Georges Dénès

SnK, PbL3, and BaL3 EXAFS, X-ray diffraction and 119Sn Mössbauer spectroscopic studies of ordered MSnF4 (M = Pb and Ba) fluoride ionic conductors with the α-PbSnF4 structure

Abstract The high performance fluoride ion conductors α-PbSnF 4 and BaSnF 4 have been studied using X-ray diffraction, 119 Sn Mossbauer spectroscopy, and EXAFS. X-ray diffraction shows the unit-cell is a tetragonally distorted fluorite-type, with ordering of the metals which results in a superstructure along the c axis. The SnK, Pb L 3 , and Ba L 3 X-ray absorption spectra of α-PbSnF 4 and BaSnF 4 as well as model compounds (SnO, β-PbF 2 , and BaF 2 ) have been recorded at 300 and 77 K. Analysis of the extended fine structure (EXAFS) of α-PbSnF 4 and BaSnF 4 indicates they have a similar local structure around the corresponding metal atoms with average nearest neighbor distances of R SnF = 2.08(3) A, R PbF = 2.50(3) A and R BaF = 2.67(2) A. The good agreement between the PbF and BaF distances derived from EXAFS with diffraction results and ion pair estimates indicates that the Pb and Ba ions determine the close-packing arrangement of the crystal structure. EXAFS shows that the local structure is much better defined around Pb and Ba than around Sn. This and the weak temperature dependence of the SnK EXAFS indicates a lower rigidity of next-neighbor fluorine shells around Sn than around Pb or Ba. In the α-PbSnF 4 structural type, the metal M (Pb or Ba) is in an eightfold coordination site similar to the cubic coordination in the fluorite-type, whereas Sn is in a unique SnF 5 E pseudooctahedral coordination, with the lone pair E being stereoactive and making the materials strongly anisotropic. This is confirmed by the large quadrupole splitting observed in Mossbauer spectroscopy, Δ = 1.52(2) mm/s for α-PbSnF 4 and 1.52(1) mm/s for BaSnF 4 . The mobile fluoride ions are probably disordered and widely spread over conduction paths, and they remain disordered even at low temperature when their long range motion is frozen. The structural results are discussed in relationship to proposed mechanisms for ionic conduction in these materials.

The “bent copper tube”: A new inexpensive and convenient reactor for fluorides of metals in suboxidation states

Abstract Bent copper tubes have been successfully used for solid state reactions at high temperature, under nonoxidizing atmosphere. They provide an inexpensive and convenient alternative to gold and platinum for reactions between fluorides, and do not reduce some elements such as tin(II), as nickel does. Solid state NMR studies of fast fluoride ion conductors Pb1−xSnxF2 prepared in copper tubes show that contamination by paramagnetic Cu2+ is minimal. 119Sn Mossbauer spectroscopy is used as a sensitive probe for the formation of SnO2, and indicates that no oxygen leaked in the tubes. Like any reaction carried out in a sealed container, those resulting in a gas being evolved and/or highly exothermic reactions can lead to explosions and thus must be avoided. An attempt to synthesize TiI4 in a sealed copper tube is described, as an example of a dangerous uncontrollable reaction by this method.

Structural Analysis for Gold Mineralization Using Remote Sensing and Geochemical Techniques in a GIS Environment: Island of Lesvos, Hellas

Exploration for epithermal Au has been active lately in the Aegean Sea of the eastern Mediterranean Basin, both in the islands of the Quaternary arc and in those of the back-arc region. The purpose of this study was the structural mapping and analysis for a preliminary investigation of possible epithermal gold mineralization, using remotely sensed data and techniques, structural and field data, and geochemical information, for a specific area on the Island of Lesvos. Therefore, Landsat-TM and SPOT-Pan satellite images and the Digital Elevation Model (DEM) of the study area were processed digitally using spatial filtering techniques for the enhancement and recognition of the geologically significant lineaments, as well as algebraic operations with band ratios and Principal Component Analysis (PCA), for the identification of alteration zones. Statistical rose diagrams and a SCHMIDT projection Stereo Net were generated from the lineament maps and the collected field data (dip and strike measurements of faults, joints, and veins), respectively. The derived lineament map and the band ratio images were manipulated in a GIS environment, in order to study the relation of the tectonic pattern to both the alteration zoning and the geomorphology of the volcanic field of the study area. Target areas of high interest for possible mineralization also were specified using geochemical techniques, such as X-Ray Diffraction (XRD) analysis, trace-element, and fluid-inclusion analysis. Finally, preliminary conclusions were derived about possible mineralization, the type (high or low sulfidation), and the extent of mineralization, by combining the structural information with geochemical information.

Structure and ionic transport of PbSnF4 superionic conductor

Abstract Although PbSnF 4 , the highest performance fluoride ion conductor, was first prepared nearly 30 years ago, structural information about it have been scarce and somewhat inadequate. We have prepared samples of tetragonal PbSnF 4 and orthorhombic PbSnF 4 under well defined conditions. Their characterization and the study of their transport properties are presented and interpreted in the light of the structural information available and the known phase transitions. The samples were characterized by X-ray diffraction, differential scanning calorimetry, differential thermal analysis, infrared spectroscopy and impedance methods. The transport properties of tetragonal and orthorhombic PbSnF 4 are very similar. The conductivity-temperature curves upon heating from 25–200 °C are characterized by the gradual slope change from a region of high activation energy (~ 55 and ~ 35 kcal/mol respectively) to that of a lower activation energy (~ 8 kcal/mol) at around 80 °C. Upon cooling, the electrical conductivity is observed to be linearly dependent on temperature with an activation energy of 30 kcal/mol. There is no symmetry change at the temperature where the conductivity curves show a gradual slope change.

A tin-119 Mössbauer study of the phase transitions in SnF2

Tin-119 Mossbauer spectra have been recorded for the three known phases of SnF2. In the monoclinic α phase the two structurally different tin environments have Mossbauer parameters which are indistinguishable from one another (δ= 3.40 mm s–1, Δ= 1.52 mm s–1): a small Goldanskii–Karyagin effect is observed. A discontinuity in the temperature dependences of the Mossbauer parameters occurs at the α→γ phase transition, ca. 425 K. The high-temperature γ phase has δ= 3.15 mm s–1 and Δ= 1.95 mm s–1, and exhibits a very large asymmetry in the intensities of the two lines of the quadrupole doublet. There is no such discontinuity at the γ⇌β second-order phase transition: progressive changes occur to give parameters for the β phase of δ= 3.11 mm s–1 and Δ= 2.10 mm s–1, and a slightly smaller line asymmetry.

Passivation of SnF2 by a coating of SnO2 formed on heating in air

X-ray powder diffraction and tin-119 Mössbauer spectroscopy were used to study samples of SnF2 heated in open teflon containers under air. The reactivity was found to be low at 100 °C and to increase dramatically with temperature. Passivation by a layer of refractory SnO2 that coats the surface of the particles is very effective at low temperature. The reactivity increases at the α→γ transition and close to the melting point, since both result in bond breaking and therefore offers opportunities for chemical attack.

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.

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.

Methods for preparing thermally stable and active acid catalysts by incorporation of lanthanum into Y, desilicated Y and X zeolites

The incorporation of La3+ ions by ion exchange into the Na forms of X, Y and desilicated Y zeolites does not alter the thermal stability of these materials, while it dramatically increases their activity in the acid-catalysed hydrocracking/ hydroisomerization of n-octane. On the other hand, lanthanum ions, when ion-exchanged into the ammonium forms of these zeolites, stabilize the acid forms currently obtained by calcination of the corresponding ammonium zeolites at high temperatures. Interestingly, high catalytic performances are obtained when the amount of La3+ ions incorporated into the Na forms of these zeolites is equal to or higher than 70% of the cationic capacity of these materials, which now become as active and selective as the LaH or ultrastabilized HY zeolites.

Mesoporous aluminosilicates: preparation from Ca-A zeolite by treatment with ammonium fluorosilicate

Ammonium hexafluorosilicate (AFS) in aqueous solution under mild conditions can modify drastically the textural properties of Ca-A zeolite. Open slit-shaped mesopores with a pore-size distribution showing a maximum at 14–20 nm are formed. The average pore diameter depends on the rate of addition of the AFS solution to the leaching medium. If such a treatment is combined with acid leaching or hydrothermal treatment, the mesoporous materials obtained exhibit similar pore-size distribution. However, their adsorption capacity for nitrogen is significantly increased.