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Exploring the Sm CaF Tysonite Solid Solution as Solid State Electrolyte: Relationships Between Structural Features and F Ionic Conductivity -

Belto Dieudonne, Johann Chable, Fabrice Mauvy, Sebastien Fourcade, Etienne Durand, Eric Lebraud, Marc Leblanc, Christophe Legein, Monique Body, Vincent Maisonneuve, and Alain Demourgues J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b05016 • Publication Date (Web): 06 Oct 2015 Downloaded from http://pubs.acs.org on October 21, 2015

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The Journal of Physical Chemistry

Exploring the Sm1-xCaxF3-x Tysonite Solid Solution as Solid State Electrolyte: Relationships Between Structural Features and F- Ionic Conductivity Belto Dieudonné,† Johann Chable,†,‡ Fabrice Mauvy,† Sebastien Fourcade,† Etienne Durand,† Eric Lebraud,† Marc Leblanc,‡ Christophe Legein,‡ Monique Body,‡ Vincent Maisonneuve,‡ and Alain Demourgues*,†

† CNRS-University of Bordeaux-ICMCB, UPR 9048, 33608 Pessac Cedex, France

‡ LUNAM Université, Université du Maine, CNRS UMR 6283, Institut des Molécules et des Matériaux du Mans (IMMM), Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France

Keywords: solid electrolytes, crystal structure, rare earth fluorides, tysonite-type structure, ionic conductivity, 19F solid state NMR

ACS Paragon Plus Environment

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ABSTRACT. Pure tysonite Sm1-xCaxF3-x solid solution for 0.05 ≤ x ≤ 0.17 have been prepared by solid-state route. For the first time, the partial Sm1-xCaxF3-x solid solution is investigated on the basis of structural features and ionic conductivity measurements. Powder XRD Rietveld refinements show an unexpected decrease of the hexagonal unit cell volume related to the creation of fluorine vacancies. The local environment of F1, which is mainly responsible of the ionic conductivity, changes with the Ca content: the distortion of the F1(Sm,Ca)4 tetrahedral site decreases with the Ca content. Fluoride-ion exchanges have been qualitatively probed on two Sm1-xCaxF3-x (x = 0.05 and x = 0.15) samples thanks to

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F MAS NMR experiments at various

spinning frequencies and temperatures. At room temperature, the ionic conductivity decreases exponentially with the Ca content and the activation energy increases monotonously with the Ca content. The highest conductivity is found for the lowest Ca content or the smallest fluorine vacancies content stabilized in the Sm1-xCaxF3-x tysonite network corresponding to Sm0.95Ca0.05F2.95 (10-4 S.cm-1, Ea = 0.36 eV at room temperature). For this composition, the largest dispersions of F2-(Sm,Ca) and F3-(Sm,Ca) distances as well as (Sm,Ca)-F1-(Sm,Ca) angles are observed. The buckling of F2/F3 sheets around z=1/4 coordinate for low Ca content contributes to affect the large F1 tetrahedral site with the strongest distortion. The higher the buckling effect into the F2/F3 sheets, the higher the F1 local site distortion and the higher the ionic mobility and conductivity.


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INTRODUCTION Rare-earth based fluorides with tysonite-type structure have been widely investigated and can be considered as fast F- ion conductors. Numerous applications can be identified and are related to energy storage systems with solid membranes or sensors with fluoride-ion selective electrodes.1-9 More recently, a new secondary battery based on a fluoride shuttle has been reported with La0.9Ba0.1F2.9 as solid state electrolyte.10 Numerous combinations of rare-earth (RE = La-Lu) and alkaline earth (AE = Ca, Sr, Ba) fluorides have been explored to form RE1-xAExF3-x with the stabilization of fluorine vacancies.11 The different ionic radii of the AE cations (Ba2+ > Sr2+ > Ca2+) as well as the RE cations (La3+ >…> Lu3+) influence the local environments of fluorine atoms and, consequently, their mobility. The best ionic conductivities on single-crystals, obtained with La, Ce, Nd and Sm combined with Ca and Sr, vary from 10-3-10-5 S.cm-1 at room temperature to 10-2-10-3 S.cm-1 at T = 150°C. The case of SmF3 is complex: the structural type depends on the synthesis route.12 SmF3 adopts either the orthorhombic YF3-type structure (SG : Pnma, Z = 4) or the LaF3 tysonite-type structure (SG: P-3c1, Z = 6). A conductivity jump for SmF3 was found around 500°C13 which corresponds to the orthorhombic (YF3)-trigonal (LaF3) phase transition14 with a higher conductivity for tysonite-type network. The incorporation of Ca2+ with a ionic size slightly larger than that of Sm3+ (at the same coordination number) into the SmF3 network has already been attempted and the tysonite-type structure is stabilized; the Sm0.94Ca0.06F2.94 single crystals exhibit a rather high ionic conductivity around 10-4 S.cm-1 at room temperature.11 In the tysonite network, fluoride anions are located in three inequivalent crystallographic sites F1, F2 and F3 with 12:4:2 multiplicities. F2 and F3 form infinite layers; F3 atoms occupy the centers of cationic triangles with D3h point symmetry and F2 atoms are above or below cationic


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triangles with C3v point symmetry. F1 atoms appear between the F2/F3 sheets and occupy a cationic distorted tetrahedral site (Figure 1). On the basis of

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F NMR investigations on single-

crystals of LaF3 and La1-xSrxF3-x, the ions of the F1 sublattice are the most mobile ions and the correlation times of the exchange motion increase in the order F1-F1 < F1-(F2,F3).15-18 A

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F

NMR MAS study on powdered LaF3 and La0.99Sr0.01F2.99 allowed to discriminate F2 and F3 sites with correlation times F1-F3 < F1-F2.19 We are currently investigating several tysonite solid solutions RE1-xAExF3-x (RE = La, Ce, Sm; AE = Ca, Sr, Ba). Surprisingly, despite the high ionic conductivity of the Sm0.94Ca0.06F2.94 single crystals,11 the Sm1-xCaxF3-x solid solution has never been explored on the basis of XRD analysis and ionic conductivity measurements in order to better understand the influence of structural features on fluorine motions and activation energies. A solid state route at high temperatures starting from binary fluorides was used to prepare Sm1-xCaxF3-x solid solution for 0.04 < x < 0.20. Powder XRD diagrams followed by Rietveld analyses were performed and the hexagonal unit cell parameters as well as atomic positions were refined.

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F MAS NMR study was undertaken

on four selected samples, i.e., orthorhombic SmF3, a mixture of orthorhombic and trigonal SmF3 and two Sm1-xCaxF3-x (x = 0.05 and x = 0.15) samples, despite their paramagnetic character, to probe the local order and motion of fluorine nuclei. The ionic conductivity versus temperature (25°C≤T≤250°C) and the activation energies were determined by impedance spectroscopy measurements for this series. Finally, the correlations between structural features and ionic conductivity in the Sm1-xCaxF3-x solid solution are discussed.


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EXPERIMENTAL SECTION Sample Preparation Solid state route synthesis in platinum tubes was used to prepare calcium-substituted samarium fluoride materials. Samarium fluoride SmF3 was purchased from CeracTM (99,9%) and calcium fluoride CaF2 from Aldrich (99%). To avoid impurities, both products were calcinated at 300°C for 8 h under argon gas flow and they were further fluorinated at 500°C under 10% F2 gas flow. XRD analysis showed that SmF3 is a phase mixture of orthorhombic (YF3-type structure) and trigonal (LaF3 tysonite-type structure) networks. The weight proportions, 51% (orthorhombic) and 49% (trigonal), were determined from the Rietveld analysis (Figure S1-Supporting Information). It can be noted that the weight ratio before fluorination was 76%/24%, respectively. Pure SmF3 phase with orthorhombic (YF3)-type structure has been obtained after annealing at T=1000°C for 24h in sealed platinum tube followed by water quenching. Intimate powder mixtures with stoichiometric amounts of starting fluorides were prepared by grinding in a glove box under argon atmosphere during 15 min. Each mixture was placed in a platinum tube which was sealed before heating to avoid oxyfluoride formation. According to SmF3-CaF2 diagram,20,21 it is possible to obtain Sm1-xCaxF3-x solid solution for x ≤ 0.20 at T = 850°C. However, in these conditions and even at 900°C for 24 h, after cooling to room temperature or even after water quenching, mixtures of the trigonal and orthorhombic phases were obtained. It was possible to prepare pure tysonite Sm1-xCaxF3-x solid solution (x = 0.05, 0.07, 0.10, 0.13, 0.15, 0.17) by increasing the synthesis temperature to 1000°C followed by water quenching. SmF3 and Sm0.975Ca0.025F2.975 with orthorhombic structure and two phase mixtures (x = 0.04, 0.20) have been also synthesized.


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The ceramics pellets of pure Sm1-xCaxF3-x compositions with tysonite-type network (x = 0.05, 0.07, 0.10, 0.13, 0.15, 0.17) were pressed in a 6 mm metallic mould by uniaxial cold pressing under a pressure of 1.5 ton for 1 min. The pellets were placed in sealed platinum tubes and then thermally heated at 1000°C for 24 h before cooling to room temperature (5°C/min heating rate and cooling). Dense ceramics (≥ 92% relative density) were obtained in these sintering conditions. XRD analysis of all pellets confirmed that only tysonite phase is present. Characterization Techniques XRD Analysis All samples were characterized by powder XRD at room temperature with a PANalytical X’Pert MPD X-ray diffractometer equipped with a linear X’celerator detector using low scan rate (20° < 2θ < 130°, steps of 0.008° and counting time of 750 s) and with Cu-Kα radiation. The structural refinements were done by using the Fullprof package of programs. All the diffraction patterns were indexed on the basis of tysonite (SG: P-3c1, Z = 6) or orthorhombic YF3 (SG : Pnma, Z = 4) type structures. The Rietveld refinement of trigonal and orthorhombic SmF3 structures was carried out from X-ray pattern of the phase mixture treated under F2 gas (see Sample Preparation). Based on the literature,11,22 the XRD Rietveld refinements of Sm1-xCaxF3-x tysonite solid solution patterns (0.05 ≤ x ≤ 0.17) were performed with fluorine vacancies only on F1 site. All atoms were refined isotropically with common displacement parameters for (Sm,Ca) atoms and for F1-F2-F3 atoms respectively and a correction of preferential orientation was applied with the (113) plane. In the trigonal-tysonite phase, the fluoride ions are distributed over three nonequivalent positions (F1:F2:F3 with 12:4:2 multiplicities). Except for x = 0.13 and x = 0.17, two structural models led to similar reliability factors: F2 in (⅓ ⅔ z) with z close to 0.18 (model 1) and disorder of F2 on two symmetry related positions (⅓ ⅔ z) and (⅓ ⅔ ½-z) and


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half site occupancy (model 2). For x = 0.13, only the model 1 led to a stable refinement; the very large standard deviations on atomic coordinates, in relation with disorder, incited us to prefer split models for x > 0.13. For x = 0.17, the model 2, completed by an equi-repartition of F1 atoms on two constrained positions with (x y z) and (x -y z) coordinates (model 3), was also tested; both models led to close reliability factors. It can be noted that the model 3 is very similar to a higher symmetrical model in the P63/mcm SG previously questioned in the literature.23,24 For the sake of clarity, only the model 1 for 0.05 ≤ x ≤ 0.13 and the model 2 for x = 0.15 and 0.17 were considered to establish Tables 2 and 3 and Figures 4, 5, 6 and 7. More detailed results are given in Supporting Information. 19

F MAS NMR

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F MAS NMR spectra were recorded on an Avance 300 Bruker spectrometer operating at 7 T

(19F Larmor frequency of 282.2 MHz), using a 1.3 mm diameter probe head allowing spinning frequencies up to 70 kHz. 1D NMR spectra were acquired using a Hahn echo sequence for which interpulse delay was synchronized with the rotor spinning frequency. A 90° pulse length of 1.55 µs was used, recycle delays of 5 s were applied and 128 transients were accumulated. Air frictional heating allowed temperature to be varied by up to 32°C from 34 to 64 kHz. Variable temperature (VT) experiments were monitored using a Bruker VT Unit.

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Pb

isotropic chemical shift of Pb(NO3)2 was used as NMR thermometer.25,26 The maximum temperature gradient over the dimension of the 1.3 mm rotor was estimated around 8°C. The 19F chemical shifts are referenced to CFCl3 at 0 ppm. Spectra reconstructions, including spinning sidebands, were achieved using the DMFIT software.27


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Electrochemical Impedance Spectroscopy EIS (Electrochemical Impedance Spectroscopy) measurements were performed on sintered pellets. A thin film of metallic electrodes was deposed on both sides of the pellets. Gold metal is used as fluoride (F-) blocking electrode. Physical vapor deposition (sputtering) during 15min was sufficient for having homogenous compact gold film. A two electrodes configuration was used for EIS measurements. EIS measurements were performed using Autolab Frequency response analyser in the frequency range of 10 Hz to 1 MHz applying 50 mV/100 mV signal amplitude. Nyquist diagrams were recorded from 25°C to 250°C under argon atmosphere. At least, two hours equilibrium time has been respected before each measurement during the heating and cooling thermal cycle in order to achieve ready state conditions. In order to modelize the impedance data, equivalent circuits were used for fitting the Nyquist diagrams with “Zview” software of Scribner Associates Inc. RESULTS AND DISCUSSION XRD Characterization and structural features The XRD patterns of Sm1-xCaxF3-x (with x = 0, 0.025, 0.04, 0.05, 0.07, 0.10, 0.13, 0.15, 0.17, 0.20) compositions are reported in Figure 2. All the patterns until x = 0.17 are indexed with the tysonite and/or YF3 type structures. A fluorite-type CaF2 impurity (SG : Fm-3m) appears for x = 0.20 and confirms the solubility limit.20,21 For SmF3 and Sm0.975Ca0.025F2.975, pure orthorhombic YF3-type phases are prepared after cooling to room temperature or water quenching (Figure 2). These cooling conditions do not allow stabilizing the high temperature tysonite form observed by Roterau et al.14 For x = 0.04, the tysonite-type structure appears after water quenching; the XRD pattern corresponds to a phase mixture of orthorhombic and tysonite types (Figure 2(c)). Pure tysonite phases are obtained in the domain x = 0.05-0.17. For increasing


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x, the XRD patterns exhibit a systematic peak shift to high angles which confirms the variation of unit cell parameters. The a parameter decreases monotonously with x whereas the c parameter remains almost constant (Figure 3, Table 1). The resulting decrease of the unit cell volume is at odd from the difference of the ionic radii of Sm3+ and Ca2+ (1.272 Å and 1.32 Å respectively for nine-fold coordination28) that should imply an expansion of the unit cell parameters. It can be thought that the unit cell volume variation is mainly driven by the creation of anionic vacancies that decreases the cationic coordination. As considered by Brown,29 Ca2+ has a preferential coordination number of 8 (ri = 1.26 Å) as in CaF2 against 9+2 for Sm3+ in tysonite-type structure.30 Then, the gradual decrease of the a parameter with x could be related to the decrease of the Ca coordination number (Sm site) by a preferential stabilization of fluoride vacancies in the vicinity of Ca2+ ions. As an example, the powder XRD Rietveld refinement of Sm0.95Ca0.05F2.95 is given on Figure 4 (other refinements are gathered in Supporting Information). It can be noted that the intensity of few diffraction peaks are not fully taken into account as a reason of preferential orientation that could not be perfectly modelized. The tysonite-type structure can be described as ∞[(Sm,Ca)F] slabs with (F2,F3) atoms around z = ¼ and ¾ and [F]2 slabs of F1 atoms around z = 0 and ½ (Figure 1). F1 atoms adopt a distorted F1(Sm,Ca)4 tetrahedral coordination while (F2,F3) atoms adopt a three-fold coordination. On the basis of atomic position refinements (Table 2), the cations keep very similar x atomic coordinates (x ≈ ⅔) whatever the Ca content. The z atomic coordinate of F2 atoms is almost constant from x = 0.05 to 0.10 (Figure 5). For x = 0.13, F2 atoms lie in the plane of F3 atoms (z = ¼) while they split on symmetrically related positions for x ≥ 0.15. However, the reliability factors of the Rietveld refinement for x = 0.13 composition remain rather high (RBragg ≈ 0.10) and consequently the interatomic distances and angles for this composition are not


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further considered. As the Ca2+ content as well as the fluorine vacancies content increase, the atomic disorder becomes more and more high. The F-(Sm,Ca) distances are reported in Table 3 and Figure 6. One should have to notice that the four F1-M distances describing a highly distorted tetrahedral site correspond to the largest bond lengths compared to F2/F3-M bond lengths. Then, the probability to stabilize fluorine vacancies on F1 site is the highest in relation with the highest ionic mobility of the F1 sublattice. The average F1-(Sm,Ca) distances do not vary significantly with x. However, the F1(Sm,Ca)4 tetrahedral site with C1 point group symmetry is strongly distorted; F1 is off-centered and a dispersion of the F1-(Sm,Ca) distances and of (Sm,Ca)-F1-(Sm,Ca) angles is observed (see Supporting Information). For x = 0.05, it can be noted that two short (2.30 Å and 2.34 Å) and two long (2.78 Å and 2.86 Å) distances F1(Sm,Ca) are related to large and low angles. The (Sm,Ca)-F1-(Sm,Ca) angle dispersion is maximum at x = 0.05 that corresponds to the optimum of the conductivity measured for Sm0.95Ca0.05F2.95 solid solution (Figure 7). It is also remarkable to note that F2-(Sm,Ca) distance drastically decreases from x = 0.05 to x = 0.07 whereas the F3-(Sm,Ca) distance increases with the same amplitude; both distances converge towards a value around 2.35 Å. The environment of F2 atoms undergoes an evolution from pyramidal C3V symmetry to planar D3h symmetry at x = 0.13. As a consequence, the thickness of the ∞[(Sm,Ca)F] slabs with (F2,F3) atoms at z = ¼ and ¾ decreases for this composition. The higher the buckling phenomena of the ∞[(Sm,Ca)F] slabs with (F2,F3) atoms, the higher the F1 site distortion into the interslabs space, the higher the F- ionic conductivity. All these structural features have a key role on the fluorine mobility in the tysonite network.


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F NMR Spectroscopy

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F MAS NMR provides an ideal method to probe the local order in fluoride solid solutions31-39

and the motion of fluorine nuclei in F- ionic conductors. Chemical exchange processes occurring between sites with distinct chemical shifts result in characteristic NMR line shapes, when the rate of the exchange process enters the so called intermediate regime of motion.40 Thus in principle, if the individual fluorine crystallographic sites are resolved in an

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F MAS NMR

spectrum of a fluoride, one-dimensional methods can be used to obtain correlation times between the different fluoride-ion sublattices, when the frequency of the motion is of the same order of magnitude as the separation in frequency between the different resonances of the sublattices.17,4145

Early 19F wide-line NMR spectra of polycrystalline rare earth fluorides REF3 (RE = La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Yb)46-51 were reported, with two distinct resolved lines,48 corresponding to the two inequivalent fluorine sites of orthorhombic HoF3, ErF3 and YbF3.

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F

NMR spectra of REF3 single crystals (RE = Ce,52 Tb and Ho,53 Nd and Pr,54 Er50) with distinct resolved lines, corresponding to the inequivalent fluorine sites were also reported. Nevertheless, due to the expected substantial strengthened by the large

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F line broadenings and NMR shifts in paramagnetic systems,

F gyromagnetic ratio, the reported

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F MAS NMR spectra of

paramagnetic fluorides (CeF3, NdF3 and SmF3) are very scarce.55 Despite expected difficulties, 19

F MAS NMR experiments were carried out on four selected samples to probe the local order

and motion of fluorine nuclei: pure orthorhombic SmF3, a mixture of orthorhombic and trigonaltysonite SmF3 (after fluorination, see Sample Preparation) and two Sm1-xCaxF3-x (x = 0.05 and x = 0.15) tysonite solid solutions.


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The 19F MAS NMR spectra of orthorhombic SmF3 (Figure 8) show an almost symmetric broad line. Assuming that, at these spinning frequencies, homonuclear

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F-19F dipolar couplings are

significantly reduced, broadening arises mainly from dipolar coupling of

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F nuclei with the

unpaired spin density on the Sm atoms. As spinning frequency (and temperature) increases, the NMR shifts slightly decrease and, as expected, the line width decreases. Two lines with close NMR shifts were used for the fits of these spectra (Figures S7 and S8, Tables S2 and S3 in Supporting Information). The lack of resolution makes it impossible to assign these NMR lines with similar shifts to the two fluorine 4d and 2a crystallographic sites (both coordinated by 3 Sm atoms). The

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F MAS NMR spectra of a mixture of orthorhombic and trigonal SmF3 (51 and 49%,

respectively) show an asymmetric broad line (Figure S9). As spinning frequency (and temperature) increases, as expected, the line width decreases. The

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F MAS NMR spectrum

recorded at 64 kHz has been fitted with four NMR lines, two of them being the two lines that were used for the fit of the spectrum of orthorhombic SmF3 (Figure 9, Table 4). The sum of the relative intensities of the two lines assigned to orthorhombic (52%) and trigonal (48%) SmF3 are similar to the fractions determined by XRD. Moreover the relative intensities of the two lines assigned to trigonal SmF3, with 2:1 ratio, can be assigned to F1 (12g, coordinated by 4 Sm atoms) and F2 and F3 (4d and 2a, coordinated by 3 Sm atoms). The values of the

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F NMR shifts of orthorhombic and trigonal SmF3 are not unusual

compared to isotropic chemical shifts in MFn diamagnetic inorganic fluorides56-61 but are significantly lower than those of LaF358 or CeF4.61 As shown by the spinning sideband intensities, the shift anisotropies are large without being outstanding (those of CeF461 are, for instance, larger). Compared to YF3 and LaF358 which are, respectively, isotypic with


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orthorhombic and trigonal SmF3, in addition to broadening, the 19F NMR shift ranges are smaller and it is not possible to discriminate fluorine atoms with the same coordination number. The isotropic lines of the 19F MAS (64 kHz) NMR spectra of trigonal SmF3, Sm0.95Ca0.05F2.95 and Sm0.85Ca0.15F2.85 are shown on Figure 10. As the CaF2 content increases, the NMR lines move toward larger shifts, i.e., toward the isotropic chemical shift of CaF2 (-108 ppm58), in agreement with fluorine environments containing more calcium atoms (Table 5). The expected line broadening associated with increasing structural disorder is observed from Sm0.95Ca0.05F2.95 to Sm0.85Ca0.15F2.85. However, the spectrum of Sm0.95Ca0.05F2.95 is narrower than that of trigonal SmF3 and the NMR resonances assigned to F1 and F2,F3 are no longer resolved. These observations are consistent with fluoride-ion exchange: exchange between two sites at a frequency greater than the separation of the two resonances results in a single resonance whose chemical shift is intermediate between both resonances. Fluoride-ion exchange is confirmed by VT experiments on Sm0.95Ca0.05F2.95 (Figure 11 (isotropic line) and Figure S10 in Supporting Information) since, as temperature increases, the width of the broad NMR line decreases, the NMR line becomes more and more symmetric and the spinning sideband intensities decrease. Fluoride-ion exchanges between F1 and F3 sites and F1 and F2 sites have been observed, at higher temperature, on 19F 1D MAS NMR spectra, in LaF3 and La0.99Sr0.01F2.99.19 The separation of the resonances assigned to F1 and F2,F3 being smaller for trigonal SmF3 (16 ppm, i.e., 4.5 kHz, between F1 and F2,F3 sites for trigonal SmF3), motions of lower frequency could affect 19F 1D MAS NMR spectra of Sm1-xCaxF3-x samples. In other words, assuming similar frequencies of motion for Sm1-xCaxF3-x and La1-xSrxF3-x, the effect of fluoride-ion exchange on the 19F 1D MAS NMR spectra would be stronger (at the same temperature) for the Sm1-xCaxF3-x samples. Moreover, in Sm1-xCaxF3-x samples, NMR shifts of the resonances increase with x. Then, the


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NMR shifts of F1 (and F2,F3) sites certainly differ significantly with their environments (FSm4, FSm3Ca or FSm2Ca2 for F1 and FSm3, FSm2Ca or FSmCa2 for F2,F3). F1-F1 exchanges being considerably faster (about two order of magnitude in La1-xSrxF3-x18) than F1-F2,3 exchanges, the 19

F MAS NMR spectra of Sm1-xCaxF3-x samples are also affected by exchanges between F1

atoms with different NMR shifts. Such VT experiments were not performed on Sm0.85Ca0.15F2.85 but the spectra recorded at various spinning frequencies (Figure S11 in Supporting Information) show the same trend. Nevertheless, for the highest spinning frequency (and highest temperature), the shoulder at high chemical shift is still present, showing that exchanges between the fluoride ions corresponding to the main line and the shoulder (separated by ~8 kHz) are not fast enough. This

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F MAS NMR study demonstrates that fluoride-ion exchanges occur at moderate

temperatures in Sm1-xCaxF3-x samples. Unfortunately, the correlation times of exchange cannot be determined since several conditions (assignment of the NMR lines on a spectrum that is not affected by exchange, coalescence reached for the highest investigated temperature) are not met. F- ionic conductivity The EIS measurements were performed from room temperature (RT) to T = 250°C under argon flow on all the samples. This narrow temperature domain was adopted in order to cope with the working temperature range of Fluoride Ion Batteries (FIB). Equivalent circuits based on resistance and constant phase element (CPE) associated in parallel (Figure 12, inset) was used to fit the impedance diagrams (experimental data). As expected, the impedance diagrams show three main contributions. In the high frequency (HF) range, the first one is related to the bulk (B). In the medium frequency range (MR), the second one is attributed to the grain boundaries (GB) and the last one in the low frequency range is assigned to the blocking electrodes (BE). The resistance (R1) is attributed to the connection circuit, the resistance (R2) and the associated


14

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capacitance (C2) to the bulk of the ceramic, the resistance (R3) and capacitance (C3) to the grain boundaries contributions and the Warburg element (Wo1) to the blocking electrode (Figure 12). The ionic conductivity of fluoride ions is determined for the ceramics by using both resistances (R2, R3) and the following equation (equation 1): σ =

1 e (1) where e is the thickness x R2 + R3 A

and A the surface of the pellet. Temperature dependence of ionic conductivity for all samples is plotted using the Arrheniustype equation (equation 2): σT = σ0 exp(-Ea / kT) (2) where σ0 is the ionic conductivity at infinite temperature T and Ea is the activation energy. Arrhenius plots of the ionic conductivity for all samples are reported on Figure 13. From T = 25°C to T = 250°C, all samples present a linear evolution of log (σT) versus T. It confirms that a hopping mechanism is responsible of the ionic conductivity in the studied temperature range. In Sm1-xCaxF3-x solid solution, the best ionic conductivity of fluoride ions in the temperature range is obtained for the Sm0.95Ca0.05F2.95 composition with an estimation of σRT = 10-4 S.cm-1 at RT. This value is very close to that of single crystals and better than values reported in the literature for fluoride ceramics.62-64 As no ionic conductivity evolution is observed after several months, it can be deduced that all previous fluorides are stable below 250°C. At RT, the conductivity decreases exponentially as a function of calcium concentration (Figure 14). Moreover, the activation energy increases linearly with x (Figure 15). The value of Ea for Sm0.95Ca0.05F2.95 seems to be lower than or equivalent to that of other solid solutions with tysonite-type structure (0.36 ± 0.02 eV).22,64-67 For Sm0.85Ca0.15F2.85, the ionic conductivity lowers to σRT = 5.4 107

S.cm-1 at RT while Ea increases to 0.57 eV. The decrease of ionic conductivity and the linear

increase of activation energy versus Ca concentration can be explained by a segregation of the


15


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Page 16 of 48

vacancies around the calcium which hampers the fluorine mobility. The variation of ∞[(Sm,Ca)F] slabs with (F2,F3) atoms versus the Ca content and, consequently, the F1 site distortion strongly contributes to the F1 ionic conductivity especially for low Ca content, as previously described in the structural features part. With regard to high ionic conductivity at RT and to the low activation energy, Sm0.95Ca0.05F2.95 seems to be the best polycrystalline solid electrolyte; then, it is susceptible to improve the cycling speed of Fluoride Ion Battery.37,68,69

CONCLUSIONS For the first time, the Sm1-xCaxF3-x solid solutions (0.05 ≤ x ≤ 0.17) with tysonite-type structure were prepared by solid state route from binary fluorides at 1000°C followed by water quenching. The orthorhombic Sm1-xCaxF3-x phase is stabilized at x < 0.04 and CaF2 impurity is detected for x > 0.17. The Rietveld XRD refinements of Sm1-xCaxF3-x patterns (0.05 ≤ x ≤ 0.17) lead to determine accurately the cell parameters and the atomic positions. Surprisingly, the decrease of a parameter is not governed by the ionic radius change from IXSm3+ to IXCa2+ but by the reduction of coordination number of metallic site that is probably satisfied by the creation of fluorine vacancies around substituting calcium.

19

F MAS NMR spectra of orthorhombic and trigonal

SmF3 are reported. Moreover, fluoride-ion exchanges have been qualitatively probed on two Sm1-xCaxF3-x (x = 0.05 and x = 0.15) samples thanks to

19

F MAS NMR experiments at various

spinning frequencies and temperatures. At room temperature, the ionic conductivity σRT of Sm1xCaxF3-x

solid solutions (x = 0.05, 0.07, 0.10, 0.15, 0.17) shows an exponential decrease with x;

the highest value (10-4 S.cm-1) is observed for the composition Sm0.95Ca0.05F2.95. Moreover, it can be noted that the activation energy increases linearly with x. The evolution of ionic conductivities with x is related to the change of F1, F2 and F3 local environments. The distortion


16

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of F1 tetrahedral site, modelized by the angle dispersion, follows the ionic conductivity change. At the highest conductivity (x = 0.05), the largest dispersion of F2-(Sm,Ca) and F3-(Sm,Ca) distances and F(Sm,Ca)-F1-(Sm,Ca) angles is observed. The trigonal symmetry of F2 site changes from pyramidal to planar for x = 0.13. Consequently, the buckling of ∞[(Sm,Ca)F] sheets at z = ¼ and ¾ shrinks and the (F2,F3) slabs become almost planar at this Ca concentration. This work demonstrates that the F1 lacunar fluorine network is the main contributor to the conductivity at low Ca content. The increase of x implies a modification of the fluorine environments and a segregation of the vacancies around the calcium can be assumed. These structural modifications hamper the fluorine mobility in the tysonite network.

ASSOCIATED CONTENT Supporting Information. Powder XRD Rietveld refinement of Sm1-xCaxF3-x (x = 0, 0.07, 0.10, 0.13 0.15, 0.17) compounds, experimental and fitted 19F MAS NMR spectra of Sm1-xCaxF3-x (x = 0, 0.05, 0.15) compounds, environments of fluorine atoms (distances and angles) of Sm1-xCaxF3-x compounds (x = 0.05, 0.07, 0.10, 0.13, 0.15, 0.17) and 19F NMR parameters deduced from the fit of the

19

F MAS (44 kHz and 64 kHz) NMR spectra of orthorhombic SmF3. This material is

available free of charge via the Internet at http://pubs.acs.org.


17


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Page 18 of 48

AUTHOR INFORMATION Corresponding Author [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The authors express their sincere gratitude to the French National Research Agency (Project FLUOBAT-ANR-12-PRGE-0009-01) for the financial support of this work and the doctoral and postdoctoral grants of J. Chable and B. Dieudonné, respectively. Acknowledgments We are grateful to Dr A. Gil Martin (IMMM, Le Mans) for quantitative analysis of orthorhombic and trigonal SmF3 phase mixture from XRD pattern.


18

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Figure 1. Representation of the tysonite-type structure. The local environment of F1 (C1 or distorted Td symmetry), F2 (C3v point group) and F3 (D3h point group) are also represented as well as the ∞[(Sm,Ca)F] sheets with (F2,F3) atoms at z = ¼ and ¾.


19


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Page 20 of 48

Figure 2. XRD patterns of Sm1-xCaxF3-x compositions ( ¤ = orthorhombic YF3-type phase, + = tysonite-type phase, ◊= CaF2).


20

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7.15

299

y = 0.0128x + 7.1209

7.10

298 7.05

a 297

y = -16.298x + 298.41

c 7.00

Volume (Å3)

Cell parameters (Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


V 296 6.95

y = -0.1967x + 6.9563 295

6.90 0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

x Figure 3. Variation of unit cell parameters and volume of the Sm1-xCaxF3-x (x = 0, 0.05, 0.07, 0.10, 0.13, 0.15, 0.17) solid solutions.


21


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Figure 4. Powder XRD Rietveld refinement of Sm0.95Ca0.05F2.95.


22

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0.35

z coordinates of F2 atoms

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.30

0.25

0.20

0.15 0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

x

Figure 5. Variation of the z coordinate of F2 atoms versus x in the Sm1-xCaxF3-x (x = 0.05, 0.07, 0.10, 0.15, 0.17) solid solutions.


23


2.E-04 2.56

F2-(Sm,Ca) distance

2.51

1.E-04

F3-(Sm,Ca) distance 2.46

F1-(Sm,Ca) average distance

2.41

σRT

σRT (S.cm-1)

F-(Sm,Ca) distance (Å)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 48

5.E-05

2.36 2.31 2.26

0.E+00 0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

x

Figure 6. Evolution of F-(Sm,Ca) distances and room temperature conductivity versus x in the Sm1-xCaxF3-x solid solutions.


24

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2.E-04

40

(Sm,Ca)-F1-(Sm,Ca) angle dispersion

38

σ(RT)

1.E-04

36 34

5.E-05

σRT (S.cm-1)

42

Angle dispersion (°)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


32 30 0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.E+00 0.18

x

Figure 7. Evolution of (Sm,Ca)-F1-(Sm,Ca) angle dispersion and room temperature conductivity versus x in the Sm1-xCaxF3-x solid solutions.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 8.

19

Page 26 of 48

F MAS NMR spectra of orthorhombic SmF3 recorded with spinning frequencies of

(a) 44 kHz (T = 40°C) and (b) 64 kHz (T = 64°C). Isotropic lines (dashed line at 64 KHz) are shown in the inset. Stars indicate spinning sidebands. Spectra are normalized to the most intense peak.


26

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Figure 9. Experimental and fitted 19F MAS (64 kHz) NMR spectra of a mixture of orthorhombic and trigonal SmF3. The individual contributions used for the fit are shown below. Isotropic lines (dashed for orthorhombic SmF3) are shown in the inset.


27


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Page 28 of 48

Figure 10. 19F MAS (64 kHz, T = 64°C) NMR spectra of trigonal (a) SmF3 (deduced from the fit of the spectrum of a mixture of orthorhombic and trigonal SmF3), (b) Sm0.95Ca0.05F2.95 and (c) Sm0.85Ca0.15F2.85. Spectra are normalized to the most intense peak.


28

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 11. VT (40°C-81°C)

19

F MAS (44 kHz) NMR spectra of Sm0.95Ca0.05F2.95. Spectra are

normalized to the most intense peak.


29


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Page 30 of 48

Figure 12. Impedance diagram and fits for Sm0.95Ca0.05F2.95 ceramic at room temperature (inset: equivalent circuit).


30

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 13. Variation of ionic conductivity versus temperature (log σT= f (1000/T)) for Sm1xCaxF3-x

(x = 0.05, 0.07, 0.10, 0.13, 0.15, 0.17) solid solution.


31


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Page 32 of 48

Figure 14. Room temperature conductivity versus x in Sm1-xCaxF3-x (x = 0.05, 0.07, 0.10, 0.13, 0.15, 0.17) solid solution.


32

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Figure 15. Activation energy as function of x in the Sm1-xCaxF3-x (x = 0.05, 0.07, 0.10, 0.13, 0.15, 0.17) solid solution.


33


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Page 34 of 48

Table 1. Unit cell parameters (Å), volume (Å3), room temperature conductivity (S.cm-1) and activation energy (eV) of the Sm1-xCaxF3-x solid solutions. x

0

a

0.05

0.07

0.10

0.13

0.15

0.17

6.9544(1) 6.9475(1)

6.9439(1)

6.9369(1)

6.9327(1)

6.9254(1)

6.9217(1)

c

7.1205(2) 7.1221(1)

7.1219(2)

7.1226(2)

7.1218(1)

7.1226(1)

7.1234(1)

Volume

298.24(2) 297.71(1)

297.39(2)

296.82(2)

296.43(1)

295.84(1)

295.56(1)

σRT

-

1.0E-4

5.2E-5

1.0E-4

1.1E-6

5.4E-7

3.1E-7

Ea

-

0.36±0.02 0.38±0.02 0.43±0.01 0.55±0.02 0.57±0.02 0.61±0.02

Table 2. Reliability factors and atomic positions (Sm,Ca: 6f (x, 0, 1/4); F1: 12g (x, y, z); F2: 4d (1/3, 2/3, z) ; F3: 2a (0, 0, 1/4) in the Sm1-xCaxF3-x solid solutions. x

0.05

0.07

0.10

0.13

0.15

0.17

Fit Model

1

1

1

1

2

2

RB

0.083

0.045

0.0468

0.102

0.085

0.0938

Rp

0.194

0.163

0.191

0.249

0.215

0.197

Rwp

0.159

0.146

0.155

0.197

0.168

0.151

0.6620(4)

0.6635(7)

0.6680(9)

0.6669(7)

0.6672(4)

0.728(3)

0.713(1)

0.6904(25)

Atom Sm,Ca

x

0.6716(3)

F1

x

0.7131(12) 0.6761(14) 0.685(3)

y

0.0616(8)

0.0530(7)

0.0412(14) 0.051(4)

0.024(4)

-0.0207(26)

z

0.5740(5)

0.5744(4)

0.5766(5)

0.5770(5)

0.5708(4)

x

0.1655(10) 0.1826(8)

0.216(3)

0.1664(10)

0.284(3)

0.3334(10)

F2

0.5540(9)

0.1880(12) 0.249(9)


34

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 3. F–(Sm,Ca) distances (Å) in the Sm1-xCaxF3-x solid solution. x

0.05

0.07

0.10

F1–(Sm,Ca)

2.299(5) 2.334(3) 2.340(3)

0.15

0.17

2.346(4)

2.301(4)

2.338(3) 2.467(6) 2.453(11) 2.405(15) 2.444(17) 2.785(8) 2.510(9) 2.601(19) 2.638(26) 2.639(11) 2.866(6) 2.946(6) 2.832(12) 2.835(16) 2.852(18)

2.57

2.57

2.56

2.56

2.56

F2–(Sm,Ca) (x3) 2.410(3) 2.348(3) 2.343(5)

2.322(5)

2.384(3)

F3–(Sm,Ca) (x3) 2.281(1) 2.347(1) 2.335(3)

2.307(3)

2.303(1)


35


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Page 36 of 48

Table 4. NMR shifts (δ, ppm), linewidths (LW, ppm) and relative intensities (I, %) of the NMR lines deduced from the fit of the

19

19

F

F MAS (64 kHz) NMR spectrum of a mixture of

orthorhombic and trigonal SmF3 and assignment of these NMR lines. δ

LW

I

Assignment

-166.1

24.0

13.0

orthorhombic

-166.0

14.9

38.8

orthorhombic

-171.3

12.7

32.1

trigonal, F1

-155.3

10.4

16.1

trigonal, F2 & F3

Table 5. Probabilities of occurrence (%) of the FSm4-yCay (F = F1) and FSm3-yCay (F = F2 or F3) species in Sm1-xCaxF3-x, based on a random distribution of Ca on the 6f site. x

F site

FSm4

FSm3Ca

FSm2Ca2

FSmCa3

FCa4

0.05

F1

81.5

17.1

1.4

0.0

0.0

0.15

F1

52.2

36.8

9.8

1.1

0.1

FSm3

FSm2Ca

FSmCa2

FCa4

0.05

F2 or F3

85.7

13.5

0.7

0.0

0.15

F2 or F3

61.4

32.5

5.7

0.3


36

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ABBREVIATIONS XRD: X-ray Diffraction NMR: Nuclear Magnetic Resonance MAS: Magic Angle Spinning SG: space group VT: variable temperature EIS: Electrochemical Impedance Spectroscopy


37


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Page 38 of 48

REFERENCES

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(9) Fjeldly, T. A.; Nagy, K. Fluoride Electrodes with Reversible Solid-State Contacts. J. Electrochem. Soc. 1980, 127, 1299-1303. (10) Anji Reddy, M.; Fichtner M. Batteries Based on Fluoride Shuttle. J. Mater. Chem. 2011, 21, 17059-17062. (11) Sorokin, N. I.; Sobolev B. P. Anionic High-Temperature Conduction in Single Crystals of Nonstoichiometric Phases R1−yMyF3−y (R = La-Lu, M = Ca, Sr, Ba) with the Tysonite (LaF3) Structure. Russ. J. Electrochem. 2007, 43, 398-409. (12) Zalkin, A.; Templeton, D. H. The Crystal Structures of YF3 and Related Compounds. J. Am. Chem. Soc. 1953, 75, 2453-2458. (13) Trnovcová, V.; Fedorov, P. P.; Furár, I. Fluoride Solid Electrolytes Containing Rare Earth Elements. J.Rare Earths 2008, 26, 225-232. (14) Rotereau, K.; Daniel, P.; Desert, A., Gesland, J. Y. The High-Temperature Phase Transition in Samarium Fluoride, SmF3: Structural and Vibrational Investigation. J. Phys.: Condens. Matter 1998, 10, 1431-1444. (15) Privalov, A. F.; Murin, I. V.; Vieth, H.-M. Disorder of Ionic Mobility in Crystalline Superionic Conductors Characterized by 19F-NMR. Solid State Ionics 1997, 101-103, 393-396. (16) Privalov, A. F.; Cenian, A.; Fujara, F.; Gabriel, H.; Murin, I. V.; Vieth, H.-M. The Distribution of Motional Correlation Times in Superionic Conductors:

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F Nuclear Magnetic

Resonance of Tysonite-Like LaF3. J. Phys.: Condens. Matter 1997, 9, 9275-9287.


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(17) Privalov, A. F.; Vieth, H.-M.; Murin, I. V. Nuclear Magnetic Resonance Study

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Table of Contents Graphic and Synopsis 2.E-04

40

(Sm,Ca)-F1-(Sm,Ca) angle dispersion

38

σ(RT)

1.E-04

36 34

5.E-05

σRT (S.cm-1)

42

Angle dispersion (°)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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32 30 0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.E+00 0.18

x


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Exploring the Sm1âxCaxF3âx Tysonite Solid ... - ACS Publications

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