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 Ba_5.66SnCl_7.30F_6.04 and thus is enriched in chlorine.     

BaSnF4 fast ion conductor: Variations versus the method of preparation and anomalous temperature variation of the quadrupole splitting

A new method of preparation of high performance fluoride ion conductor, BaSnF₄, by water leaching of newly discovered barium tin(II) chloride fluorides, has been designed, and the materials have been studied and compared to the solid prepared by the usual dry method. The unit-cell parameters and crystallite dimensions were found to vary with the method of preparation. In addition, the crystallite dimensions were found to be highly anisotropic for the samples obtained by the wet method. The Mössbauer spectrum is made of a large tin(II) quadrupole doublet, and a broad tin(IV) oxide peak due to surface oxidation. The tin(II) spectrum is in agreement with covalently bonded tin(II) having a strongly stereoactive lone pair. An unusually high dependence of the quadrupole splitting at low temperatures was observed (5.8 times larger than for α-SnF₂).     

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 τ _i?>?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.     

Bonding in the Doubly Disordered Ba1−x Sn x Cl1+y F1−y Solid Solution

New materials were prepared in the SnF₂/BaCl₂ system by precipitation, and in the SnF₂/BaCl₂/BaF₂ system by direct reactions at high temperature in dry conditions. Stoichiometric BaSn₂Cl₂F₄ and BaSnClF₃?0.8H₂O, and a wide Ba_1?x Sn _x Cl_1+y F_1?y solid solution were prepared for the first time. Elemental analysis, X-ray diffraction and ¹¹⁹Sn Mössbauer spectroscopy were used for the characterization and study of bonding in the new materials. Mössbauer spectroscopy was shown to be an excellent method for probing both the type of bonding at tin(II) (ionic or covalent) and the bond strength at the tin sublattice. Tin(II) is covalently bonded in the stoichiometric phases and ionic (Sn²⁺ stannous ion) in the precipitated Ba_1?x Sn _x Cl_1+y F_1?y solid solution. The case of Ba_1?x Sn _x Cl_1+y F_1?y prepared in dry conditions is more complex. At negative y values (Cl: F <1) and more particularly at high x (solid solution rich in tin), a mixture of Sn²⁺ and covalent Sn(II) is observed, with a normal sublattice strength for Sn(II). At positive y values (Cl:F >1) and more particularly at low x (poor in tin), all the tin(II) is in the ionic form. Furthermore, at high x and high y, the tin(II) sublattice strength decreases so drastically that the tin recoil free fraction at ambient temperature is nearly zero. The bonding type and tin sublattice strength can be explained in terms of preference of covalent bonding with F and when tin clustering occurs, whereas an excess Cl around Sn(II) forms ionic bonding and tin rattling due to ionic size mismatch.     

Using Mössbauer spectroscopy to choose the sites that can be occupied by divalent tin

Mössbauer spectroscopy can be a useful structural tool to assist crystallographic methods for site assignment when the compound under investigation contains divalent tin. The goal of this work was to show that the structure of tin(II) fluoride, also know as stannous fluoride, SnF₂, could have been solved 14 years earlier if Mössbauer spectroscopic results, already known, had been used. A first attempt to solve the crystal structure, carried out by Bergerhoff in 1962 seemed to find the tin positions, however, it failed to find the positions of fluorine. Further extensive studies by Dénès et al. in the mid 1970s yielded the same results as those of Bergerhoff, despite the use of a Nonius CAD-4 automatic diffractometer, in contrast with Bergerhoff’s film work. The tin positions yielded a residual of 0.23, and Fourier difference maps showed significant electron density that could be fluorine atoms, however, their number did not match the number of fluorine atoms expected and several F-F distances were way too short. In addition, refinement using these possible fluorine positions led to no improvement of the residual factor. Finally, the crystal structure was published by McDonald et al. in 1976. It was found that the tin sublattice determined by Bergerhoff was basically correct, except that half of the tin atoms found by Bergerhoff to be on the (4b) and (4e) special Wyckoff sites were actually on the (8f) general site. A translation of the origin of the unit-cell by the [1/8, 0, 3/16] vector allows to change the tin Wyckoff sites from (4b), (4e) and (8f) to two (8f) sites, while keeping the basic spatial distribution of tin. A method has now been designed, using ¹¹⁹Sn Mössbauer spectroscopy, to test the suitability of some Wyckoff sites for divalent tin, using the Mössbauer spectrum. The tin(II) doublet (δ = 3.430(3) mm/s, Δ = 1.532(3) mm/s) shows that the lone pair is on a hybrid orbital, therefore, it is stereoactive, and it results that tin cannot be on either the (4b) or (4e) tin site since both an inversion center and a 2-fold axis would generate a second lone pair unless the 2-fold axis were along the tin-lone pair axis.     

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

Hyperfine Interactions - 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...     

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 =3.1 mm/s and =1.9 mm/s.     

Search for the stannous ion in a chloride/fluoride matrix: cases of Ba1−x Sn x Cl1+y F1−y and of Ba2SnCl6

With the exception of anhydrous SnCl₂, in divalent tin fluorides and chlorides, tin(II) is always covalent bonded, i.e. its valence orbitals are hybridized and the tin lone pair is located in one of the hybrid orbitals. This lone pair is highly stereoactive and generates a large efg, resulting in a large quadrupole splitting. A doubly disordered Ba_1?x Sn _x Cl_1+y F_1?y solid solution has been prepared and found to contain either ionic tin(II) (Sn²⁺ ions) or a mixture of ionic and covalent tin(II), depending on x, y and the method of preparation. The ionic tin(II) spectrum in Ba_1?x Sn _x Cl_1+y F_1?y gives a Mössbauer single line that is broadened by the lattice efg, like in SnCl₂. Now, Sn²⁺ has been found to be present in an undistorted octahedral coordination in a newly isolated compound, Ba₂SnCl₆. It should be the first example of Sn²⁺ that is fully ionic and has a perfectly spherical lone pair.     

Phase transition of the doubly disordered Ba1−x Sn x Cl1+y F1−y solid solution at high temperature

The Ba_1−x Sn _x Cl_1+y F_1−y solid solution has the BaClF structure, with full disorder of the cations and partial disorder of the anions. It can be prepared either by the wet method or the dry method. Most samples prepared by the aqueous route give a tin(II) single line at ca. 4.1 mm/s characteristic of the Sn²⁺ stannous ion, and therefore of ionic bonding. In some rare cases, a very weak quadrupole doublet indicates the presence of a small amount of covalently bonded tin(II). The solid solution prepared by the dry method has different unit-cell parameters, and it contains a larger amount of covalently bonded tin, except at very low x values or highly positive y values, and the Sn²⁺ recoil-free fraction is much smaller. In the present work, a study of the phase transition that takes place when precipitated Ba_1−x Sn _x Cl_1+y F_1−y is heated has been undertaken. A dramatic increase of the amount of covalently bonded tin(II) occurred on heating.     

The first barium tin(II) bromide fluoride

In an effort to prepare barium tin(II) bromide fluorides for the first time, possibly similar to the chloride fluorides obtained earlier in our laboratory, precipitation reactions were carried out by mixing aqueous solutions of SnF₂ and of BaBr₂.2H₂O. In contrast with the chloride fluoride system, a single powdered phase was obtained throughout the SnF₂ – BaBr₂ system, with the yield being maximum at X ≈ 0.25, where X is the molar fraction of barium bromide in the reaction mixture. Phase identification with the JCPDS database failed to produce a match, confirming that a new phase had been produced. The exact chemical composition of the new compound has not been obtained yet. Based on the X value for the maximum yield, the Sn/Ba ratio is likely to be 3/1 or 2/1. The Mössbauer spectrum at ambient conditions shows that bonding to tin(II) is covalent, therefore with the tin lone pair being stereoactive. The Mössbauer parameters (δ = 3.68 mm/s, Δ = 0.99 mm/s) are similar to those of SnBrF and of Sn₂BrF₅, thereby showing that tin is bonded to both fluorine and bromine. The larger isomer shift and lower quadrupole splitting than in tin(II) fluorides show that the stereoactivity of the tin lone pair is lower than in the fluorides. The Mössbauer parameters fit well the linear correlation of the quadrupole splitting versus the isomer shift” that has been shown to be present in other series of tin(II) compounds. The linear decrease on this correlation shows that the contribution of non-spherical orbitals (p and d) to the lone pair is a much larger contributor to the quadrupole splitting than lattice distortions. The structure is likely made of Ba²⁺ cations and tin(II) fluoride bromide polyatomic anions, with covalent bonding withinthe anions.