IV) oxidation states; (2) Sn-F binding on mineral surfaces with no evidence of SnO. The surface binding is, however, not pure Sn-F but contains contributions of other ligands, probably oxygens from surface phosphates or hydroxyl groups. Results also suggest that surface reacted stannous tin is oxidized with time, even when bound as a layer on the tooth surface. This study demonstrates for the first time the presence of Sn-F on tooth enamel post treatment and the contribution of passivation to long term stannous chemistry on tooth surfaces. The study also illustrates the practical applications of the Mössbauer technique.

Reactivity and Stability of Superionics MSnF 4 at High Temperature in Various Media

Superionic MSnF 4 are the highest performance fluoride ion conductors, with PbSnF 4 being the best. Prototypes of devices using PbSnF 4 have been constructed and tested. Since the fluoride ion mobility is thermally activated, some devices might be used more efficiently above ambient temperature. Therefore, it is of prime importance that the stability of these materials be tested under potential conditions of use, since thermal degradation and phase transitions are likely to alter the conducting properties. Tetragonal SrSnF 4 , α-PbSnF 4 and BaSnF 4 , and orthorhombic o-PbSnF 4 , are stable at ambient conditions, even in air. However, all undergo significant deterioration when heated in air: the color changes from white to yellowish, and tin hydrolysis and oxidation takes place. They are much more stable under inert conditions (nitrogen or argon). However, PbSnF 4 undergoes several phase transitions at high temperatures: o to α starting at ca. 100°C, α to β (reversible, but the reverse reaction is very sluggish) starting at 250°C, β to γ (reversible) at 390°C. β-PbSnF 4 can be quenched to ambient temperature and is metastable for long times, however, eventually it starts changing to stable α-PbSnF 4 and this change is uncontrollable and is faster above ambient temperature. Microcrystalline μγ-PbSnF 4 , obtained by ball milling any other phase of PbSnF 4 , gives rapidly a-PbSnF 4 at 200°C. In addition, for all MSnF 4 , hydrolysis of the Sn-F bonds to Sn-O occurs with traces of moisture.

The Ba1-xSnxCl1+yF1-y Solid Solution

A tetragonal distortion (P4/nmm) of the β-PbF2 fluorite type is obtained by 50% substitution of F by Cl, and it gives the PbClF structure. In PbClF, both anions contribute to the conduction, however it is a poor conductor, but it is purely ionic. We have found that one can substitute up to 25% Ba by Sn and, up to 15% Cl by F (y 0), to give a Ba1-XSnxCl1+yF1-y solid solution (0 ≤ x≤ 0.25) which has the same structure as BaCIF (PbCIF type), with full Ba/Sn disorder, whereas the anion sites remain ordered like in unsubstituted BaClF. However, there is disorder between -yF and (1+y)Cl on the Cl site if y 0. Surprisingly, tin(II) is present in the ionic Sn2+ stannous ion form instead of being covalently bonded. This is established from the Mossbauer parameters (δ≈ 4.05mm/s, Δ≈0). This also makes it possible to have a significant electron conductivity from the non-hybridized tin (II) lone pair.

Reaction of stannous fluoride in hydrogen peroxide

The reaction of SnF2 stannous fluoride with aqueous solutions of H2O2 hydrogen peroxide was studied as a function of the molar ratio H2O2/SnF2 in the range 0.02 to 5.00. The products were characterized by thermal analysis, X-ray diffraction and tin119 Mössbauer spectroscopy. The X-ray diffraction pattern of all samples shows only highly broadened lines, characteristic of microcrystalline SnO2 (average particle diameter: 39 Å). Thermal analyses show that the material is hydrated. Mössbauer spectroscopy gives a broad single line at approximately 0 mm/s, characteristic of SnO2 for all samples, and in some cases a tin(II) doublet with δ=3.1 mm/s and Δ=1.9 mm/s.

A Mössbauer Spectroscopic Study of Self-Passivation in Tin(II) Chloride Fluorides: The Special Case of the Ba1−xSnxCl1+yF1−y Solid Solution

A wide disordered Ba1−xSnxCl1+yF1−y solid solution has been prepared by wet and dry methods. Mossbauer spectroscopy has been used to study the oxidation of tin(II) to tin(IV) and the phenomenon, or lack of, self-passivation, and how it varies with the method of preparation, with the type of bonding, and with the lattice strength.