Copper Weberites: Crystal Structure and Magnetic Investigation of Na2CuGaF7 and Na2CuInF7

The crystal structures of two new copper weberites Na₂CuGaF₇ and Na₂CuInF₇ have been determined. Na₂CuGaF₇ has the monoclinic space group C2/c: a = 1232.5(5) pm, b = 731.8(1) pm, c = 1278.0(5) pm, β = 109.29(2)° and Z = 8. Na₂CuInF₇ crystallizes in the orthorhombic space group Pmnb: a = 731.8(1) pm, b = 1060.2(2) pm, c = 771.2(1) pm and Z = 4. The structures have been refined from 1175 reflections to R = 0.043 (wR = 0.035) for Na₂CuGaF₇, and from 1917 reflections to R = 0.034 (wR = 0.025) for Na₂CuInF₇. The structures consist of [CuF₅]_n^3n? chains which are parallel the a-axis in Na₂CuInF₇ and oriented in two alternating directions in Na₂CuGaF₇. Sodium atoms exhibit either seven-fold or eight-fold coordination. Although strong antiferromagnetic interactions are observed inside the chains, there is no evidence for three-dimensional ordering.     

Analysis of the refractive indices of TiO2, TiOF2, and TiF4: concept of optical channel as a guide to understand and design optical materials

In an effort to gain insight into how optical properties of insulating materials are affected by a change in chemical composition, we investigated the dielectric functions of titanium dioxide, TiO(2), and its fluorine-substituted phases, TiOF(2) and TiF(4), by electron energy loss spectroscopy measurements and density functional theory electronic band structure calculations. The refractive indices of these compounds are found to be inversely proportional to their cell volumes per formula unit. This observation was explained by employing the concept of optical channels. Our study indicates that the light-scattering properties of insulating compounds can be controlled by modifying their cell volumes.     

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.     

Fluorinated transition alumina with Al2−x/3O3−xFx compositions: Thermal, chemical, structural and morphological investigations

Fluorinated transition aluminas (Al_2−x/3O_3−xF_x) with hexagonal platelet shape were synthesized via decomposition of α-AlF₃ under air; they are thermally stable up to 1000 °C and exhibit at 1150 °C a weight loss with volume reduction caused by fluorine departure corresponding to a phase transition toward corundum alumina. The different characterizations performed in this study are structural (XRD), chemical (TGA-MS and microprobe analysis) and morphological (SEM, TEM and dilatometry). The evidence provided from this study is consistent with the formation of an Al-O-F phase as an intermediate compound in the pyrohydrolysis of an aluminium trifluoride phase to α-Al₂O₃.     

Access to Heteroleptic Fluorido-Cyanido Complexes with a Large Magnetic Anisotropy by Fluoride Abstraction

Silicon-mediated fluoride abstraction is demonstrated as a means of generating the first fluorido-cyanido transition metal complexes. This new synthetic approach is exemplified by the synthesis and characterization of the heteroleptic complexes, trans-[M^IVF₄(CN)₂]²⁻ (M=Re, Os), obtained from their homoleptic [M^IVF₆]²⁻ parents. As shown by combined high-field electron paramagnetic resonance spectroscopy and magnetization measurements, the partial substitution of fluoride by cyanide ligands leads to a marked increase in the magnetic anisotropy of trans-[ReF₄(CN)₂]²⁻ as compared to [ReF₆]²⁻, reflecting the severe departure from an ideal octahedral (O_h point group) ligand field. This methodology paves the way toward the realization of new heteroleptic transition metal complexes that may be used as highly anisotropic building-blocks for the design of high-performance molecule-based magnetic materials.     

Structural phase transitions in elpasolite- and cryolite-type fluorides

The occurrence of phase transitions in elpasolite- and cryolite-type fluorides depends mainly on two factors: the electronic structure of the involved transition element and/or the value of the tolerance factor (t) close to the lower limit of the stability range. The phase transitions have been followed using low- and hightemperature X-ray diffraction. EPR signals of Ni¹¹¹ and Pd¹¹¹ compounds are characteristic of a doublet ground state. The trivalent species are located in axially elongated octahedra and a static ← dynamic Jahn-Teller transition can be observed in some of these phases.     

Ferroelastic phase transition in elpasolite Tl2KInF6

A new elpasolite Tl₂KInF₆ has been synthesized. This compound undergoes at 228 K a ferroelastic phase transition from a room-temperature Fm3m variety to a monoclinic variety. X-ray, optical, calorimetric and DTA studies under hydrostatic pressure have been performed and compared with other elpasolite-type fluorides.     

Structural architecture and physical properties of some inorganic fluoride series: a review

Many structural networks of transition metal fluorides and oxide fluorides may be derived from simple units of octahedral MF₆ units, connected by common corners, generally in a 3D way: e.g., perovskite, ReO₃, elpasolite, HTB, TTB, pyrochlore, etc. The accurate knowledge of these structures, together with the stabilities of these phases vs. temperature, allows deeper interpretation and fitting of many of their physical properties. Among the numerous fluoro-compounds which derive from these structural arrangements, several fluorinated and oxyfluorinated series have been chosen to illustrate the great range of outstanding physical properties, including ferromagnetism, piezoconductivity, ferroelasticity, high T_C superconductivity, surface functionalization, that can be better understood with the help of structural considerations. The concept of nanometric particles in materials science has also drastically changed the investigations in solid state fluorine chemistry, not only for finding original synthetic routes, but also new characterization tools. As an example, in nanostructured M(OH,F)₃ metal hydroxyfluorides that can be used as acidic heterogeneous catalysts, the morphological nature and specific area of the materials can be tuned by changing the experimental parameters of synthesis. In particular, the number and strength of Brønsted/Lewis acidic sites and the thermal stability can be correlated to the structural arrangements.