[show abstract][hide abstract] ABSTRACT: The transport properties of molten LiF-YF3 mixtures have been studied by pulsed field gradient nuclear magnetic resonance spectroscopy, potentiometric experiments, and molecular dynamics simulations. The calculated diffusion coefficients and electric conductivities compare very well with the measurements across a wide composition range. We then extract static (radial distribution functions, coordination numbers distributions) and dynamic (cage correlation functions) quantities from the simulations. Then, we discuss the interplay between the microscopic structure of the molten salts and their dynamic properties. It is often considered that variations in the diffusion coefficient of the anions are mainly driven by the evolution of its coordination with the metallic ion (Y(3+) here). We compare this system with fluorozirconate melts and demonstrate that the coordination number is a poor indicator of the evolution of the diffusion coefficient. Instead, we propose to use the ionic bonds lifetime. We show that the weak Y-F ionic bonds in LiF-YF3 do not induce the expected tendency of the fluoride diffusion coefficient to converge toward one of the yttrium cation when the content in YF3 increases. Implications on the validity of the Nernst-Einstein relation for estimating the electrical conductivity are discussed.
The Journal of chemical physics 05/2013; 138(18):184503. · 3.09 Impact Factor
[show abstract][hide abstract] ABSTRACT: In this paper we address the effects of fission products on the speciation in molten fluoride salts. Numerous systems with cross-connections have been investigated in order to better identify the influence of CsF in a fluoride melt containing rare earth and oxides : LaF3–AF (A = Li, Na, K, Rb and Cs), LaF3–LiF–CsF, LaF3–LiF–CaF2, LaF3–LiF–CaO, LaF3–LiF–CaO–CsF. In this goal, we performed high temperature NMR experiments and followed in situ the evolution of 19F, 23Na, 85Rb, 133Cs and 139La NMR chemical shifts. In LaF3–AF–CsF and LaF3–AF–CaF2 systems, the coordination number of lanthanum cation ranges from 6 to 8 depending on the LaF3 concentration and on the polarizability of the other cations. The addition of oxide (CaO) in the latter mixtures leads to the formation of lanthanum oxyfluoride species that precipitate in LaOF when CaO concentration is increased. The addition of CsF to LaF3–LiF–CaO yields to a displacement of the dissolved versus precipitated LaOF proportion.
[show abstract][hide abstract] ABSTRACT: The electrical conductivities of molten LiF–YF3 and LiF–NaF–ZrF4 have been obtained from impedance measurements and molecular dynamics simulations, and discussed through the analysis of the local structure of the liquid. The conductivity decreases with addition of MFn (Mn+ = Y3+ or Zr4+) in the melt due to the formation of MFxn−x units, which can be either isolated or linked via bridging fluorine. Structural differences have been put in evidence depending on the nature of the multivalent metallic species (Y3+ and Zr4+) added in the alkali fluoride solvent regarding the stability of the MFxn−x units and the length m of [MFxn−x]m chains. These differences are at the origin of a more pronounced decrease of the electrical conductivity in the Zr-containing fused salts in the low MFn concentration régime.
Journal of Fluorine Chemistry 08/2012; 134:44–48. · 1.94 Impact Factor
[show abstract][hide abstract] ABSTRACT: The synthesis and structure resolution of RbLaF(4) are described. RbLaF(4) is synthesized by solid-state reaction between RbF and LaF(3) at 425 °C under a nonoxidizing atmosphere. Its crystal structure has been resolved by combining neutron and synchrotron powder diffraction data refinements (Pnma,a = 6.46281(2) Å, b = 3.86498(1) Å, c = 16.17629(4) Å, Z = 4). One-dimensional (87)Rb, (139)La, and (19)F MAS NMR spectra have been recorded and are in agreement with the proposed structural model. Assignment of the (19)F resonances is performed on the basis of both (19)F-(139)La J-coupling multiplet patterns observed in a heteronuclear DQ-filtered J-resolved spectrum and (19)F-(87)Rb HMQC MAS experiments. DFT calculations of both the (19)F isotropic chemical shieldings and the (87)Rb, (139)La electric field gradient tensors using the GIPAW and PAW methods implemented in the CASTEP code are in good agreement with the experimental values and support the proposed structural model. Finally, the conductivity of RbLaF(4) and luminescence properties of Eu-doped LaRbF(4) are investigated.
[show abstract][hide abstract] ABSTRACT: This review covers the local structure of molten metal halide, where a somewhat wide definition of “local structure” is employed, i.e. from the first shell of interacting neighbours up to correlations arising at the nanometer length scale. These particular liquids are indeed strongly organized at unusually long distances due to the predominance of coulombic interactions. Many of them are also characterized by the formation of intermediate range ordering. It is therefore impossible to describe the local structure of these liquids omitting this longer-ranged correlation. Finally, a deeper attention is given to the systems on which recent progresses have been made in the last decade, as for instance molten fluorides and rare earth halides.
[show abstract][hide abstract] ABSTRACT: Measuring internuclear distances through dipolar interaction is a major challenge for solid-state nuclear magnetic resonance (NMR) spectroscopy. Obtaining reliable interatomic distances provides an access to the local structure in ordered or disordered solids. We show that at magic angle spinning (MAS) frequencies larger than ca. 50 kHz, some of the three-spin terms of the homogeneous homonuclear dipolar Hamiltonian can be used to promote the creation of double-quantum coherences between neighbouring (1)H or (19)F spins without using dipolar recoupling pulse sequences in the Dipolar Homonuclear Homogeneous Hamiltonian (DH(3)) double-quantum/single-quantum correlation experiment. This makes it possible to probe inter-nuclear spatial proximity with limited risk of probe or sample damage from radio-frequency (RF) irradiation, and is fully appropriate for fast repetition rate offering sensitivity gains in favourable cases. Experimental demonstrations are supported by multi-spin numerical simulations, which points to new possibilities for the characterization of spin-system geometries.
Physical Chemistry Chemical Physics 03/2011; 13(17):8024-30. · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: Dialkylimidazolium chlorometallate molten salts resulting from the combination of zirconium or hafnium tetrachloride and 1-butyl-3-methylimidazolium chloride, [C(1)C(4)Im][Cl], have been prepared with a molar fraction of MCl(4), R = n(MCl4)/n(MCl4) + n([C1C4IM][Cl]) equal to 0, 0.1, 0.2, 0.33, 0.5, 0.67. The structure and composition were studied by Differential Scanning Calorimetry (DSC), (35)Cl (263 to 333 K), (1)H and (13)C solid state and solution NMR spectroscopy, and electrospray ionisation (ESI) mass spectrometry. The primary anions of the MCl(4)-based ILs were [MCl(5)], [MCl(6)] and [M(2)Cl(9)], whose relative abundances varied with R. For R = 0.33, pure solid [C(1)C(4)Im](2)[MCl(6)], for both M = Zr and Hf are formed (m.p. = 366 and 385 K, respectively). For R = 0.67 pure ionic liquids [C(1)C(4)Im][M(2)Cl(9)] for both M = Zr and Hf are formed (T(g) = 224 and 220 K, respectively). The thermal dissociation has been attempted of [C(1)C(4)Im](2)[HfCl(6)], and [C(1)C(4)Im](2)[ZrCl(6)] monitored by (35)Cl and (91)Zr solid NMR (high temperature up to 551 K).
[show abstract][hide abstract] ABSTRACT: Dialkylimidazolium chlorometallate molten salts resulting from the combination of zirconium or hafnium tetrachloride and 1-butyl-3-methylimidazolium chloride, [C1C4Im][Cl], have been prepared with a molar fraction of MCl4, R = nMCl4/nMCl4 + n[C1C4IM][Cl] equal to 0, 0.1, 0.2, 0.33, 0.5, 0.67. The structure and composition were studied by Differential Scanning Calorimetry (DSC), 35Cl (263 to 333 K), 1H and 13C solid state and solution NMR spectroscopy, and electrospray ionisation (ESI) mass spectrometry. The primary anions of the MCl4-based ILs were [MCl5], [MCl6] and [M2Cl9], whose relative abundances varied with R. For R = 0.33, pure solid [C1C4Im]2[MCl6], for both M = Zr and Hf are formed (m.p. = 366 and 385 K, respectively). For R = 0.67 pure ionic liquids [C1C4Im][M2Cl9] for both M = Zr and Hf are formed (Tg = 224 and 220 K, respectively). The thermal dissociation has been attempted of [C1C4Im]2[HfCl6], and [C1C4Im]2[ZrCl6] monitored by 35Cl and 91Zr solid NMR (high temperature up to 551 K).
[show abstract][hide abstract] ABSTRACT: We have developed a new experimental setup for the measurement of self-diffusion coefficients at high temperature (up to 1500K) in corrosive liquids. It is based on pulsed field gradient NMR using a 10mm liquid probe with two channels F-H/X modified and coupled with laser heating. The values obtained thanks to this setup are in good agreement with the available data obtained using the reference method, i.e., the capillary method. We present here results on 19F, 7Li, 23Na, 27Al in molten fluorides. In alkali fluoride mixtures, the self-diffusion coefficients depend weakly on the composition (nature and concentration of the different alkali) but mainly on temperature. In cryolite (Na3AlF6), the diffusion evidences AlFx3−x species.
Comptes Rendus Chimie - C R CHIM. 01/2010; 13(4):399-404.
[show abstract][hide abstract] ABSTRACT: The local structure and the dynamics of molten LiF-KF mixtures have been studied by nuclear magnetic resonance (NMR) and molecular dynamics simulations. We have measured and calculated the self-diffusion coefficients of fluorine, lithium and potassium across the full composition range around the liquidus temperature and at 1123 K. Close to the liquidus temperature, D(F), D(Li) and D(K) change with composition in a way that mimics the phase diagram shape. At 1123 K D(F), D(Li) and D(K) depend linearly on the LiF molar fraction. These results show that the composition affects the self-diffusion of anions and cations more weakly than the temperature. The activation energy for diffusion was also determined and its value can be correlated with the strength of the anion-cation interaction in molten fluoride salts.
Physical Chemistry Chemical Physics 12/2009; 11(48):11501-6. · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: Self-diffusion is a fundamental property of liquid dynamics that also provides important structural information. To explore the dynamics in inorganic melts with high liquidus temperature, we propose a new setup based on pulsed field gradient NMR combined with laser heating that makes possible in situ self-diffusion coefficient measurements up to 1500 K. Applied to several corrosive molten fluorides in a wide range of compositions and temperature, we have evidenced the different key parameters of their motion along with their structural characteristics. In alkali fluorides, the self-diffusion coefficient of fluorine depends slightly on the composition compared to the temperature, displaying these systems as an ideal bath of polarizable hard spheres. In contrast, self-diffusion in rare earth and alkali fluorides mixtures presents a complicated balance between temperature and the network-forming process of the ionic long-lived units. These results open wide perspectives in the study of high temperature liquids.
[show abstract][hide abstract] ABSTRACT: In situ high temperature nuclear magnetic resonance in molten fluoride mixtures gives some structural picture of the complexes existing in the melt, i.e. of their nature and relative proportion. Thanks to the development of a laser heating system associated with a close crucible in boron nitride, we can describe experimentally the evolution of these complexes from the anions and the cations point of view. By 19F NMR, we have shown the existence of three kinds of fluorine atoms depending on the composition: free fluorine like in pure LiF (non-bonded), bridging fluorine in melts rich in LnF3 in addition with terminal fluorine singly bonded to one rare earth. Data obtained by NMR spectroscopy are also combined with EXAFS measurements, again thanks to a specific development of the sample holder adapted with molten fluorides and high temperature. This study is a part of our systematic investigation of the different Alk–LnF3 systems by NMR and EXAFS spectroscopy.
Journal of Fluorine Chemistry - J FLUORINE CHEM. 01/2009; 130(1):45-52.
[show abstract][hide abstract] ABSTRACT: The local structures of molten lanthanum alkali fluoride binaries have been studied using HT NMR technique. The chemical shifts of (19)F, (23)Na and (139)La in solid and in liquid have been compared for AF (A = alkali) and LaF(3). In pure molten alkali fluorides, the polarisability of anion-cation pairs appears to be a key parameter to depict the observed evolution of (19)F chemical shifts. The influence of the composition has also been studied by measuring the chemical shifts in molten LaF(3)-AF as a function of LaF(3) concentration. A strong influence of the alkali influence is observed. The coordination number of lanthanum is decreased versus AF amount all the more since the alkali atomic number is high. Moreover, the more polarisable the alkali, the less bridging fluorines between the LaF(x) units.
Physical Chemistry Chemical Physics 07/2008; 10(22):3222-8. · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: We have studied the self-diffusion properties of butyl-methyl-imidazolium bis(trifluoromethylsulfonyl)-imide ([BMIM][TFSI]) + water system. The self-diffusion coefficients of cations, anions, and water molecules were determined by pulsed field gradient NMR. These measures were performed with increased water quantity up to saturation (from 0.3 to 30 mol %). Unexpected variations have been observed. The self-diffusion coefficient of every species increases with the quantity of water but not in the same order of magnitude. Whereas very similar evolutions are observed for the anion and cation, the increase is 25 times greater for water molecules. We interpret our data by the existence of phase separation at microscopic scale.
The Journal of Physical Chemistry B 11/2007; 111(41):11888-91. · 3.61 Impact Factor
[show abstract][hide abstract] ABSTRACT: The objective of this paper is to describe the local structure in fluoride melts of nuclear interest. Our experimental approach combines NMR and EXAFS spectroscopies both sensitive to the microstructure of solid and liquid materials. These techniques allow identifying the complex formation in molten media. Thanks to the development of specific cells adapted to molten fluorides up to 1300 °C, we give some structural description of LiF–LnF3 systems (Ln = Y, Lu, Ce) and discuss the evolution of 19F spectra in terms of different fluorine configurations depending on the composition. This approach is further extended to the characterization of LiF–ThF4 system by 19F NMR in melts from 0 to 100 mol% ThF4 at temperatures ranging from 600 to 1100 °C. We report also very first EXAFS spectra obtained in situ at high temperature, at the Thorium LIII edge, in LiF–ThF4 molten mixtures.
Journal of Nuclear Materials 01/2007; 360(1):43-48. · 1.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: A casting process has been studied for charged polymers: the sulfonated polyimide ionomer membrane. The formation of the membrane has been followed by X-ray reflectivity as a function of temperature. The effect of equivalent weight has been also investigated. The thickness loss presents two regimes: the first one is linear vs time indicating that the models developed for noncharged polymer may be suitable for ionomers in the early period of drying. The second one corresponds to the loss of X-ray reflectivity signal. Moreover, the X-ray reflectivity signal seems to be correlated to the characteristic time of the sample drying. In complement, we have studied the influence of casting on the properties of the dried ionomer membranes. The transport coefficients of N(CH(3))(4)(+) ions confined in two kinds of membranes that were differently cast were measured. The results show that shearing the ionomer solution during casting may lead to an enhancement of the anisotropy of structure and of transport. Moreover, we have studied the effect of both interfaces on the ion transport properties through the dried membranes.
The Journal of Physical Chemistry B 07/2005; 109(22):11332-9. · 3.61 Impact Factor
[show abstract][hide abstract] ABSTRACT: The structure of a sulfonated polyimide (sPI) ionomer membranes was investigated via the transport properties of various confined cations (7Li+, 23Na+, 87Rb+, 133Cs+). Their NMR spectra show large residual quadrupolar splitting depending on the orientation of the film in the static magnetic field B0. This behavior is the fingerprint of a macroscopic nematic ordering of charged interfaces. This is also confirmed by the anisotropy of the self-diffusion tensor measured by 1H and 7Li PGSE experiments on N(CH3)4+ and Li+ cations, respectively.
Magnetic Resonance Imaging 03/2005; 23(2):367-8. · 2.06 Impact Factor
[show abstract][hide abstract] ABSTRACT: The local structure in molten YF3–LiF and LaF3–LiF binaries has been investigated by NMR and EXAFS spectroscopy. For YF3–LiF, the high-temperature 19F and 7Li NMR spectra have been recorded in situ in the molten state for compositions ranging from pure LiF to pure YF3. The LaF3–LiF system has been studied by 19F and 139La NMR and EXAFS spectroscopy at the LIII edge of La. The local environment around the La3+ in the molten state does not depend on the composition, while 19F chemical shift evolution confirms the progressive formation of bridged species. To cite this article: A.-L. Rollet et al., C. R. Chimie 7 (2004).
Comptes Rendus Chimie - C R CHIM. 01/2004; 7(12):1135-1140.
[show abstract][hide abstract] ABSTRACT: In charged systems, the counterion distribution is an important information to understand their equilibrium structure. Small angle neutron scattering used with a contrast variation method is a suitable technique to analyze this distribution in aqueous and confined media. We had applied this method to Nafion membranes, a complex ionomer and disordered charged system. In this paper, the same approach is used for a lyotropic liquid crystal (LC) system: a lamellar phase of perfluoropolyether surfactants. As for Nafion system, we show strong counterion condensation at the surfactant–water interface that depends on the long-range period characterizing such an LC system.
Physica B Condensed Matter 01/2004; 350(1). · 1.33 Impact Factor
[show abstract][hide abstract] ABSTRACT: A new cell with simple design has been developed for high temperature X-rays absorption measurements in both solid and molten lanthanide fluorides. Two plates of pyrolitic boron nitride are fixed hermetically together around the samples in order to avoid any evaporation and atmosphere interaction. EXAFS spectra of molten mixtures of LiF–LaF3 measured at the La LIII absorption edge are reported up to 900 °C, and show the ability of this cell to keep the salt and to perform long time acquisition improving the signal to noise ratio.
Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms - NUCL INSTRUM METH PHYS RES B. 01/2004; 226(3):447-452.