Dielectric and thermocurrent studies of some fluoride and oxide fluoride compounds with a (NH4)3FeF6−related structure

Dielectric and thermocurrent measurements have been carried out on (NH₄)₃AlF₆,(NH₄)₃FeF₆ and Rb₃FeF₆ ceramic samples. A maximum of permittivity is observed close to the transition temperature (T_tr(NH₄)₃AlF₆ = 217 K; T_tr(NH₄)₃FeF₆ = 264 K; T_tr(Rb₃FeF₆) = 629 K). This non cubic-cubic transition is of the ferroelastic type. The high Curie temperature of the rubidium iron fluoride causes large dielectric losses at the transition and thermocurrent measurements cannot be performed. On the contrary, in the low-temperature phase of the ammonium compounds a polarization current of about 10-9A/cm² is obtained and can be reversed when the sign of the polarization field is changed. This property could correspond to a ferroelectric behavior. However no pyroelectric current is detected when the temperature decreases from T_tr. Another hypothesis, based on the field-induced polarization, has been also considered.In connection with the ferroelectric properties of Rb₃MoO₃F₃, the system Rb₃MoO₃F₃Rb₃FeF₆ has been performed. Extended domains of solid solution with Rb₃(Mo_1?xFe_x)O_3?3xF_3+3x formula and having a (NH₄)₃FeF₆-related structure have been pointed out. The replacement of oxygen by fluorine leads to a strong decrease of the ferroelectric Curie temperature: T_C(x=0) = 538 K, T_C(x=0.4) < 80 K. This result is in good agreement with the lack of ferroelectricity in A₃MF₆ fluorides. In any case, the low-temperature phase is ferroelastic and is characterized by a remanent polarization.     

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.     

Study of the fluorination of carbon anode in molten KF-2HF by XPS and NMR investigations

During the electrolysis of molten KH-2HF, a solid CF film is formed on carbon anodes. The influence of the potential applied to the anode on the composition of the CF surface film was studied by XPS and NMR. Results obtained with carbon electrochemically passivated in molten KF-2HF have been compared with those obtained with carbon simply immersed in molten KF-2HF without any polarization. Whatever the potential, and even in the case of carbon sample immersed in molten KF-2HF without any polarization, the presence of covalent and semi-ionic C–F bonds has been pointed out both by XPS and NMR. For polarized samples, the higher the potential applied to the anode in KF-2HF, the higher the fluorination level. From investigations carried out with carbon activated at 40 V in molten KF-2HF, it was concluded that the electropolishing of the surface induced by this treatment allows enhancing drastically the fluorine evolution reaction.     

Electrochemical properties and structures of surface-fluorinated graphite for the lithium ion secondary battery

Surface modification of graphite powder has been performed by elemental fluorine and radiofrequency (rf) plasma fluorination. Both methods give rise to an enlargement of the surface areas of graphite samples and a change of the pore volume distribution. The capacities of surface-fluorinated graphite samples are higher than those of original samples and even more than the theoretical capacity of graphite, 372 mAh g⁻¹, without any reduction of the first colombic efficiencies. The increments of the capacities are ∼5, 10, and 15% for graphite samples with average particle diameters of 7, 25 and 40 μm, respectively.     

Perovskite-like fluorides and oxyfluorides: Phase transitions and caloric effects

An analysis of the effect that chemical and hydrostatic pressures have on the thermodynamic properties of perovskite-like fluorine-oxygen compounds A ₂ A′MeO _x F_{6 − x} has revealed that materials under-going order-disorder transitions and having significant external-pressure compliance have the highest caloric efficiency. Some of the fluorides and oxyfluorides under study can be considered promising solid coolants.