M. Cecilia Madamba

Understanding the phase transitions and texture in superionic PbSnF4: The key to reproducible properties☆

Abstract The recent use of high-performance fluoride ion conductor PbSnF4 for the fabrication of a solid state room temperature amperometric oxygen sensor implies that the materials used for its fabrication is well defined and undergoes no transformation over the lifetime of the sensor in the conditions of use. There are many ways to prepare PbSnF4, and subtle differences can lead to different PbSnF4 phases. Furthermore, moderate applications of heat and mechanical energy (ball milling), or minor changes in the composition of the reaction medium, can result in phase transitions, including order/disorder phenomena. In addition, it was also found that several phases, that appear stable, are in fact metastable and undergo transformations over prolonged periods of time. Furthermore, most phases of PbSnF4 show a considerable amount of preferred orientation due to the layered structure, causing highly anisotropic properties of polycrystalline samples. We have discovered methods for eliminating the preferential orientation of the crystallites and for enhancing it, close to the situation of a single crystal, in one direction. The adequate choice of the method of preparation, proper control of the preferred orientation, and knowledge of the phase transitions, should make possible the production of a material with stable and reliable properties.

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.

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.

Phases Driven Far From Equilibrium by Applying Mechanical Energy: Phase Transformations to γ-PbSnF4 Upon Ball Milling

The technique of ball milling has been applied to various phases of superionic PbSnF 4 , namely on (i): highly stressed tetragonal α-PbSnF 4(aq1) obtained by precipitation from aqueous solutions, (ii): highly stressed tetragonal α-PbSnF 4(aq2) obtained by reaction of a solid with an aqueous solution, (iii): stressed orthorhombic o-PbSnF 4 obtained by precipitation from aqueous solutions, (iv): non-stressed tetragonal α-PbSnF 4 (ssr) obtained by direct reaction between SnF 2 and PbF 2 at high temperature, and on (v): non-stressed tetragonal β-PbSnF 4 obtained by direct reaction between SnF 2 and PbF 2 at high temperature. In all cases, transformation to microcrystalline cubic γ-PbSnF 4 is observed very rapidly. This is a unique method for stabilizing high temperature γ-PbSnF 4 at ambient temperature, which cannot be done by conventional methods, such as quenching. The phases obtained are totally disordered, microcrystalline, and have the memory of their origin.

Phase Stability and Properties of Superionic PbSnF 4 as a Function of the Method of Preparation

Superionic PbSnF 4 can be prepared using a variety of different methods. It crystallizes in various different unit-cells. All structures are closely related to that of the fluorite type structure, with various degrees of order/disorder and different types of superstructures. Reaction of lead(ll) nitrate and stannous fluoride in water gives highly stressed and highly oriented tetragonal α-PbSnF 4 , however, in HF/H 2 O, it gives orthorhombic O-PbSnF 4 , which is also stressed but less. Reactions of solid α-PbF 2 with an aqueous solution of SnF 2 , in excess SnF 2 , also gives stressed α-PbSnF 4 . Solid state reactions of SnF 2 and PbF 2 give unstressed and much less oriented α-PbSnF 4 at 250°C, whereas tetragonal β-PbSnF 4 is obtained above 270°C and is quenchable to ambient temperature. At 390°C, cubicy-PbSnF 4 is obtained, however, this is not quenchable. Ball milling of o-PbSnF 4 gives stressed α-PbSnF 4 , and ball milling of a- and β-PbSnF 4 gives microcrystalline γ-PbSnF 4 . Annealing the latter gives unstressed and non oriented α-PbSnF 4 . Stirring a slurry of microcrystalline γ-PbSnF 4 in water gives α-PbSnF 4 except the one originating from o-PbSnF 4 , which gives back o-PbSnF 4 . The texture of these phases, and therefore the macroscopic properties related to it, can be drastically modified by modifying the method of preparation.

Kinetics of Phase Transitions in Superionic Pbsnf4 Versus Temperature

PbSnF 4 is the highest performance fluoride ion conductor known to date and is starting to be used for the fabrication of chemical sensors. Although several phase transitions have been reported, with conflicting results from different groups, the exact sequence of phase transitions versus temperature and the kinetics of the phase transformations remain unclear. We have prepared the three phases that can be stabilized at ambient temperature, i.e. o-, α- and β-PbSnF 4 , and studied the stability of each phase versus temperature. It appears that some of the phases transform very slowly to another under prolonged heating at constant temperature. This is very important since the conductivity is not necessarily the same for each phase, and therefore, a phase transition taking place slowly in the PbSnF 4 used for the fabrication of a device, might not be detectable by a short time evaluation but, it could well alter its properties after prolonged use.

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.