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Mixed-valence induced during the intercalation of lithium ions into uranium fluorides
Journal of Physics and Chemistry of Solids 65 (2004) 591–596 www.elsevier.com/locate/jpcs
Mixed-valence induced during the intercalation of lithium ions into uranium fluorides E. Mapemba, M. Dubois*, M. Josse, M. El-Ghozzi, D. Avignant Laboratoire des Mate´riaux Inorganiques, UMR 6002 CNRS, Universite´ Blaise Pascal, 24, Avenue des Landais, Aubiere 63177, France
Abstract The mixed-valence in uranium fluorides LiU4F16 and UF4 can be induced (or eliminated) by the intercalation (deintercalation) of lithium ions into these compounds. These processes can be electrochemically performed both with LiU4F16 (or LiTh4F16) and with monoclinic UF4 as starting materials using a liquid electrolyte composed of propylene carbonate as solvent and LiClO4 as salt. In the cases of UF4 and LiU4F16, the electrochemical intercalation of Liþ into the host matrix is reversible and a maximum of three Liþ ions per U4F16 unit can be reversibly intercalated. Additional experimental evidences of the mixed-valence in these fluorides are presented such as the Electron Paramagnetic Resonance characterization of the starting materials and of those electrochemically modified. q 2003 Elsevier Ltd. All rights reserved. Keywords: A. Inorganic compounds; B. Chemical synthesis; D. Electron paramagnetic resonance (EPR); D. Electrochemical properties
1. Introduction In connection with a more general study of actinide and lanthanide fluorides, we have been interested in the phases appearing in the binary systems LiF – MF4 (M ¼ Ce, U and Th) and particularly those corresponding to the stoichiometry MF4/LiF ¼ 4. The phase diagrams of the systems LiF –UF4 [1] and LiF – ThF4 [2,3] show the existence of the compounds LiM4F17. Previous crystal structure analyses of these compounds have revealed an ambiguity concerning their stoichiometry; after a new structural investigation the initial LiTh4F17 composition was found to be in fact Th6F24·H2O [4]. These compounds have been prepared as single-crystals using the flux-growth method (ZnCl2 – LiCl eutectic composition was used as the flux) [5]. The authors of this work claim that although no lithium ions are present in this hydrated tetrafluoride, their presence is necessary for the preparation of the compound. The water molecules were certainly introduced in the crystal structure during the abundant washing of the sample with water carried out in order to eliminate the chlorides ZnCl2 and LiCl. An exchange of Liþ by H2O certainly occurred; this one is facilitated by the open-framework of * Corresponding author. Tel.: þ 33-04-73-40-71-05; fax: þ 33-04-73-4071-08. E-mail address: [email protected] (M. Dubois). 0022-3697/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2003.10.036
the structure and is accompanied by the modification of the oxidation state of a part of the thorium ions. Nevertheless, preliminary single-crystal X-ray diffraction investigation suggests mixed-valence for the actinide and lanthanide ions leading to the more likely formula LiM4F16 (better explicit as LiMIIIMIV 3 F16). Indeed, a partial reduction of the tetravalent metal occurs. Such a mixed-valence induced by alkaline ions was already encountered in terbium fluorides RbAl2Tb4F22 (RbAl2TbIIITbIV 3 F22) and Rb2AlTb3F16 (Rb2AlTbIIITbIV 2 F16) [6]. The crystal-chemical properties of tetravalent terbium and uranium fluorides exhibit some similitudes. As it has been observed for these compounds, a statistical distribution between M3þ and M4þ ions occurs in LiM4F16 as shown by the process of the crystallographic data (M3þ and M4þ ions are located and mixed on two crystallographic sites). The three dimensional framework of this structure delimits channels running along the k110l directions where the lithium ions lie. In other words, these lithiated phases result from the intercalation of the lithium ions into MF4 tetrafluorides which simultaneously undergo a tetragonal modification of their monoclinic structure. The accommodation of the lithium ions induces the partial reduction of the tetravalent metal. Such a process was investigated using electrochemical techniques. The electrochemical intercalation, or deintercalation, into these compounds can induce, or remove, the mixed-valence of the uranium and thorium ions by modifying their oxidation
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states. Both LiM4F16 (M ¼ U and Th) compounds and a commercial uranium tetrafluoride UF4 were studied as starting materials using a liquid electrolyte composed of propylene carbonate (PC) as solvent and LiClO4 as salt. The starting compounds and those electrochemically modified were characterized by Electron Paramagnetic Resonance (EPR) in order to evidence the modification of the oxidation state of the uranium ions.
2. Experimental Polycrystalline samples of LiM4F16 (M ¼ U, Th and Ce) were prepared by reacting mixtures of uranium, thorium or cerium tetrafluoride MF4 and lithium fluoride LiF in platinum tubes sealed under dry argon gas. The solid state reactions were carried out in two steps: First, the samples were heated at 300 8C for 24 h and secondly at 650 8C for 72 h. Under these conditions, single-crystals of LiU4F16 were obtained contrary to the case of the thorium phase. Electrochemical measurements were carried out using a conventional two electrodes cell (Swagelok), where lithium was used both as reference and counter electrode. Then, the potentials refer to a Liþ/Li electrode. The working electrode was formed by LiM4F16 or UF4 (about 86% by weight (w/w)), carbon black (CB, 5%, w/w) to insure electronic conductivity, and polyvinylidene difluoride (PVDF, 9%, w/w) as the binder. The mass of active material (LiU4F16 or UF4) was close to 2.0 mg. After stirring in PC, this mixture was spread thinly onto a collector by evaporation of PC. Then it was vacuum dried for 1 h at 150 8C. A PVDF microporous film wetted with LiClO 4 electrolyte (1 mol l21) dissolved in PC was sandwiched between the working electrode and a lithium metal foil. The cells were assembled in an argon-filled glove box. Prior to use LiClO4 (Aldrich, purity 99%) was outgassed under dynamic vacuum at 150 8C and PC was doubly distilled to remove traces of water. Cyclic voltammetry was performed at room temperature between 4.5 and 1.5 V vs Liþ/Li with a linear potential sweeping of 0.17 mV s21. Galvanostatic reduction and oxidation were carried out in the same potentials range; the current density was equal to 10 mA mg21. EPR spectra were recorded using an X Band Bruker EMX spectrometer equipped with a standard variable temperature device and operating at 9.653 GHz. Diphenylpicrylhydrazyl was used to calibrate the resonance frequency ðg ¼ 2:0036 ^ 0:0002Þ: LiM4F16 and UF4 were characterized by EPR spectroscopy in the initial state and after a complete reduction or oxidation. These samples were prior dried under dynamic vacuum, ground and set into a glass tube of 0.5 mm in diameter; the preparation of the samples was carried out in a glove box under inert atmosphere (argon). The presence of lithium ions into the compound LiTh4F16 was checked by 7Li-MAS/NMR using a Bruker Avance DSX 300 spectrometer operating at 116.64 MHz. The spectrum was recorded at a spin rate of 10 kHz using
4 mm in diameter zirconia rotors. The processing and acquisition parameters were 6 ms single pulse duration, recycle time 1 s and 4500 scans.
3. Results and discussion The 7Li-MAS/NMR spectrum of LiTh4F16 exhibits a narrow single line (DH ¼ 8:7 ^ 0:1 ppm) centered at 0 ppm vs a reference of solid LiCl and its spinning side bands indicating that the lithium ions are present in the structure and occupy only one crystallographic site. Fig. 1a shows the first voltammogram of the first cycle for the composite electrode LiU4F16/PVDF/CB. A strong peak centered at 3.7 V is present in the first oxidation process and disappears almost entirely in the following cycles (Fig. 1a and b). This transformation may be related to the irreversible decomposition of the electrolyte PC/ LiClO4. As this anodic peak was not observed with an electrode composed with carbon black and PVDF (CB/ PVDF 80/20%, w/w), this side-reaction seems to be catalyzed by the active material LiU4F16. The products of this reaction were not identified by X-ray diffraction measurements, which were performed with the cycled materials. Further investigations must be performed to precise if the decomposition involves the salt LiClO4 or/ and the solvent. On and after the second cycle, an anodic peak at around 4.1 V was observed. This one could be assigned to the oxidation of the U3þ ions consecutive to the deintercalation of Liþ from the host matrix according to the reaction III IV þ 2 LiUIII UIV 3 F16 ! Li12x U12x U3þx F16 þ xLi þ x e
During the first cycle, the deintercalation of Liþ ions was limited (only a shoulder near 4.0 V was observed); this is certainly due to the difficulties in initiating the deintercalation of Liþ from the raw material. The main cathodic peak present at 2.7 V is assigned to the intercalation of lithium IV ions into the host fluoride matrix Li12x UIII 12x U3þx F16 : A multiphase behavior appears clearly on the voltammograms of the third and fifth cycles (Fig. 1b); the reduction process occurred in two steps associated with the two peaks at 2.6 and 2.1 V. A large hysteresis is observed between the reduction and the oxidation processes since the deintercalation of the Liþ ions occurred at 4.1 and 3.6 V (the attribution of this last peak is ambiguous because it coincides with the potential of the decomposition of the electrolyte). The oxidation process was stopped before its achievement at 3.75 V during the fifth cycle; consecutively, the intensity of the reduction peak at 2.6 V decreased: this confirms that the processes in reduction and in oxidation are correlated. Due to their broad shapes, the two anodic peaks were superimposed in the range of potentials 3.6 – 3.9 V. When the intercalation process was stopped at 3.75 V, the second process associated with the anodic peak at around 4.0 V was
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Fig. 1. Voltammograms of the composite electrode LiU4F16/PVDF/CB in PC/LiClO4 (1.0 mol l21) recorded with a linear scan rate of 0.17 mV s21: first and second cycles (a), third, fourth and fifth cycles (b). The reference markers ða; b and gÞ show the selected potentials at which the EPR spectra of the samples were recorded.
started. This explains the presence of two cathodic peaks for the fifth cycle (Fig. 1b). In order to estimate the capacity x in LixU 4F16 galvanostatic measurements were performed after the voltammograms of the sixth cycle (Fig. 2). Three lithium ions per U4F16 unit were reversibly intercalated. The galvanostatic mode confirms the large hysteresis and the multiphase behavior: two plateaus were present in the reduction process near 2.5 and 2.0 V which are associated
with two transformations in the oxidation run at 3.7 and 4.0 V. The irreversible capacity Dx ¼ xoxidation 2 xreduction ¼ 1 is related to the amount of current used during the decomposition of the electrolyte. The EPR signal of LiU4F16 at room temperature, shown in Fig. 3, is slightly anisotropic and broad (the linewidth DHpp was found equal to 1100 ^ 10 G). The value of the g-factor is equal to 2.3400 ^ 0.0005; this large g-value may reflect the magnetic exchange between
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Fig. 2. Galvanostatic curve of the composite electrode LiU4F16/PVDF/CB after six cycles in the voltammetric mode in PC/LiClO4 (1.0 mol l21, the current density was equal to 10 mA mg21).
uranium ions as proposed by Hinatsu and Fujino [7] in the case of Ce0.1U0.9O2 solid solution ðg ¼ 2:43Þ: The signal is attributed to U3þ and not to U4þ because of the EPR silence of U4þ in spite of the two single electrons of its 5f2 configuration. The EPR parameters DHpp ; the g-factor
Fig. 3. EPR spectra at room temperature of the starting material LiU4F16 and the electrochemically modified ones during the intercalation of Liþ. Insert: temperature dependence of the linewidth and g-factor.
and the ratio A=B (A and B are the magnitudes of the positive and negative peaks, respectively) exhibit a linear temperature dependence (Fig. 3). DHpp and g-factor decrease with increasing the temperature contrary to the A=B ratio for which a positive temperature dependence is observed (A=B ¼ 1:0 and 0.8 at 300 and 110 K, respectively). A motional narrowing of the linewidth is observed; by analogy with other inorganic materials exhibiting mixed-valence such as transition-metal oxide glasses [8], doped-borates Mg1þx Ti12x BO4 [9] and Cu2V2O5 [10], the existence of a hopping conduction mechanism between U3þ and U4þ cation could be expected. The decrease of the g-factor with increasing the temperature indicates an improvement of the electronic mobility, which is clearly thermally activated. When the active material was electrochemically modified by intercalation of Liþ ions (compound noted a; obtained after a complete reduction until 1.5 V) or by deintercalation (the oxidation process was stopped at 4.5 V, the resulting sample is noted g), its EPR spectrum changed drastically. In the fully oxidized sample ðgÞ; except a narrow line attributed to the carbon black ðDHpp ¼ 10:0 ^ 0:2 GÞ; no EPR signal was detected at ambient temperature indicating that the U3þ ions were almost totally oxidized to U 4þ; the formation of paramagnetic U5þ ions (5f1) is limited because of the absence of signal in the limits of detection of the spectrometer. On the contrary, the signal of the sample reduced until 1.5 V is similar to the one of LiU4F16: the linewidth ðDHpp ¼ 1100 ^ 10 GÞ and the position ðg ¼ 2:34Þ are close in the two cases indicating the coexistence of U3þ and U4þ as expected. When the working electrode was partly oxidized until 3.8 V (sample noted b) the intensity of the ESR line and the linewidth ðDHpp ¼ 820 ^ 10 GÞ were lower than for the starting material LiU4F16. The decrease of the spin density, which is related to the U3þ/U4þ ratio, results in the lowering of the dipolar broadening. This explains the line narrowing of the sample b in comparison with LiU4F16 and the compound fully reduced. Fig. 4a displays the voltammogram of the first cycle for the composite UF4/PVDF/CB. The active material was firstly reduced (Fig. 4a) or oxidized (Fig. 4c). By analogy with the case of LiU4F16, the strong peak centered in the range 3.5 –3.6 V in the oxidation wave is related to the decomposition of the electrolyte. This peak is shifted by 200 mV in comparison with the case of LiU4F16. The active material seems to catalyst the irreversible reaction of the electrolyte, the potential of this side-reaction is then dependent of the active material. Moreover, the experimental conditions of the cycling differ in the two cases: the working electrode UF4/PVDF/CB was prior reduced whereas LiU4F16 was firstly deintercalated. When the working electrode composed with UF4 was prior oxidized, the strong anodic peak was then centered at around 3.65 V (Fig. 4c). In this case, the intercalation and deintercalation of Liþ into UF4 take place at 2.6 and 4.1 V, respectively (Fig. 4c).
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the second cycle); such a phenomenon was also observed for the first deintercalation of Liþ into LiU4F16. On the other hand, the oxidation peak at 4.1 V was absent when the electrode was firstly oxidized. This fact confirms the attribution of the peaks at 2.6 and 4.1 V to the intercalation and deintercalation processes. The electrochemical behavior of UF4 (Fig. 4) and LiU4F16 were similar: (i) the total disappearance of the peak related to the electrolyte decomposition occurred after four cycles and (ii) the intercalation and deintercalation processes take place in the same range of potentials. Nevertheless, the difference between the potentials of the two reduction peaks was lower for UF4 than for LiU4F16 (DE ¼ 0:3 and 0.7 V, respectively, for UF4 and LiU4F16). The reversible IV capacity y in Liy UF4 ðLiy UIII y U12y F4 Þ; which was measured with galvanostatic conditions, decreased upon cycling from 0.52 to 0.27 in the second and tenth cycles, respectively. These capacities were calculated from the reduction curve because the decomposition of the electrolyte modifies the oxidation capacity. Two lithium ions can then be reversibly intercalated into four UF4 IV units leading to the composition Li2UIII 2 U2 F16. The fully reduced tetrafluoride UF4 until 1.5 V (noted a) exhibits a broad anisotropic EPR line (Fig. 5, DHpp ¼ 1100 ^ 10 G; g ¼ 2:34) contrary to the sample oxidized until 4.5 V ðbÞ and to the commercial UF4, which were EPR silent. In these ones, U4þ ions are predominant. As for LiU4F16, the line of the sample a and its linewidth have originated from the simultaneous presence of U3þ and U4þ ions and their interaction. The hysteresis between the potentials of the reduction and the oxidation processes into LiTh4F16 is lower than for the homologous uranium sample: the intercalation and the deintercalation of the lithium ions into LiTh4F16 occurred around 2.2 and 2.7 V, respectively. Whereas the þ IV oxidation state is exclusively obtained in aqueous solution, the existence of Th3þ ions in solid compounds were
Fig. 4. Voltammograms of the composite electrode UF4/PVDF/CB in PC/LiClO4 (1.0 mol l21) recorded with a linear scan rate of 0.17 mV s21: first and second cycles (a), third, fourth and fifth cycles (b) during which the working electrode was prior reduced and first and second cycles of UF4/PVDF/CB electrode firstly oxidized (c) The EPR spectra of the samples were recorded at potentials shown by the reference markers ða; bÞ:
Two cycles are necessary to reach the equilibrium potential of the intercalation process because of difficulties to initiate the processes (the cathodic peak is centered near 2.0 V during the first cycle and it shifts to 2.6 V for
Fig. 5. EPR spectra at room temperature of the compounds obtained after a complete reduction ðaÞ or oxidation ðbÞ of the electrode UF4/PVDF/CB during the intercalation-deintercalation of Liþ into UF4.
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previously reported, for example in Ti2S3 sulfide [11] and in Th4F15 hydrate [12].
4. Conclusion The uranium fluoride LiU4F16, falsely reported as LiU4F17 result from the intercalation of lithium ions into UF4 tetrafluorides. The accommodation of lithium ions into open-channels present in the three-dimensional framework of the tetrafluoride structure induces the partial reduction of the tetravalent metal. Such a processes lead to mixed-valence U3þ/U4þ in these fluorides, which can be electrochemically induced or removed. This technique can then be used as a probe to better understand the particular crystal-chemical properties of these compounds. The mixed-valence can be also induced into monoclinic uranium tetrafluoride UF4. During the intercalation of Liþ into tetravalent uranium IV phases Lix UIII x U42x F16 ; a wide range of compositions can be IV obtained from UF4 ðx ¼ 0Þ to Li3UIII 3 U F16 ðx ¼ 3Þ: A new variety of tetragonal tetrafluoride UF4 could be also expected by the complete deintercalation of the lithium ions from IV tetragonal Lix UIII x U42x F16 solid solution. This new variety structurally differs from monoclinic tetrafluoride but also from the tetragonal form a of ZrF4 [13]. Neutron diffraction investigations are in progress in order to determine the location of Liþ ions in the crystal structure of these new phases obtained either by electrochemical and chemical intercalation of Liþ ions into lanthanide or actinide MF4 or IV LixMIII x M4-xF16 (M ¼ U, Th and Ce).
References [1] E.M. Levin, C.R. Robbins, H.F. McMurdie, Phase diagrams for ceramists, The American Ceramic Society, 1964, p. 422. [2] R.E. Thoma, H. Insley, B.S. Landau, H.A. Friedman, W.R. Grimes, Phase equilibria in the fused salt systems LiF–ThF4 and NaF–ThF4, J. Phys. Chem. 63 (1959) 1266–1274. [3] E.M. Levin, C.R. Robbins, H.F. McMurdie, Phase diagrams for ceramists, The American Ceramic Society, 1964, p. 421. [4] A. Cousson, M. Pages, R. Chevalier, Crystal structure of Th6F24·H2O, Acta Crystallogr. B35 (1979) 1763–1765. [5] A. Cousson, M. Pages, J.C. Cousseins, A. Vedrine, Flux growth of fluorides of lithium and thorium or uranium, J. Cryst. Growth 40 (1977) 157 –160. [6] M. Josse, M. Dubois, M. El-Ghozzi, D. Avignant, Synthesis and crystal structure of Rb2AlTb3F16: a new mixed-valence terbium fluoride, Solid State Sci 5 (2003) 1141–1148. [7] Y. Hinatsu, T. Fujino, Study of electron paramagnetic resonance on Ce0.1O0.9O2 solid solution, Chem. Phys. Lett. 172 (1990) 131 –136. [8] L. Bih, M. El Omari, J.-M. Re´au, M. Haddad, D. Boudlich, A. Yacoubi, A. Nadiri, Electronic and ionic conductivity of glasses inside the Li2O–MoO3 – P2O5 system, Solid State Ionics 132 (2000) 71– 85. [9] J. Dumas, M. Continentino, J.J. Capponi, J.L. Tholence, Electron paramagnetic study of the warwickites Mg1 þ xTi1 2 xBO4, Solid State Commun. 106 (1998) 35–38. [10] G. Sperlich, W.D. Laze´, G. Bang, Localized d electrons in the ‘one dimensional’ electronic conductor CuxV2O5 (ESR investigation), Solid State Commun. 16 (1975) 489– 492. [11] W. Zachariasen, Crystal-chemical studies of the 5f series of elements. X. Sulfides and oxysulfides, Acta Crystallogr. 2 (1949) 291–296. [12] W. Zachariasen, Crystal chemical studies of the 5f series of elements. XIX. The crystal structure of the higher thorium hybride, Acta Crystallogr. 6 (1953) 393–395. [13] R. Papiernik, D. Mercurio, B. Frit, Structure du tetrafluorure de zirconium, ZrF4 a, Acta Crystallogr. B38 (1982) 2347– 2353.
Mixed-valence induced during the intercalation of lithium ions into uranium fluorides E. Mapemba, M. Dubois*, M. Josse, M. El-Ghozzi, D. Avignant Laboratoire des Mate´riaux Inorganiques, UMR 6002 CNRS, Universite´ Blaise Pascal, 24, Avenue des Landais, Aubiere 63177, France
Abstract The mixed-valence in uranium fluorides LiU4F16 and UF4 can be induced (or eliminated) by the intercalation (deintercalation) of lithium ions into these compounds. These processes can be electrochemically performed both with LiU4F16 (or LiTh4F16) and with monoclinic UF4 as starting materials using a liquid electrolyte composed of propylene carbonate as solvent and LiClO4 as salt. In the cases of UF4 and LiU4F16, the electrochemical intercalation of Liþ into the host matrix is reversible and a maximum of three Liþ ions per U4F16 unit can be reversibly intercalated. Additional experimental evidences of the mixed-valence in these fluorides are presented such as the Electron Paramagnetic Resonance characterization of the starting materials and of those electrochemically modified. q 2003 Elsevier Ltd. All rights reserved. Keywords: A. Inorganic compounds; B. Chemical synthesis; D. Electron paramagnetic resonance (EPR); D. Electrochemical properties
1. Introduction In connection with a more general study of actinide and lanthanide fluorides, we have been interested in the phases appearing in the binary systems LiF – MF4 (M ¼ Ce, U and Th) and particularly those corresponding to the stoichiometry MF4/LiF ¼ 4. The phase diagrams of the systems LiF –UF4 [1] and LiF – ThF4 [2,3] show the existence of the compounds LiM4F17. Previous crystal structure analyses of these compounds have revealed an ambiguity concerning their stoichiometry; after a new structural investigation the initial LiTh4F17 composition was found to be in fact Th6F24·H2O [4]. These compounds have been prepared as single-crystals using the flux-growth method (ZnCl2 – LiCl eutectic composition was used as the flux) [5]. The authors of this work claim that although no lithium ions are present in this hydrated tetrafluoride, their presence is necessary for the preparation of the compound. The water molecules were certainly introduced in the crystal structure during the abundant washing of the sample with water carried out in order to eliminate the chlorides ZnCl2 and LiCl. An exchange of Liþ by H2O certainly occurred; this one is facilitated by the open-framework of * Corresponding author. Tel.: þ 33-04-73-40-71-05; fax: þ 33-04-73-4071-08. E-mail address: [email protected] (M. Dubois). 0022-3697/$ - see front matter q 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2003.10.036
the structure and is accompanied by the modification of the oxidation state of a part of the thorium ions. Nevertheless, preliminary single-crystal X-ray diffraction investigation suggests mixed-valence for the actinide and lanthanide ions leading to the more likely formula LiM4F16 (better explicit as LiMIIIMIV 3 F16). Indeed, a partial reduction of the tetravalent metal occurs. Such a mixed-valence induced by alkaline ions was already encountered in terbium fluorides RbAl2Tb4F22 (RbAl2TbIIITbIV 3 F22) and Rb2AlTb3F16 (Rb2AlTbIIITbIV 2 F16) [6]. The crystal-chemical properties of tetravalent terbium and uranium fluorides exhibit some similitudes. As it has been observed for these compounds, a statistical distribution between M3þ and M4þ ions occurs in LiM4F16 as shown by the process of the crystallographic data (M3þ and M4þ ions are located and mixed on two crystallographic sites). The three dimensional framework of this structure delimits channels running along the k110l directions where the lithium ions lie. In other words, these lithiated phases result from the intercalation of the lithium ions into MF4 tetrafluorides which simultaneously undergo a tetragonal modification of their monoclinic structure. The accommodation of the lithium ions induces the partial reduction of the tetravalent metal. Such a process was investigated using electrochemical techniques. The electrochemical intercalation, or deintercalation, into these compounds can induce, or remove, the mixed-valence of the uranium and thorium ions by modifying their oxidation
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states. Both LiM4F16 (M ¼ U and Th) compounds and a commercial uranium tetrafluoride UF4 were studied as starting materials using a liquid electrolyte composed of propylene carbonate (PC) as solvent and LiClO4 as salt. The starting compounds and those electrochemically modified were characterized by Electron Paramagnetic Resonance (EPR) in order to evidence the modification of the oxidation state of the uranium ions.
2. Experimental Polycrystalline samples of LiM4F16 (M ¼ U, Th and Ce) were prepared by reacting mixtures of uranium, thorium or cerium tetrafluoride MF4 and lithium fluoride LiF in platinum tubes sealed under dry argon gas. The solid state reactions were carried out in two steps: First, the samples were heated at 300 8C for 24 h and secondly at 650 8C for 72 h. Under these conditions, single-crystals of LiU4F16 were obtained contrary to the case of the thorium phase. Electrochemical measurements were carried out using a conventional two electrodes cell (Swagelok), where lithium was used both as reference and counter electrode. Then, the potentials refer to a Liþ/Li electrode. The working electrode was formed by LiM4F16 or UF4 (about 86% by weight (w/w)), carbon black (CB, 5%, w/w) to insure electronic conductivity, and polyvinylidene difluoride (PVDF, 9%, w/w) as the binder. The mass of active material (LiU4F16 or UF4) was close to 2.0 mg. After stirring in PC, this mixture was spread thinly onto a collector by evaporation of PC. Then it was vacuum dried for 1 h at 150 8C. A PVDF microporous film wetted with LiClO 4 electrolyte (1 mol l21) dissolved in PC was sandwiched between the working electrode and a lithium metal foil. The cells were assembled in an argon-filled glove box. Prior to use LiClO4 (Aldrich, purity 99%) was outgassed under dynamic vacuum at 150 8C and PC was doubly distilled to remove traces of water. Cyclic voltammetry was performed at room temperature between 4.5 and 1.5 V vs Liþ/Li with a linear potential sweeping of 0.17 mV s21. Galvanostatic reduction and oxidation were carried out in the same potentials range; the current density was equal to 10 mA mg21. EPR spectra were recorded using an X Band Bruker EMX spectrometer equipped with a standard variable temperature device and operating at 9.653 GHz. Diphenylpicrylhydrazyl was used to calibrate the resonance frequency ðg ¼ 2:0036 ^ 0:0002Þ: LiM4F16 and UF4 were characterized by EPR spectroscopy in the initial state and after a complete reduction or oxidation. These samples were prior dried under dynamic vacuum, ground and set into a glass tube of 0.5 mm in diameter; the preparation of the samples was carried out in a glove box under inert atmosphere (argon). The presence of lithium ions into the compound LiTh4F16 was checked by 7Li-MAS/NMR using a Bruker Avance DSX 300 spectrometer operating at 116.64 MHz. The spectrum was recorded at a spin rate of 10 kHz using
4 mm in diameter zirconia rotors. The processing and acquisition parameters were 6 ms single pulse duration, recycle time 1 s and 4500 scans.
3. Results and discussion The 7Li-MAS/NMR spectrum of LiTh4F16 exhibits a narrow single line (DH ¼ 8:7 ^ 0:1 ppm) centered at 0 ppm vs a reference of solid LiCl and its spinning side bands indicating that the lithium ions are present in the structure and occupy only one crystallographic site. Fig. 1a shows the first voltammogram of the first cycle for the composite electrode LiU4F16/PVDF/CB. A strong peak centered at 3.7 V is present in the first oxidation process and disappears almost entirely in the following cycles (Fig. 1a and b). This transformation may be related to the irreversible decomposition of the electrolyte PC/ LiClO4. As this anodic peak was not observed with an electrode composed with carbon black and PVDF (CB/ PVDF 80/20%, w/w), this side-reaction seems to be catalyzed by the active material LiU4F16. The products of this reaction were not identified by X-ray diffraction measurements, which were performed with the cycled materials. Further investigations must be performed to precise if the decomposition involves the salt LiClO4 or/ and the solvent. On and after the second cycle, an anodic peak at around 4.1 V was observed. This one could be assigned to the oxidation of the U3þ ions consecutive to the deintercalation of Liþ from the host matrix according to the reaction III IV þ 2 LiUIII UIV 3 F16 ! Li12x U12x U3þx F16 þ xLi þ x e
During the first cycle, the deintercalation of Liþ ions was limited (only a shoulder near 4.0 V was observed); this is certainly due to the difficulties in initiating the deintercalation of Liþ from the raw material. The main cathodic peak present at 2.7 V is assigned to the intercalation of lithium IV ions into the host fluoride matrix Li12x UIII 12x U3þx F16 : A multiphase behavior appears clearly on the voltammograms of the third and fifth cycles (Fig. 1b); the reduction process occurred in two steps associated with the two peaks at 2.6 and 2.1 V. A large hysteresis is observed between the reduction and the oxidation processes since the deintercalation of the Liþ ions occurred at 4.1 and 3.6 V (the attribution of this last peak is ambiguous because it coincides with the potential of the decomposition of the electrolyte). The oxidation process was stopped before its achievement at 3.75 V during the fifth cycle; consecutively, the intensity of the reduction peak at 2.6 V decreased: this confirms that the processes in reduction and in oxidation are correlated. Due to their broad shapes, the two anodic peaks were superimposed in the range of potentials 3.6 – 3.9 V. When the intercalation process was stopped at 3.75 V, the second process associated with the anodic peak at around 4.0 V was
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Fig. 1. Voltammograms of the composite electrode LiU4F16/PVDF/CB in PC/LiClO4 (1.0 mol l21) recorded with a linear scan rate of 0.17 mV s21: first and second cycles (a), third, fourth and fifth cycles (b). The reference markers ða; b and gÞ show the selected potentials at which the EPR spectra of the samples were recorded.
started. This explains the presence of two cathodic peaks for the fifth cycle (Fig. 1b). In order to estimate the capacity x in LixU 4F16 galvanostatic measurements were performed after the voltammograms of the sixth cycle (Fig. 2). Three lithium ions per U4F16 unit were reversibly intercalated. The galvanostatic mode confirms the large hysteresis and the multiphase behavior: two plateaus were present in the reduction process near 2.5 and 2.0 V which are associated
with two transformations in the oxidation run at 3.7 and 4.0 V. The irreversible capacity Dx ¼ xoxidation 2 xreduction ¼ 1 is related to the amount of current used during the decomposition of the electrolyte. The EPR signal of LiU4F16 at room temperature, shown in Fig. 3, is slightly anisotropic and broad (the linewidth DHpp was found equal to 1100 ^ 10 G). The value of the g-factor is equal to 2.3400 ^ 0.0005; this large g-value may reflect the magnetic exchange between
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Fig. 2. Galvanostatic curve of the composite electrode LiU4F16/PVDF/CB after six cycles in the voltammetric mode in PC/LiClO4 (1.0 mol l21, the current density was equal to 10 mA mg21).
uranium ions as proposed by Hinatsu and Fujino [7] in the case of Ce0.1U0.9O2 solid solution ðg ¼ 2:43Þ: The signal is attributed to U3þ and not to U4þ because of the EPR silence of U4þ in spite of the two single electrons of its 5f2 configuration. The EPR parameters DHpp ; the g-factor
Fig. 3. EPR spectra at room temperature of the starting material LiU4F16 and the electrochemically modified ones during the intercalation of Liþ. Insert: temperature dependence of the linewidth and g-factor.
and the ratio A=B (A and B are the magnitudes of the positive and negative peaks, respectively) exhibit a linear temperature dependence (Fig. 3). DHpp and g-factor decrease with increasing the temperature contrary to the A=B ratio for which a positive temperature dependence is observed (A=B ¼ 1:0 and 0.8 at 300 and 110 K, respectively). A motional narrowing of the linewidth is observed; by analogy with other inorganic materials exhibiting mixed-valence such as transition-metal oxide glasses [8], doped-borates Mg1þx Ti12x BO4 [9] and Cu2V2O5 [10], the existence of a hopping conduction mechanism between U3þ and U4þ cation could be expected. The decrease of the g-factor with increasing the temperature indicates an improvement of the electronic mobility, which is clearly thermally activated. When the active material was electrochemically modified by intercalation of Liþ ions (compound noted a; obtained after a complete reduction until 1.5 V) or by deintercalation (the oxidation process was stopped at 4.5 V, the resulting sample is noted g), its EPR spectrum changed drastically. In the fully oxidized sample ðgÞ; except a narrow line attributed to the carbon black ðDHpp ¼ 10:0 ^ 0:2 GÞ; no EPR signal was detected at ambient temperature indicating that the U3þ ions were almost totally oxidized to U 4þ; the formation of paramagnetic U5þ ions (5f1) is limited because of the absence of signal in the limits of detection of the spectrometer. On the contrary, the signal of the sample reduced until 1.5 V is similar to the one of LiU4F16: the linewidth ðDHpp ¼ 1100 ^ 10 GÞ and the position ðg ¼ 2:34Þ are close in the two cases indicating the coexistence of U3þ and U4þ as expected. When the working electrode was partly oxidized until 3.8 V (sample noted b) the intensity of the ESR line and the linewidth ðDHpp ¼ 820 ^ 10 GÞ were lower than for the starting material LiU4F16. The decrease of the spin density, which is related to the U3þ/U4þ ratio, results in the lowering of the dipolar broadening. This explains the line narrowing of the sample b in comparison with LiU4F16 and the compound fully reduced. Fig. 4a displays the voltammogram of the first cycle for the composite UF4/PVDF/CB. The active material was firstly reduced (Fig. 4a) or oxidized (Fig. 4c). By analogy with the case of LiU4F16, the strong peak centered in the range 3.5 –3.6 V in the oxidation wave is related to the decomposition of the electrolyte. This peak is shifted by 200 mV in comparison with the case of LiU4F16. The active material seems to catalyst the irreversible reaction of the electrolyte, the potential of this side-reaction is then dependent of the active material. Moreover, the experimental conditions of the cycling differ in the two cases: the working electrode UF4/PVDF/CB was prior reduced whereas LiU4F16 was firstly deintercalated. When the working electrode composed with UF4 was prior oxidized, the strong anodic peak was then centered at around 3.65 V (Fig. 4c). In this case, the intercalation and deintercalation of Liþ into UF4 take place at 2.6 and 4.1 V, respectively (Fig. 4c).
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the second cycle); such a phenomenon was also observed for the first deintercalation of Liþ into LiU4F16. On the other hand, the oxidation peak at 4.1 V was absent when the electrode was firstly oxidized. This fact confirms the attribution of the peaks at 2.6 and 4.1 V to the intercalation and deintercalation processes. The electrochemical behavior of UF4 (Fig. 4) and LiU4F16 were similar: (i) the total disappearance of the peak related to the electrolyte decomposition occurred after four cycles and (ii) the intercalation and deintercalation processes take place in the same range of potentials. Nevertheless, the difference between the potentials of the two reduction peaks was lower for UF4 than for LiU4F16 (DE ¼ 0:3 and 0.7 V, respectively, for UF4 and LiU4F16). The reversible IV capacity y in Liy UF4 ðLiy UIII y U12y F4 Þ; which was measured with galvanostatic conditions, decreased upon cycling from 0.52 to 0.27 in the second and tenth cycles, respectively. These capacities were calculated from the reduction curve because the decomposition of the electrolyte modifies the oxidation capacity. Two lithium ions can then be reversibly intercalated into four UF4 IV units leading to the composition Li2UIII 2 U2 F16. The fully reduced tetrafluoride UF4 until 1.5 V (noted a) exhibits a broad anisotropic EPR line (Fig. 5, DHpp ¼ 1100 ^ 10 G; g ¼ 2:34) contrary to the sample oxidized until 4.5 V ðbÞ and to the commercial UF4, which were EPR silent. In these ones, U4þ ions are predominant. As for LiU4F16, the line of the sample a and its linewidth have originated from the simultaneous presence of U3þ and U4þ ions and their interaction. The hysteresis between the potentials of the reduction and the oxidation processes into LiTh4F16 is lower than for the homologous uranium sample: the intercalation and the deintercalation of the lithium ions into LiTh4F16 occurred around 2.2 and 2.7 V, respectively. Whereas the þ IV oxidation state is exclusively obtained in aqueous solution, the existence of Th3þ ions in solid compounds were
Fig. 4. Voltammograms of the composite electrode UF4/PVDF/CB in PC/LiClO4 (1.0 mol l21) recorded with a linear scan rate of 0.17 mV s21: first and second cycles (a), third, fourth and fifth cycles (b) during which the working electrode was prior reduced and first and second cycles of UF4/PVDF/CB electrode firstly oxidized (c) The EPR spectra of the samples were recorded at potentials shown by the reference markers ða; bÞ:
Two cycles are necessary to reach the equilibrium potential of the intercalation process because of difficulties to initiate the processes (the cathodic peak is centered near 2.0 V during the first cycle and it shifts to 2.6 V for
Fig. 5. EPR spectra at room temperature of the compounds obtained after a complete reduction ðaÞ or oxidation ðbÞ of the electrode UF4/PVDF/CB during the intercalation-deintercalation of Liþ into UF4.
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previously reported, for example in Ti2S3 sulfide [11] and in Th4F15 hydrate [12].
4. Conclusion The uranium fluoride LiU4F16, falsely reported as LiU4F17 result from the intercalation of lithium ions into UF4 tetrafluorides. The accommodation of lithium ions into open-channels present in the three-dimensional framework of the tetrafluoride structure induces the partial reduction of the tetravalent metal. Such a processes lead to mixed-valence U3þ/U4þ in these fluorides, which can be electrochemically induced or removed. This technique can then be used as a probe to better understand the particular crystal-chemical properties of these compounds. The mixed-valence can be also induced into monoclinic uranium tetrafluoride UF4. During the intercalation of Liþ into tetravalent uranium IV phases Lix UIII x U42x F16 ; a wide range of compositions can be IV obtained from UF4 ðx ¼ 0Þ to Li3UIII 3 U F16 ðx ¼ 3Þ: A new variety of tetragonal tetrafluoride UF4 could be also expected by the complete deintercalation of the lithium ions from IV tetragonal Lix UIII x U42x F16 solid solution. This new variety structurally differs from monoclinic tetrafluoride but also from the tetragonal form a of ZrF4 [13]. Neutron diffraction investigations are in progress in order to determine the location of Liþ ions in the crystal structure of these new phases obtained either by electrochemical and chemical intercalation of Liþ ions into lanthanide or actinide MF4 or IV LixMIII x M4-xF16 (M ¼ U, Th and Ce).
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