Abdualhafed Muntasar

Spontaneous Oxidation of Barium Tin(II) Chloride Fluorides

Covalently bonded BaSnF4, BaSn2F6 and new barium tin(II) chloride fluorides BaSn2Cl2F4, BaSnClF3⋅0.8H2O, and non-stoichiometric Ba1−xSnxCl1+yF1−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 Ba1−xSnxCl1+yF1−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).

A Thin Passivating Tin(IV) Oxide Layer on Tin(II)-Containing Fluoride Particles, or not? The M1-xSnxF2 Solid Solutions

Abstract Tin-119 Mossbauer spectroscopy shows that all powdered tin(II) compounds give a small tin(IV) signal that has been attributed to the presence of a very thin layer of SnO 2 on the surface of the particles, that passivates against further oxidation, and is undetectable by X-ray diffraction. In a new development of this work, efficient passivation against further oxidation of the M 1-x Sn x F 2 (M = Ca and Pb) solid solution poor in tin was found to be as high as for phases rich in tin, even though the amount of tin is way too low to be able to produce full coating of the particles by a thin layer of SnO 2 . This observation required developing a new model to explain the sluggish oxidation of phases poor in tin, based on bonding type and strength.

To Passivate or not to Passivate, that is the Question: The Case of Barium Tin(II) Chloride Fluorides

Abstract Barium tin(II) chloride fluorides were found to have a diversified behavior with respect to oxidation. Three types of situations were observed: (i) slow “fluoridelike” oxidation, (ii) moderate speed of oxidation, (iii) “fast” oxidation. The two latter categories were found only for the Ba1-xSnxCl1+yF1-y solid solution, that has the BaClF structure, with full Ba/Sn disorder, and also disorder between y Cl and (1-y) F (for y>0), or between –y F and (1+y) Cl (for y

THE UNIQUE CASE OF TIN(II) FLUORIDE CONTAINING UNEXPECTED SUBSTITUTIONAL SOLID SOLUTIONS: LOCAL STRUCTURE VERSUS GLOBAL STRUCTURE

Solid solutions provide the means of tailoring the properties of materials by adjusting their chemical composition without phase demixing. A solid solution is a compound with a variable composition. Substitutional solid solutions are formed by replacing some atoms/ions by other atoms/ions. In order to be energetically favored, this replacement is possible only if certain criteria are met: (1) size criterion: there should be no more than 15% difference between the radius of the atoms/ions that are replacing each other, and (2) bonding type criterion: the replacing atom/ion should be able to accept the bonding type of the host. If these criteria are not met, the substitution will create an unacceptable level of stress at and around the substitution sites. In our studies of the SnF2/MF2 systems, where (M = Ca, Sr, Ba or Pb), we have found a M1−xSnxF2 solid solution for M = Ca and Pb, with a wide range of composition in the case of Pb. In addition, the investigation of the SnF2/MF2/MCl2 systems revealed an even more complicated type of solid solution. All should be forbidden according to accepted criteria: the alkaline earth metal fluorides and chlorides have ionic structures, while SnF2 has three allotropes, all which are characterized by strongly covalent bonding with a significant amount of polymerization. In addition, the similar size criterion is grossly violated, Sn2+ being much smaller than the alkaline earth metal, well beyond the accepted limit of about 15% for ion substitution. Furthermore, the wider solid solutions are formed with PbF2 and BaClF, i.e. for the cations that have the largest size difference. The Ba1−xSnxCl1+yF1-y solid solution is by far the most unusual, and it is probably unique since it is a doubly disordered solid solution (simultaneous disorder on the cationic sita and on the anionic sites). The combined use of X-ray diffraction and Mössbauer spectroscopy was necessary in order to understand these solid solutions.

Site distortions created by the stereoactive lone pair of Tin(II) in highly symmetric structures

Several fluoride compounds containing divalent tin that have a fluorite (CaF2-type) unit cell have been prepared and studied. Some are stoichiometric compounds while others are solid solutions. The cubic symmetry of the unit-cell (no lattice distortion and no superstructure) and the unique metal ion site of the fluorite structure make it that tin and the other metal have to be disordered on the normal metal site of the fluorite unit-cell. However, that site has the m3m-Oh point symmetry, and the metal ion is located in the center of a cube having fluoride ions in all its corners. Therefore, the same coordination should apply to tin. However, tin(II) possesses a non-bonding pair of electrons called a “lone pair”, and in order for tin(II) to have a cubic symmetry, its lone pair has to be located on the unhybridized 5s orbital, that is spherical and thus does not distort the coordination. In such a case, the lone pair is said to be “non-stereoactive”. This would make tin present in the form of the Sn2+ stann...

Doublet asymmetry in divalent tin Mössbauer spectra

In randomly oriented polycrystalline samples, the probability of the two transitions for quadrupolar interactions of 119Sn is the same and therefore the two lines should have the same intensity, resulting in a symmetric doublet. Several cases taken from the divalent tin materials prepared in our laboratory depart from the expected symmetric doublet: (i) case of a non-stereoactive electron lone pair on tin: single line spectrum; (ii) case of a stereoactive lone pair resulting in highly preferred orientation: strongly asymmetric doublet, varying with the orientation of the sample in the γ-ray beam, similar to the case of a single crystal; (iii) case of a stereoactive lone pair resulting in high bonding anisotropy: asymmetric doublet, varying with temperature (Goldanskii-Karyagin effect); (iv) case of a mixture of the two kinds of tin(II), i.e., with a non-stereoactive lone pair and with a stereoactive lone pair, giving spectra varying from a very highly asymmetric doublet to a single line, without significa...

When crystallography can use help from tin-119 Mössbauer spectroscopy

Crystallography, the most powerful method for obtaining structural data, can benefit from help from other techniques. In this work, 119Sn Mössbauer spectroscopy was used to assist crystallography, for finding the tin(II) positions in the unit-cell and determine a tin(II) coordination in agreement with both the diffraction data and the tin electronic structure. Even high quality single crystal data do not guarantee that the right solution will be obtained. A first attempt at the structure of α−SnF2 yielded the tin positions with very reasonable R and Rw residuals, 0.23-0.25. However, the fluorine positions could not be found (Bergerhoff, 1962). After many other attempts, the full crystal structure was finally solved 14 years later (R.C. McDonald et al. 1976). The difference in the tin position with the initial solution (1962) was that, in the latter, half of the tin atoms were on special sites, however, the tin sublattice was identical. Because the tin sites in the initial solution gave very reasonable residuals, 14 years of hopeless efforts were wasted. The presentation will show that this could have been avoided using 119Sn Mössbauer spectroscopy. This was possible since the spectrum had already been recorded (A.J.F. Boyle et al., 1962). Mössbauer spectroscopy can also help determine the tin coordination, when combined with powder diffraction data, in case of disordered structures. The presence of tin(II), disordered with a metal ion in cubic coordination, when diffraction shows there is no lattice distortion and no superstructure, suggests that tin has also a cubic coordination. This would require the tin lone pair to be non-stereoactive; however Mössbauer spectroscopy shows it is stereoactive.

Combined Use Of Mössbauer SpectroscopyAnd X-ray Diffraction For The Study OfOrder-disorder In Tin(II)-containingFluoride Ion Conductors

In some materials, cationic or anionic disorder can take place, and the knowledge of the extent of this disorder and how it varies with preparation conditions and temperature can be essential, since key properties of the materials can vary drastically with the presence of disorder. In addition, we have discovered a method for disordering, by ball-milling, fluoride-ion conductors, the structure of which is derived from the fluorite-type. The presence of the disorder raises critical questions about the electronic structure of tin(II) and the possibility of mixed conduction due to the unhybridized tin(II) non-bonded pair having the option to add electron motion to the fluoride-ion conductivity. The study of disordered materials by X-ray diffraction does not reveal a full understanding of the tin situation, particularly its electronic structure. In addition, in many of the ordered tin(II) containing phases, very highly enhanced preferred orientation put some limits on the usefulness of X-ray diffraction, while, however, generating new knowledge. A unique method has been designed in our laboratory to determine unambiguously the lone pair stereoactivity in disordered systems, and to use it in highly oriented systems, by use of Sn Mössbauer spectroscopy. Examples taken from our work on high fluoride ion conductors in the SnF2/MF2 system (M = Ca, Sr, Ba and Pb) have been studied.