A study of the phase diagrams of (NH4)3Ga1−x ScxF6 ammonium cryolites

This paper reports on the results of analyzing p-T and x-T phase diagrams and calorimetric properties of solid solutions in (NH₄)₃Ga_1?x Sc_xF₆ cryolites with scandium concentrations x=0.0, 0.1, 0.35, 0.4, 0.6, 0.8, and 1.0. The thermodynamic parameters of the phase transitions observed in the studied compounds are determined. The generalized phase diagram and successive structural transformations in a series of (NH₄)₃ Me ³⁺F₆ ammonium cryolites are discussed.     

New 1-D magnetic systems in fluorides with the usovite-type structure

New compounds with the usovite-type structure have been prepared which exhibit different types of magnetic behavior depending on the magnetic ions involved. In BaMnAlF₇ and BaMnGaF₇ antiferromagnetic chains are observed. For Ba₂CaCoFe₂F₁₄ and Ba₂CaMnFe₂F₁₄ the magnetic behavior is characteristic of ferrimagnetic chains. For the cobalt compound a magnetic transition with applied field has been observed below 30 K. For Ba₂CaCuFe₂F₁₄ the magnetic properties have been analyzed in terms of isolated Fe³⁺ -Cu²⁺ -Fe³⁺ trimeric units.     

Palladium Compounds with +III Oxidation State

Trivalent oxidation state of palladium has been unambiguously characterized in elpasolites of A₂BPdF₆ formulation (A, B = alkali metal). At room temperature K₂LiPdF₆ is cubic (Fm3m space group, a = 8.154 Å) while all other A₂BPdF₆ compounds possess a tetragonal F4/mmm symmetry with c/a > 1. Phase transitions in Pd elpasolites have been followed using low-and hightemperature X-ray diffraction. Between 200 and 500 K every A₂BPdF₆ compound undergoes a tetragonal ? cubic phase transition. The ESR signals are characteristic of a doublet ground state; they bring up the unequivocal evidence of a te configuration of Pd(+III). This configuration is stabilized versus the te high -spin alternative by an important Jahn-Teller splitting of the ²E_g-state. ESR results confirm that the PdF₆ octahedra are tetragonally elongated in most cases; an additional orthorhombic component is found for Cs₂KPdF₆ and Cs₂RbPdF₆. The occurence of Pd(+III) in a low-spin state is corroborated by magnetic measurements.     

Modification of surface properties of carbon-based and polymeric materials through fluorination routes: From fundamental research to industrial applications

The outstanding characteristics of fluorine gas, e.g., extreme reactivity and oxidizing power, and the utmost electronegativity of F⁻ ion, lead to very strong bonds between fluorine and most of the other elements of the periodical table. Treatments involving F₂, fluorinated gases and rf plasma-enhanced fluorination (PEF) constitute exceptional tools for modifying the surface properties of materials. Many advantages of these techniques can be indeed outlined, when compared to more conventional methods: low-temperature reactions (even at room temperature), chemical modifications limited to surface only without changing the bulk properties, possible non-equilibrium reactions. Depending on the type of starting materials and employed techniques, the improved properties may concern wettability, adhesion, chemical stability, barrier properties, biocompatibility, grafting, mechanical behavior. Several examples of surface fluorination will be given on various types of carbon-based materials, elastomers and polymers.     

Optical properties of the (CrF6)3- complex in A2BMF6:Cr3+ elpasolite crystals: variation with M-F bond distance and hydrostatic pressure

This work investigates the photoluminescence (PL) properties of the Cr (3+)-doped and Cr (3+)-pure fluoroelpasolites along the A 2BMF 6 series and as a function of pressure. In particular, we focus on the variation of the crystal-field spectrum and the associated PL. The results are explained on the basis of the octahedral (CrF 6) (3-) complex subjected either to external pressure or the internal pressure exerted by different crystal hosts. We have established structural correlations between the crystal-field parameter 10Dq and the Cr-F distance, R Cr-F, from which we have determined the local structure around the Cr (3+) impurity, allowing the host material effect on the Cr-F bonds to be studied. As salient features, we show, first, a weak dependence of the first excitation energy, E 1, usually identified as 10Dq, with R Cr-F as E 1 = KR Cr-F (-3.3), and, second, an increase of the Stokes shift upon R Cr-F reduction or with increasing pressure. We associate this unusual behavior with the existence of state mixing among (4)T 2g(F), (2)E g(G), and (2)T 1g(G) states in the first excitation band of Cr (3+). Finally, high-pressure experiments performed on Rb 2KCrF 6 indicate that the excited-state spin crossover, (2)E g(G) <--> (4)T 2g(F), takes place around 7 GPa. The results indicate the suitability of the selected A 2BMF 6:Cr (3+) elpasolites to establish structural correlations between PL and R Cr-F.     

Discharge reaction mechanism in graphite fluoride-lithium batteries

The discharge reaction mechanism of graphite fluoride-lithium batteries utilizing a 1M LiClO₄-propylene carbonate (PC) system was studied by means of X-ray diffraction, ESCA, NMR and transmission electron microscopy, differential thermal analyses and thermal gravimetry. Two types of graphite fluoride flakes, (CF)_n and (C₂F), and (C₂F)_n fibers were used as cathode materials. Based on the full characterization of discharge products of graphite fluoride cathodes, it has been revealed that the discharge reaction proceeds through the formation of an intermediate phase, solvated ternary compound of C-F-Li·PC or C₂-F-Li·PC, and they then decompose to graphite with low crystallinity and LiF. The decomposition process is not involved in the electrochemical reaction, and this discharge reaction infers the open-circuit voltage of batteries and the stable discharge potential with high utility of graphite fluoride cathode.     

One-dimensional magnetism in N2H6FeF5,

A new Fe(III) fluoride N₂H₆FeF₅ has been investigated by magnetic susceptibility measurements and Mössbauer resonance. It has a one-dimensional magnetic behaviour. The intrachain exchange integral ($\text{J}\text{k}\text{= -10.2}\text{K}$) has been determined by fitting the χ^-1 = f(T) curve and the ratio between the inter- and intrachain exchange integrals evaluated with Oguchi's formula. Below T_N = 9 K, N₂H₆FeF₅ shows a long range three-dimensional anti-ferromagnetic ordering.     

Etude d'un nouveau type de fluorure antiferromagnetique a caractere bidimensionnel: NaFeF4

A new type of two-dimensional antiferromagnetic structure has been investigated by magnetic susceptibility measurements, neutron diffraction and Mössbauer resonance. The magnetic cell of NaFeF₄ is doubled along the a axis and the spins lie along the b axis. The κ^-1 vs T curve shows a sharp minimum at about 105 K close to the three-dimensional transition temperature determined by Mössbauer spectrometry (111.5 K). A calculation of z. snfc;J/kz. snfc; has been performed using a high temperature series expansions technique (J/k = -23 K). The variation of the hyperfine field in the range 0.6 ?T/T_N? 1 gave the value of the critical exponant β = 0.25.     

A cell for dielectric measurements in fluorinated atmospheres

A device is described which is suitable for dielectric measurements from 20 to 600°C in various fluorinated atmospheres. The cell has been tested with series of natural and synthetic oxide fluorides and fluorides derived from (NH₄)₃FeF₆, chiolite and weberite structural types.