Alain Demourgues

Dual Lithium Insertion and Conversion Mechanisms in a Titanium-Based Mixed-Anion Nanocomposite

The electrochemical reaction of lithium with a vacancy-containing titanium hydroxyfluoride was studied. On the basis of pair distribution function analysis, NMR, and X-ray photoelectron spectroscopy, we propose that the material undergoes partitioning upon initial discharge to form a nanostructured composite containing crystalline Li(x)TiO(2), surrounded by a Ti(0) and LiF layer. The Ti(0) is reoxidized upon reversible charging to an amorphous TiF(3) phase via a conversion reaction. The crystalline Li(x)TiO(2) is involved in an insertion reaction. The resulting composite electrode, Ti(0)-LiF/Li(x)TiO(2) ⇔ TiF(3)/ Li(y)TiO(2), allows reaction of more than one Li per Ti, providing a route to higher capacities while improving the energy efficiency compared to pure conversion chemistries.

Electron microscopy study of electrochemically prepared La2NiO4+δ (0.17≤δ≤0.26)

Abstract A new material with composition of La2NiO4.26 exhibiting a large interstitial oxygen amount with regard to the basic K2NiF4- type structure has been prepared using an electrochemical process. This material has been examined by TEM and various super-structures either commensurate or incommensurate have been found. On the basis of the obtained ED patterns a structural model is proposed to account for the commensurate superstructures; they correspond to different oxygen stoichiometry, gd=0.17 and δ=0.25. In this structural model the interstitial oxygen atoms are located along the [111] directions of the K2NiF4-type unit cell. Two triclinic unit cells have been proposed for these two phases. Moreover an idealized structure involving an ordering of the nickel sites is depicted for La2NiO4.25. The ED patterns of the incommensurate structures have been described using a q ∗ modulation vector in addition to the three fundamental vectors q t ∗ , b t ∗ and c t ∗ of the I4/mmm structure. It was then pointed out that the modulation vector of the incommensurate structures lies between the two q 1 ∗ and q 2 ∗ modulation vectors that can be defined for the commensurate phases (δ=0.17 and δ=0.25 respectively) in the reciprocal plane corresponding to the [311] zone axis. We assume that a direct relation exists between q ∗ and δ.

Coupling sol-gel synthesis and microwave-assisted techniques: a new route from amorphous to crystalline high-surface-area aluminium fluoride.

A non-aqueous sol-gel Al-based fluoride has been subjected to the microwave solvothermal process. The final material depends on the temperature heat treatment used. Three types of material have been prepared: 1) for low temperature heat treatment (90 degrees C) X-ray amorphous alkoxy fluoride was obtained; 2) for the highest temperature used (200 degrees C) the metastable form beta-AlF3 was obtained with a very large surface area of 125 m2 g(-1). The mechanism of the amorphous=crystalline transformation has been rationalised by the occurrence of a decomposition reaction of the gel fluoride induced by the microwave irradiation. 3) Finally, at intermediate temperature (180 degrees C) a multi-component material mixture exhibiting a huge surface area of 525 m2 g(-1) has been obtained and further investigated after mild post-treatment fluorination using F2 gas. The resulting aluminium-based fluoride still possesses a high-surface-area of 330 m2 g(-1). HRTEM revealed that the solid is built from large particles (50 nm) identified as alpha-AlF3, and small ones (10 nm), relative to an unidentified phase. This new high-surface-area material exhibits strong Lewis acidity as revealed by pyridine adsorption and catalytic tests. By comparison with other materials, it has been shown that whatever the composition/structure of the Al-based fluoride materials, the number of strong Lewis acid sites is related to the surface area, highlighting the role of surface reconstruction occurring on a nanoscopic scale on the formation of the strongest Lewis acid sites.

Investigation of Ga Substitution in ZnO Powder and Opto-Electronic Properties

Two sets of Ga-doped ZnO powders were synthesized via solid-state and Pechini routes with a substitution rate varying from 0 to 4 mol %. The gallium solubility limit is strongly dependent on the synthesis history. Indeed, a low temperature annealing allows incorporating about 1.5 mol % (X-ray diffraction (XRD), inductive coupled plasma spectroscopy (ICP), optical properties) whereas under 0.1% of dopant is introduced after thermal treatment at high temperature: 1500 degrees C (from XRD and pellets conductivity). The incorporation of gallium leads to an anisotropic distortion of the zincite crystal lattice (a and c parameters increase and decrease, respectively, versus the Ga content leading to a decrease of the c/a ratio) which can be explained from the valence bond model. XRD analysis, chemical titration by ICP, and conductivity measurements (on pellets obtained at high temperature) allow determining accurately the maximum Ga content in the zincite. The optical properties (IR absorption efficiency) linked to electron carriers are directly correlated to the gallium rate introduced in ZnO oxide; nevertheless, the non linear correlation between these two parameters tends to show that the concentration of charge carriers in the system is not equal to the amount of Ga(3+) atoms inserted per ZnO volume unit. A saturation regime is observed and was here explained once again on the basis of the valence band model by the increase of inhibiting p type defects with the increase of (n-type donors) Ga(3+) concentration.

Synthesis and characterization of Al3+, Cr3+, Fe3+ and Ga3+ hydroxyfluorides: correlations between structural features, thermal stability and acidic properties

The synthesis of aluminium, chromium, iron and gallium hydroxyfluorides in their hexagonal tungsten bronze (HTB) β-form has been undertaken by sol precipitation followed by thermal treatments. These solids, which could be used in heterogeneous catalysis, have been firstly characterized by chemical analysis, X-ray diffraction and FTIR spectroscopy in order to determine their composition and structural features. In the HTB hydroxyfluorides series, there is competition between the formation of M–F and M–OH bonds, which depends on the type of cation, the nature of precursor and the route of synthesis. FTIR spectroscopy study has shown the presence of both free- and linked- OH− groups. The nature of cations, the decomposition kinetics of the M(H2O)63+ aquo-complex and the size of tunnels in the HTB framework account for the thermal stability of these compounds. For instance, a comparison between Al3+ and Fe3+ hydroxyfluorides shows that the Al–(F,OH) bond is more stable than the Fe–(F,OH) bond, with a difference of more than 200 K in their thermal stabilities. The substitution of Fe3+ by Cr3+, which gives rise to an increase in the content of H2O/OH groups preferentially around Cr3+, allows the improvement of the M–F bonding stability. The template effect of water has also been pointed out. The acidic character of these solids has been evaluated by FTIR analysis using probe molecule adsorption and leads to the conclusion that the strongest Lewis acidity is found in Al3+ and Ga3+ homologous compounds with respect to that of iron hydroxyfluoride. These characteristics can be directly related to the strength of the M–(F,OH) chemical bond and the thermal stability of these solids. The use of the ratio χ/r2 between the electronegativity χ and the ionic radius r, which can be ascribed to an electrical field gradient around the cation, has been proposed. This parameter allows a more accurate approach of both the acidic strength and the thermal stability in the hydroxyfluoride series and accounts for the experimentally observed sequence.

Rare earth fluorosulfides LnSF and Ln2AF4S2 as new colour pigments

Abstract Rare earth fluorosulfides LnSF and Ln2AF4S2 (A=Ca, Sr) have been prepared and characterized by X-ray diffraction. The structures can be described as the succession of various sheets of rare earth or alkaline earth, fluorine and sulfur. Thus in these compounds, rare earth is at the center of a distorted square antiprism with four F atoms on one base, four S in the other and a fifth Ln–S bond parallel to the c axis. These compounds exhibit interesting colour from yellow (Sm, Gd) to red (Ce). The chromatic properties have been correlated to structural features and mechanisms at the origin of the colour have been proposed. The variation of energy of absorption edge as a function of rare earth has been explain in terms of ionization energies.

Structural features of new rare earth-based mixed anions (O, S, F) compounds: relationships between optical absorption and rare earth environment

In the structure of α-LnSF and Ln2AF4S2 (A=Ca, Sr) compounds which can be considered as formed of layer stacking, the rare earth and fluorine atoms form [Ln2F2]4+ or [Ln2AF4]4+ fluorite-type blocks which alternate with double [S2]4− layers along the c-axis. Then, it appears possible to modify the size of the fluorite-type block without any modification of its charge. In the case of rare earth oxyfluorosulfides, two new classes of compounds Ln3OF3S2 (Ln=La, Ce) and La2O1.5FS were identified. The structures of these compounds are related to α-LnSF and Ln2O2S networks. In these frameworks, the charge of the blocks containing the rare earth ions can be 2+ or 4+. These blocks alternate with single or double layers of sulfur atoms. The local geometry (number of neighbors, bond distances and angles) around the rare earth varies as a function of the number of the sulfur sheets. Absorption properties in UV-visible range are correlated with the structural features.

Local organization of Fe3+ into nano-CeO2 with controlled morphologies and its impact on reducibility properties

Ce and Fe are the main metal additives which have been tested by many laboratories for their catalytic activity at low temperature for soot oxidation in diesel engines.The key role and impact of Fe on the reducibility properties of ceria have been investigated on the basis of composition and structural features considering various synthesis routes. Two different procedures were used to prepare iron-substituted cerium dioxide Ce1−xFexO2−x/2: a classical co-precipitation route followed by annealing at T = 600 °C and an unusual microwave-assisted hydrothermal synthesis at T = 200 °C. The highest surface areas around 100 m2 g−1, have been obtained for oxides containing the largest Fe amount and prepared by the microwave-assisted route. Solid solutions were obtained up to x = 0.15 and 0.20 through microwave-assisted and co-precipitation routes, respectively. The highest decrease of the lattice parameter is observed for compounds prepared by the microwave-assisted route. The TEM analysis reveals a nano-cubic shape (with mainly {001} planes) for compounds prepared by the microwave-assisted route and containing low Fe amount. For the other compounds prepared also by the co-precipitation process, a nano-polyhedron shape corresponding to a thermodynamically stable morphology is observed. EPR (T = 5 K, very low Fe rates), Mossbauer (room temperature, x = 0.10) and XANES–EXAFS (RT and T = 20 K) spectroscopies studies showed that Fe ions adopt the trivalent state and are located in isolated distorted (orthorhombic and axial distortions) octahedra or form Fe clusters. The local organization of Fe3+ plays a key role in the oxygen vacancy distribution and consequently the reducibility properties of the Ce1−xFexO2−x/2 solid solution. For the compounds prepared by the microwave-assisted route, a large proportion of Fe3+ is located in isolated distorted octahedral sites that contribute to affect a high number of Ce4+ nearest neighbors with a high mobility of oxygen vacancies. In this case, the oxygen vacancies are well distributed around Ce4+ and Fe3+. In the case of compounds obtained by the co-precipitation route, Fe clusters abundance is higher and consequently oxygen vacancies are mainly located around Fe clusters. Such a distribution should explain the lower solubility limit and the larger variation of the cell parameter versus x Fe content in the series prepared by the microwave-assisted route. It is shown that these last compounds exhibit better reducibility properties with a doubled reduction rate (comparison between x(Fe) = 0.05 and pure CeO2) equal to 60% at T = 550 °C due to the iron local organization, a nano-cubic morphology and a higher surface area.

Preparation and structural properties of new series of mixed-anion compounds: rare earth fluorosulfides

Abstract Rare earth fluorosulfides with the following compositions: LnSF and Ln 2 AF 4 S 2 have been prepared by solid state routes. These structures can be described as a stacking of fluoride-based slabs ([Ln 2 F 2 ] 4+ in LnSF and [Ln 2 AF 4 ] 4+ in Ln 2 AF 4 S 2 ) and double layers [S 2 ] 4− . The fluoride blocks are related to fluorite-type structure. The relative compactness of the fluoride blocks leads to a relaxation of the LnS bond lengths.

Understanding the Relationships between Structural Features and Optical/Magnetic Properties When Designing Fe1–xMgxMoO4 as Piezochromic Compounds

Fe1-xMgxMoO4 compounds with x = 0, 0.25, 0.5, 0.75, and 1.0 were obtained after annealing under inert gas at T = 700 °C. All of the compounds exhibit a pressure-induced and/or temperature-induced phase transition between the two polymorphs adopted by AMoO4 compounds (A = Mn, Fe, Co, and Ni). For the FeMoO4 compound, for both the α and the β allotropic forms, the structural features have been correlated to the magnetic properties, the Mössbauer signals, and the optical absorption properties to gain a better understanding of the phenomena at the origin of the piezo(thermo)chromic behavior. The different contributions of the Mössbauer signals were attributed to the different Fe(2+) ions or Fe(3+) ions from the structural data (Wyckoff positions, bond distances and angles) and were quantified. Furthermore, the low Fe(3+) concentration (9 and 4 mol %, respectively, in the α and the β allotropic forms) was also quantified based on the magnetic susceptibility measurements. The net increase in the Fe(3+) quantity in the α-form in comparison to the β-form, which is associated with the occurrence of Fe-Mo charge transfer, is at the origin of the important divergence of coloration of the two forms. To design new piezo(thermo)chromic oxides and to control the pressure (temperature) of this first-order phase transition, FeMoO4-MgMoO4 solid solutions were synthesized. The optical contrast between the two allotropic forms was increased due to magnesium incorporation, and the phase transition (β → α) pressure increased steadily with the Mg content. A new generation of nontoxic and chemically stable piezochromic compounds that are sensible to various pressures was proposed.