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International audience; A crystallographic approach incorporating multinuclear high field solid state NMR (SSNMR), X-ray structure determinations, TEM observation, and density functional theory (DFT) was used to characterize two polymorphs of rubid-ium cryolite, Rb3AlF6. The room temperature phase was found to be ordered and crystallizes in the Fddd (n°70) space group with a = 37.26491(1) Å, b = 12.45405(4) Å, c = 17.68341(6) Å. Comparison of NMR measurements and computational results revealed the dynamic rotations of the AlF6 octahedra. Using in-situ variable temperature MAS NMR measurements, the chemical exchange between rubidium sites was observed. The β-phase, i.e. high temperature polymorph, adopts the ideal cubic double-perovskite structure, space group 3 ̅ , with a = 8.9930(2) Å at 600 °C. Additionally, a series of polymorphs of K3AlF6 has been further characterized by high field high temperature SSNMR and DFT computation.

Precise research on the RbF-Al2O3 system was carried out by means of combining X-ray powder diffraction, high-field solid-state NMR spectroscopy, and thermal analysis methods. α-Rb3AlF6, RbAlO2, Rb2Al22O34, and new phase, Rb2Al2O3F2, were identified in the system. The structure of this new rubidium oxofluoroaluminate was determined. It is built up from single layers of oxygen-connected AlO3F tetrahedra, those layers beeing separated by fluorine atoms. This type of structure exhibits a decent ionic conductivity at ambient temperature, 1.74 × 10-6 S cm-1. The similar structural arrangement of O3Al-O-AlO3 and FO2Al-O-AlO2F tetrahedra of the conduction planes in Rb2Al22O34 and Rb2Al2O3F2 were confirmed by 27Al NMR measurements. A thermal analysis of the RbF-Al2O3 system revealed that it can be defined as a pseudobinary subsystem of the more general quaternary RbF-AlF3-Al2O3-Rb2O phase diagram. From a phase analysis of individual phase fields, the mutual metastable behavior of all founded phases can be considered. It was observed that fluoro- and oxoaluminates exist together. Rb2Al2O3F2 is more stable under high temperature. Rubidium fluoro- and oxoaluminates are metastable precursors of the thermodynamically more stable structure of rubidium oxofluoroaluminate.

We examined the kinetic and transport processes involved in Mg production from MgO via electrolysis at ca 1250 K with in a eutectic mixture of MgF 2 –CaF 2 , using a Mo cathode, and carbon anode. Exchange current densities, transfer coefficients, and diffusion coefficients of the electroactive species were established using a combination of cyclic and linear sweep voltammetry, chronoamperometry and electrochemical impedance spectroscopy. The cathode kinetics are described by a concentration dependent Butler–Volmer equation. The exchange current density and cathodic transfer coefficient are 11±4 A cm −2 and 0.5±0.12 respectively. The kinetics of the anode are described by two Tafel equations: at an overvoltage below 0.4 V, the exchange current density is 0.81±0.2 mA cm −2 with an anodic transfer coefficient of 0.5±0.1; above 0.4 V overvoltage the values are 0.14±0.05 mA cm −2 and 0.7±0.2 respectively. The diffusion coefficients of the electroactive species are D (Mg 2+ )=5.2±0.6E−5 cm 2 s −1 and D ( Mg 2 OF 4 2 - )=7.2±0.2E−6 cm 2 s −1 . The ionic conductivity of the electrolyte is ca 2.6 S cm −1 . A 3D finite element model of a simple cell geometry incorporating these kinetic and transport parameters suggest that up to 27% of the energy required to drive the electrolysis reaction can be supplied thermally for a current density of 0.5 A cm −2 , enabling a reduction in operating cost if the thermal energy is substituted for valuable electric work.

Precise investigation of part of the phase diagram of KF–Al2O3 system was performed in an experiment combining different techniques. Solidified mixtures of KF–Al2O3 were studied by X-ray powder diffraction and high-field solid-state NMR spectroscopy over a wide range of compositions. To help with the interpretation of the NMR spectra of the solidified samples found as complex admixtures, we synthesized the following pure compounds: KAlO2, K2Al22O34, α-K3AlF6, KAlF4, and K2Al2O3F2. These compounds were then characterized using various solid-state NMR techniques, including MQ-MAS and D-HMQC. NMR parameters of the pure compounds were finally determined using first-principles density functional theory calculations. The phase diagram of KF–Al2O3 with the alumina content up to 30 mol % was determined by means of thermal analysis. Thermal analysis was also used for the description of the thermal stability of one synthesized compound, K2Al2O3F2.

The X-ray diffraction (XRD) phase analysis of different solidified uranium-based fluoride systems ((LiF–NaF)eut–UF4; (KF–LiF–NaF)eut–UF4; (LiF–NaF)eut–UF4–ZrF4 and (KF–LiF–NaF)eut–UF4–ZrF4) were examined in order to provide the basis for pyro-electrochemical extraction of uranium in molten fluorides. Several uranium-based species (Na2UF6, Na3UF7, K2UF6, K3UF7, UO2, K3UO2F5) were identified in the solidified melts. The role of oxygen in argon atmosphere was found to be critical in the formation of uranium species during the melting and solidification. In order to reduce the accumulated level of free oxygen traces in our experiments, zirconium (in the form of ZrF4) was used inside the melt as an oxygen buffer. It was found that ZrF4 can really stabilize the uranium species by complexation and protects them against the oxygenation. The results of this work highlight the importance of oxygen removal for obtaining pure deposit in the electrorefinning of uranium.

Electrochemistry of gadolinium and uranium in LiF–CaF2 (79–21 mol%) melt was studied using reactive Ni electrode and alloying reactions were observed. Deposits of gadolinium and uranium in the form of Gd–Ni and U–Ni intermetallic alloys were obtained after electrolysis by modulated current. Electrolysis of the same parameters was used also in the complex system of LiF–CaF2–UF4–GdF3 to demonstrate feasibility of selective deposition of uranium and therefore its separation from the system. Compact deposit of U–Ni alloy containing only traces of gadolinium was obtained.

The electrochemical behaviour of samarium was investigated in LiF-BeF2 system on an inert (Mo) electrode by cyclic voltammetry and chronopotentiometry at 804, 833, 847 and 872 K. Redox process Sm3++e−→Sm2+ was recognized and analysed. Cyclic voltammetry data suggested that at potential sweep rates lower than 0.25 V/s, the reduction was limited by the diffusion of Sm3+ ions. It was not possible to observe reduction process of Sm2++2e−→Sm0 due to insufficient electrochemical stability of LiF-BeF2. Diffusion coefficients of Sm3+ ions in LiF-BeF2 were calculated from voltammetric and chronopotentiometric data in the temperature range 804–872 K. Diffusion coefficient values obeyed Arrhenius law. Activation energy was calculated to be 102.5 kJ/mol.

Sulphur doped nanoparticles of titanium dioxide were prepared by the reaction of titanium butoxide and thiourea in methanol. X-ray powder diffraction and High resolution transmission electron microscopy were used to investigate the crystallinity of the prepared nanoparticles. Based on the X-ray photoelectron spectroscopy measurements sulphur is present in the form of S 6+ which substitutes Ti 4+ in the titania lattice. The band gap energies of synthesised photocatalyst were calculated from the fitting of experimental dependences by double Boltzmann symmetrical functions. The photocatalytic activity of the samples was determinated by the measurement of the 4-chlorophenol (4-CP) degradation in the aqueous suspensions in the ultraviolet and visible region. Electron paramagnetic resonance spectroscopy confirmed the generation of OH radicals from S doped TiO 2 under vis and UV irradiation.

d ) Institute of Physics, Slovak Academy of Sciences, Dubravska cesta 9, SK-845 36 Bratislava e ) Instituto de Ciencia de Materiales de Aragon, CSIC-University of Zaragoza, ES-50009 Zaragoza The rapid-solidification processing (by a cooling rate of 105 -1 06 K/s) was used for the preparation of deeply undercooled cryolite/alumina (Na3AlF 6/Al2O3) melts. We found a mass of nanotubes on the surface of these undercooled melts. The nanotubes were preferentially located on the defect places of the surface with the following approximate dimensions: base � 100 � 100 nm, length � 1000 nm. The solidified samples with the nanotubes on the surface were analyzed by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and infrared spectroscopy (IR).

The interfacial tension (IFT) between aluminum and cryolite melts containing different salt additions (AlF3, NaF, Na2SO4, NaVO3) has been measured by the capillary depression method under the condi...

The interfacial tension (IFT) between aluminium and cryolite melts containing different salt additions (AlF3, NaF,Na2SO4) has been measured during electrolysis by the capillary depression method. The technique is based on the measurement of the capillary depression occurring when a capillary, which is moved vertically down through the molten salt layer, passes through the metal/salt interface. The depression is measured by simultaneous video recording of the immersion height of the alumina capillary. The interfacial tension is strongly dependent on the n(NaF)/n(AlF3) ratio. The addition of Na2SO4 decreases the IFT of the aluminium/electrolyte interface. We also found the different influence of the conditions of electrolysis on the IFT in systems with and without Na2SO4. In systems without Na2SO4 the IFT decreases with increasing current density, and in systems with Na2SO4 it increases.

The cathodic concentration overvoltage in Na 3 AlF 6 –AlF 3 –Al 2 O 3 (sat.) melts was investigated at the temperature of 1000°C. The melts contained 10 and 20 wt.% of AlF 3 . Experimental data on the cathodic overvoltage agreed well with values calculated from the content of sodium in aluminium. The dependence of the cathodic overvoltage on temperature was studied as well. It was shown that the addition of 5 wt.% CaF 2 or 5 wt.% MgF 2 does not influence the overvoltage significantly, while the addition of 2 wt.% LiF decreases the overvoltage by 20 mV at a cathodic current density of 0.25 A cm −2 and by 50 mV at 0.75 A cm −2 . These results are consistent with the influence of these additions on the content of sodium in aluminium.