Electrochemical Formation of Divalent Samarium Cation and Its

Jun 28, 2018 - Synopsis. Herein, we have electrochemically prepared the stable divalent samarium (Sm) cation in LiCl-KCl eutectic melt. We investigate...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Electrochemical Formation of Divalent Samarium Cation and Its Characteristics in LiCl−KCl Melt Sang-Eun Bae,*,†,‡ Tae Sub Jung,§,∥ Young-Hwan Cho,† Jong-Yun Kim,†,‡ Kyungwon Kwak,*,§,∥ and Tae-Hong Park*,†,‡

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Nuclear Chemistry Research Division, Korea Atomic Energy Research Institute, 989-111 Daedeok-daero, Yuseong-gu, Daejeon 34057, Korea ‡ Department of Radiochemistry & Nuclear Nonproliferation, University of Science and Technology, 217 Gajeong-ro Yuseong-gu, Daejeon 34057, Korea § Center for Molecular Spectroscopy and Dynamics, Institute for Basic Science (IBS), Korea University, Seoul 02841, Korea ∥ Department of Chemistry, Korea University, Seoul 02842, Korea ABSTRACT: The electrochemical reduction of trivalent samarium in a LiCl−KCl eutectic melt produced highly stable divalent samarium, whose electrochemical properties and electronic structure in the molten salt were investigated using cyclic voltammetry, UV−vis absorption spectroscopy, laser-induced emission spectroscopy, and density functional theory (DFT) calculations. Diffusion coefficients of Sm2+ and Sm3+ were electrochemically measured to be 0.92 × 10−5 and 1.10 × 10−5 cm2/s, respectively, and the standard apparent potential of the Sm2+/3+ couple was estimated to be −0.82 V vs Ag|Ag+ at 450 °C. The spectroelectrochemical study demonstrated that the redox behavior of the samarium cations obeys the Nernst equation (E°′ = −0.83 V, n = 1) and the trivalent samarium cation was successfully converted to the divalent cation having characteristic absorption bands at 380 and 530 nm with molar absorptivity values of 1470 and 810 M−1 cm−1, respectively. Density function theory calculations for the divalent samarium complex revealed that the absorption signals originated from the 4f6 to 4f55d1 transitions. Additionally, laser-induced emission measurements for the Sm cations in the LiCl− KCl matrix showed that the Sm3+ ion in the LiCl−KCl melt at 450 °C emitted an orange color of fluorescence, whereas a red colored emission was observed from the Sm2+ ion in the solidified LCl-KCl salt at room temperature.

1. INTRODUCTION The pyrochemical process enables separation of actinides from other FPs in spent nuclear fuel in a proliferation-resistant manner and provides new types of fuels for next-generation nuclear systems such as a fast reactor, where separated longlived actinides and fission products are recycled and transmuted.1 It uses electrochemical methods in a medium of molten salts, which provide several advantages for the separation of actinides from fission products in spent nuclear fuel, such as the following. (i) High thermal and radiation stability of the inorganic molten salt allows effective dissolution of the highly radioactive spent nuclear fuel and treatment of the hot fuel with a shorter cooling time. (ii) The compact equipment and recyclability of the molten salt lead to costeffective reduction of radioactive waste from the process. (iii) The incapability to recover pure plutonium with a liquid electrode fulfills proliferation resistance.2−4 Therefore, further development of the pyrochemical process is crucial to achieving a closed fuel cycle that aims to reduce the volume and long-term radiotoxicity of spent nuclear fuel as well as attain sustainable nuclear energy. The central unit operation of the pyrochemical process is the electrorefining stage, where spent nuclear fuel is dissolved in a molten chloride salt, uranium (U) is selectively deposited on a solid electrode, and then transuraniums (TRUs) and residual © XXXX American Chemical Society

U are collectively partitioned in a liquid electrode from other fission products (FPs). Since lanthanides comprise ∼1 wt % of FPs in the spent nuclear fuel,5 they can significantly accumulate in the reaction medium and modify the electrolyte characteristics during the electrorefining process, which may also lead to codeposition of TRUs and lanthanides in the liquid electrode.6 Because lanthanides tend to be segregated under thermal treatment during TRU fuel fabrication and their high neutron capture cross section damages the neutron economy of the next-generation reactors,1 the lanthanide concentration needs to be well controlled during the process. Therefore, understanding of the electrochemical and chemical behavior of the lanthanides as well as actinides in the molten salt is critical for achieving successful sustainable nuclear energy systems. Samarium (Sm),3,7 europium (Eu),8 and ytterbium (Yb)9 form stable and soluble divalent ions, as well as trivalent ones, in the molten salt unlike other lanthanides, which exist exclusively as trivalent ions in the melt. Moreover, these divalent ions can be electrochemically deposited as alloys in reactive solid electrodes3 and liquid electrodes,10 though their electrodeposition on a stable solid electrode is beyond the potential window offered by the molten salt. Since Sm, one of Received: April 4, 2018

A

DOI: 10.1021/acs.inorgchem.8b00909 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry the most abundant lanthanide elements in the fission products, exhibits a neutron poisoning effect,3 particular attention should be paid to the concentration of Sm in the nuclear fuel to avoid malfunction of an advanced nuclear reactor.1 Therefore, the electrochemical and chemical behaviors of divalent as well as trivalent Sm in the molten salt need to be well understood and controlled in the pyrochemical process. Divalent Sm has also attracted attention in the fields of organic synthesis11,12 and photoluminescence,13,14 where it has shown its high reducing power and unique optical properties. Since metal−ligand interactions influence the reactivity and chemistry of divalent lanthanide complexes,15,16 understanding electronic structures of the ligand environments can provide valuable information on the nature of the lanthanide−ligand bond17 and the chemical behavior of the complexes in condensed media.18 Spectroscopic studies of Sm ions have been exclusively performed to investigate their electronic properties19 as well as ligand structures.20 In particular, many studies on divalent Sm in solid matrices have been reported because of its stability and useful emission features.15,21 However, only a few reports have discussed the preparation and electronic structure of the divalent Sm ion in liquid phases22−27 because the divalent Sm ion may not be stable in solutions.19 In this study, the reduction of trivalent Sm in a LiCl−KCl eutectic produced divalent Sm stable in the molten salt solution. The oxidation state of the Sm ion was controlled electrochemically. The redox properties and diffusion coefficients of both divalent and trivalent Sm ions were determined using cyclic voltammetry. The optical properties of the divalent Sm ion in the LiCl−KCl eutectic melt were investigated using UV−vis absorption spectroscopy. The intense electronic absorption bands of divalent Sm distinguished it from trivalent Sm, allowing for the spectroelectrochemical monitoring of the redox reaction. A computational study revealed the origins of the electronic transitions of the divalent Sm ion in the LiCl−KCl melt. In addition, emission properties of the Sm cations in LiCl−KCl were examined using laser-induced emission spectroscopy at 450 °C and room temperature.

Figure 1. Schematic of a spectroelectrochemical cell used in this study. assembly composed of QE65 Pro, DH-2000, and optical fiber connections. To measure the emission of Sm ions in a molten LiClKCl eutectic, the third harmonic beam (wavelength of 355 nm) of a Nd:YAG laser (Quantel, Brilliant, pulse width of ∼6 ns and repetition rate of 10 Hz) was used as an excitation light source. The laser pulse energy used in this experiment was 10 mJ. The emission signal was detected with a photomultiplier tube (Hamamatsu Photonics, R928) coupled with a 300 mm focal length monochromator (Acton Research Corp., Czerny-Turner type SpectraPro-2300i). A holographic grating (1200 grooves/mm) was installed in the monochromator. Emission perpendicular to the propagation direction of the laser beam was delivered to the entrance slit of the monochromator through a specially designed optical fiber bundle (2 × 8 mm2, 345 fibers). The slit width was set at 0.5 mm for all measurements in this experiment. Emission spectra were measured at 450 °C using the photon counting system (Acton Research Corp.) to investigate the dependence of emission on the concentration of SmCl3. The experiments and sample preparation were undertaken in a glovebox under argon atmosphere, where the oxygen content and moisture levels were maintained at less than 1 ppm.

3. COMPUTATIONAL DETAILS All density functional theory (DFT) calculations were performed using the Gaussian 09 program package.29 For [SmCl6]3− and [SmCl6]4− complexes, the basis set chosen for the Cl ion was 6-31G(d′).30−33 The Wood-Boring quasirelativistic effective core potential method with a 28 core electron system (MWB28)34 and the corresponding basis sets were used for the Sm ion. The complexes were energetically optimized by Becke 3-parameter exchange and Lee−Yang− Parr correlation functionals with the Coulomb-attenuating method (CAM-B3LYP).35 Time-dependent DFT calculations for [SmCl6]4− were performed to obtain a simulated UV−vis spectrum with the same basis sets and functionals.36−39 To consider the effect of the surrounding LiCl-KCl melt, a polar continuum solvation model was adopted with a dielectric constant (ε) of 2.3741, because it is known that the dielectric constant of ionic liquids ranges from 2 to 3.40 We performed a natural bonding orbital (NBO) analysis using NBO 3.141 equipped within the Gaussian 09 program package and a natural transition orbital (NTO) analysis to assign UV−vis spectrum signals.

2. EXPERIMENTAL SECTION Lithium chloride−potassium chloride (LiCl−KCl) eutectic salts (anhydrous beads) and samarium trichloride (SmCl3) were obtained from Sigma-Aldrich Co. Ltd. (purity ≥ 99.99%). Silver chloride (AgCl) was purchased from Alfa Aesar (purity ≥ 99.998%). All the chemicals were used as received. A spectroelectrochemical reaction vessel (Figure 1) was made with a quartz cell (Hellma Co.) and a quartz tube (350 mm in length, 15 mm in outer diameter, and 2 mm in wall thickness) using a glass blowing technique. Tungsten (W) wire and foil electrodes were used as working electrodes for electrochemical and spectroelectrochemical measurements, respectively. As illustrated in Figure 1, the reference and counter electrodes were encased in Pyrex tubes containing the LiCl−KCl eutectic melt to prevent electrical contact. A silver (Ag) wire was immersed in the melt containing 1.0 wt % of AgCl for the reference electrode and a coil of silver wire was used for the counter electrode.28 All potentials are reported with respect to the Ag|Ag+ reference electrode unless otherwise specified. Finally, the top of the reference electrode was sealed with Teflon tape to prevent evaporation of the molten salt at high temperatures. The temperature of the molten salt was measured using a calibrated K type Chromel−Alumel thermocouple wire. Cyclic voltammograms (CVs) were obtained using a Gamry Reference 3000 interfaced with a computer. UV−vis spectra were obtained using an Ocean Optics

4. RESULTS AND DISCUSSION 4.1. Electrochemical Measurements. To investigate the electrochemical behavior of the trivalent and divalent Sm cations, we carried out cyclic voltammetry in LiCl-KCl melts. Figure 2a shows a CV of Sm3+ in a LiCl-KCl eutectic (96 mM) B

DOI: 10.1021/acs.inorgchem.8b00909 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

also confirmed that the redox reactions were reversible. Figure 2c shows the respective cathodic and anodic peak currents of Sm3+ and Sm2+ as a function of scan rates. The scan rate dependence for Sm2+ was obtained after converting Sm3+ to Sm2+ in a LiCl-KCl melt by applying a cathodic potential of −1.2 V for an hour.9,19 It is known that Sm2+ tends to disproportionate into Sm3+ and Sm0 in solution phases including ionic liquids.19,22 Similar results have also been observed for Nd2+ in a LiCl-KCl molten salt.43 However, Sm2+ generated in the LiCl-KCl melt at 450 °C was found to be stable during the whole electrochemical and spectroscopic measurements (vide infra). The peak currents of the Sm cations increased linearly proportional to the square root of the scan rates, implying that the redox reactions of the Sm cations were limited by its diffusion rates. The diffusion coefficients of the Sm cations in the LiCl-KCl melt can be determined by using the Randles−Sevcik equation42 ij F 3 yz zz i p = 0.4463jjj z k RT {

1/2

n3/2AD01/2C0*v1/2

(2)

where ip is the peak current, F is the Faraday constant, R is the gas constant, T is the temperature, n is the number of electrons, A is the electrode area, D0 is the diffusion coefficient of the ion, C0 is the bulk concentration of the ion, and ν is the scan rate. The diffusion coefficients of Sm2+ and Sm3+ were estimated to be 0.92 × 10−5 and 1.10 × 10−5 cm2/s, respectively, from the slopes of the linear regressions. The value for Sm3+ was close to one previously reported7 and Sm2+ exhibited a slightly smaller diffusion constant than Sm3+. Therefore, the apparent standard reduction potential for the Sm2+/3+ redox reactions was determined to be −0.82 V vs Ag| Ag+, using eq 3.44 i Ep,1a + Ep,1c yz RT ijj DLn2+ zz − lnjj E 0′ = jjjj z 2 nF jk DLn3+ k {

with a W working electrode at 450 °C. The redox currents of Sm2+/3+ were observed around −0.85 V vs Ag|Ag+. The Li deposition/oxidation and chlorine evolution currents appeared at −2.4 V and +1.2 V, respectively, well consistent with the CV results reported previously.3 The separation between the anodic and cathodic peak potentials was estimated to be 0.15 V, which is close to the value for reversible soluble−soluble reactions (i.e., 0.14 V) as given by the equation RT nF

(3)

where E0′ is the apparent standard reduction potential for Sm2+/3+ redox reactions, Ep,1a and Ep,1c are the peak potentials for the anodic and cathodic reactions, respectively, and DLn is the diffusion coefficient of the Ln ion. 4.2. Spectroelectrochemical Study of Sm3+. The Sm3+ ion has five electrons occupying the 4f orbitals and their f−f transitions appear at around 400 nm19 and 1600 nm.20,45 Although the transitions between f orbitals of the lanthanide are strictly parity forbidden and many f−f transitions are also spin forbidden, they exhibit weak and narrow adsorption bands because spin−orbit coupling moderately attenuates the forbiddenness.46 However, these intraconfigurational transitions associated with Sm3+ are too weak to be observed at a solution concentration such as 2.6 mM, as seen in Figure 3a, which shows electronic absorption spectra obtained from a LiCl-KCl melt containing Sm3+ with varying potentials in the region where the redox currents of Sm2+/3+ appear in the CV (Figure 2a). Under the potential applied, the individual spectrum was recorded when it became unchanged, which normally took around 60 min. As the applied potentials progressed toward a negative direction, the intensities of new broad bands at 380 and 530 nm gradually increased and their absorbance was maximum at the more negative potential than −0.97 V (Figure 3b). This implies that nearly all of the Sm3+ ions turned into Sm2+ and thus the new bands were ascribed to the electronic transitions of Sm2+ in the LiCl-KCl melt. The

Figure 2. Cyclic voltammograms (CVs) with a wide potential range (a) and at various scan rates (b) obtained using a W electrode in a LiCl-KCl melt containing 96 mM SmCl3 at 450 °C. (c) Peak currents as a function of scan rates. Cathodic peak currents were obtained from CVs in panel b while the anodic ones were measured from CVs performed in a LiCl-KCl melt containing Sm2+.

(Epa − Epc)/mV = 2.22

yz zz zz {

(1)

where Epa and Epc are the peak potentials for anodic and cathodic reactions, respectively, R is the gas constant, T is the temperature, n is the number of electrons, and F is the Faraday constant.42 Figure 2b shows the CVs of Sm3+ at different scan rates. The peak potentials were unchanged at various scan rates, which C

DOI: 10.1021/acs.inorgchem.8b00909 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

determined according to the Nernst equation, as shown below42 E = E0 +

RT ijj αO yzz RT ijj γO[O] zyz zz lnjj zz = E 0 + lnjj j z nF k αR { nF jk γR [R] z{

(5)

where E is the standard reduction potential, αO and αR are the activities, γO and γR are the activity coefficients, and [O] and [R] are the concentrations of the oxidized and reduced forms, respectively. The activities can be expressed by the activity coefficients and concentrations of the ions. The constant terms, E0 and activity coefficients, are substituted with the apparent standard reduction potential, E0′. 0

E = E0 +

RT ijj γO yzz RT ijj [O] yzz lnjj zz + lnj z nF jk γR z{ nF jk [R] z{

= E 0′ +

where E 0′ = E 0 +

(6)

RT ijj γO yzz lnjj zz nF jk γR z{

The concentration ratio of Sm3+ and Sm2+ can be derived from Figure 3a. A relationship between the applied potentials and the logarithm of the concentration ratio is depicted in Figure 3c. It was noted that the logarithm of the concentration ratio of Sm3+ and Sm2+ was linearly proportional to the applied potentials. The slope of linear regression (RT/nF) was 65 mV, which is very close to the ideal value of 62 mV when the number of the electrons (n) involved in the reaction is one. Therefore, it was confirmed that the redox reactions of the Sm cations in the LiCl−KCl melt take place through a oneelectron pathway, as shown in eq 4. Furthermore, the apparent standard reduction potential obtained directly from the spectroelectrochemical method was −0.83 V, in excellent agreement with the one obtained using the electrochemical method (−0.82 V) in Figure 2. 4.3. DFT Quantum Calculations. To simulate the UV− vis spectrum of the divalent Sm ion, apparently dissimilar to that of the trivalent Sm ion, in the LiCl−KCl eutectic melt, we assumed that the Sm ions exist in this melt as octahedral complexes with six chloride ions. Several spectroscopic studies have suggested that trivalent lanthanide chloride complexes possess the octahedral symmetry of [LnCl6]3− in the LiCl− KCl molten salt.20,48 In addition, 6-coordinate octahedral [LnCl6]3− complexes have also been found in chloride-rich ionic liquid environments.19 The reversible redox behavior of Sm3+/2+ (Figure 2) and their similar diffusion coefficients in the LiCl−KCl melt suggested that a significant change in the coordination environment of the Sm complex, such as ligand dissociation or association, was unlikely to occur during the reduction of [SmCl6]3− under the condition of this work. The optimized structure showed octahedral symmetry for both [SmCl6]4− and [SmCl6]3− complexes. Table 1 shows the structural information for the complexes, such as the distance between the metal center and ligand, which was 3.160 Å for [SmCl6]4− and 2.772 Å for [SmCl6]3− complexes. The computed Sm−Cl distance for [SmCl6]3− was very close to that determined by the single crystal X-ray diffraction study of (NEt4)2SmCl6 (2.71 ± 0.01 Å).17

Figure 3. UV−vis absorption spectra obtained at 450 °C from LiCl− KCl containing Sm cations at various potentials (a), absorbance at 380 and 530 nm as a function of applied potentials (b), and relationship between the concentration ratio (Sm3+/2+) and the applied potentials obtained from panel a (c).

molar absorptivity values of these two bands were estimated to be 1460 and 810 M−1 cm−1, respectively. Figure 3b shows the photos of the melt before and after the reduction of Sm3+, demonstrating that the colorless LiCl-KCl melt containing Sm3+ changed to the reddish brown solution of Sm2+ by the reduction reaction. Absorbance recorded during an electrochemical reaction provides information about the concentrations of the absorbing ions that undergo the redox reaction.9,47 The absorbance observed in Figure 3 is proportional to the concentrations of Sm3+ and Sm2+ according to Beer’s law. At the potentials applied, the electrochemical reaction involving the Sm cations on a W electrode is as shown below. Sm 3 + + e− F Sm 2 +

RT ijj [O] yzz lnj z nF jk [R ] z{

(4) 2+

Therefore, the production of Sm on the electrode exhausts an equivalent of Sm3+. By measuring the peak absorbance of Sm2+ in LiCl−KCl at each potential, it is possible to find the concentration ratio of Sm3+ and Sm2+. An apparent standard reduction potential of the Sm2+/3+ redox reaction can be D

DOI: 10.1021/acs.inorgchem.8b00909 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Bond Length Obtained from the Simulated Samarium Chloride Complexes and Their Electronic Configuration

Table 2. Electronic Transition Orbitals Contributing to the UV−vis Spectrum of Sm2+ in LiCl−KCl

electron configuration

complex 4−

[SmCl6] [SmCl6]3−

Sm−Cl bond length (Å) 3.160 2.772

Cl−

Smn+(n=2or3) 6.00

0.37

0.17

0.31

0.01

[Xe]4f 5d 6s 6p 6d [Xe]4f5.095d1.166s0.276p0.556d0.04

[Ne]3s1.963p5.90 [Ne]3s1.893p5.76

Electron populations for each atomic orbital were determined using NBO population analysis and are listed in Table 1. In the [SmCl6]4− complex, the central Sm2+ atom gained one electron from six Cl− ions and the electron from the ligands filled the vacant orbitals of Sm2+ composed of the 5d, 6s, and 6p orbitals. For the [SmCl6]3− complex, however, two electrons from six Cl− ions were transferred and filled the Sm3+ orbitals composed of mainly 5d orbitals and also 6s and 6p orbitals. From this numerical analysis, we determined more electron transfer from the ligand to the metal in the case of [SmCl6]3− than [SmCl6]4−, and demonstrated that the 5d orbitals of [SmCl6]3− were involved in the covalency of the Sm−Cl bond with the 4f orbitals playing a minimal role.17 Figure 4 shows the simulated UV−vis spectrum of [SmCl6]4− (black line) obtained from the time-dependent DFT calculations. From the calculation results, we could obtain the peak position and oscillator strength for each peak, which are listed in Table 2 and are represented as bars in the spectrum (red line). For direct comparison between the calculated and experimental spectra, a UV−vis spectrum was simulated by convolution with Gaussian line shape functions for each transition. The Gaussian line width for each transition was set to 0.4 eV and the calculated oscillator strength determined the peak area for each transition. As shown in Figure 4, the simulation result explained the absorption spectrum of Sm2+ in the LiCl-KCl melt, showing two comparable absorption peaks at around 300 and 455 nm, though there were some differences in the peak position. To investigate the electronic transitions and the related electronic structure, we performed natural transition orbital (NTO) analysis. Table 2 describes the NTO analysis results that show

the electronic transition orbitals contributing to the electronic transitions in the UV−vis spectrum. The two peaks at 300 and 455 nm in the [SmCl6]4− spectrum were assigned to the transitions from 4f-originated a2u, t1u, and t2u to 5d-originated eg* and t2g*.19,46,49 Johnson et al. reported the Sm2+ spectrum in a LiCl−KCl molten salt, where only the band at 530 nm was observed.22 SmX2 (X = I, Br, Cl, and OTf) showed charge transfer bands at 500−800 nm in THF with different oxidation potentials,12,50 implying that the coordination environment significantly affects the electronic structure of Sm2+ and thus its optical property as well as reactivity. In this work, Sm2+ in the LiCl− KCl melt was obtained by reducing Sm3+ and exhibited a dramatic change in the absorption spectrum (Figures 3). The

Figure 4. Simulated UV−vis absorption spectrum obtained using the TDDFT calculations for the [SmCl6]4− complex. The red bars represent the wavelength and oscillator strength for the calculated transitions. E

DOI: 10.1021/acs.inorgchem.8b00909 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Sm2+ ion showed two broad and intense absorption bands centered at 380 and 530 nm. The TDDFT result of [SmCl6]4− has shown that these bands could be attributed to the transitions from 4f6 to 4f55d. The molar absorptivities for two bands of Sm2+ in the LiCl−KCl melt at 450 °C were measured to be 1470 and 810 M−1 cm−1. These values are comparable to those obtained for a trivalent uranium cation in LiCl−KCl melt, ranging between 500 and 1900 M−1 cm−1.51 4.4. Laser-Induced Emission Spectroscopy. Emission spectroscopy also provides information on the species of the lanthanide ion in a LiCl−KCl melt and their electronic structure.15,52,53 Figure 5a shows emission spectra obtained from a LiCl−KCl melt containing Sm3+ at 450 °C with excitation at 355 nm. Sm3+ exhibited an orange melt under irradiation (Figure 5 inset) and emitted at the wavelengths of 565, 600, and 645 nm, which were assigned to the transitions from the excited state 5G5/2 to the ground states 6Hj, j = 5/2, 7/2, and 9/2, respectively.54−56

For investigation of the emission of Sm2+, 1.0 wt % of Sm3+ in a melt was electrochemically reduced to Sm2+ by applying a potential of −1.2 V. However, the reduction reaction only decreased the emission signals of Sm3+, which completely disappeared after 840 s, and no other emission signals were observed (Figure 5b), indicating that Sm2+ does not emit in LiCl−KCl at 450 °C. However, the solidified LiCl−KCl salt showed a broad emission band (Figure 5c) ranging from 650 to 850 nm at room temperature. A similar spectrum has been found for Sm2+-doped potassium halide57 and the origin of the luminescence can be assigned to a mixture of the transitions from 5D0 to 7FJ or 5D1 to 7FJ (J = 0, 1, 2, 3, 4).21 However, the room-temperature emission for Sm3+ from the solidified LiClKCl salt was not observed, which has been reported to appear at 563, 600, 650, and 708 nm.56 Therefore, Sm2+ electrochemically generated in the LiCl-KCl melt maintained the oxidation state during the solidification. Because the resulting Sm2+-LiCl-KCl salt was stable at room temperature, it may be used as a Sm2+ source for spectral hole burning crystals or glasses.58,59

5. CONCLUSIONS The electrochemical and spectroscopic studies of the samarium cations demonstrated that the electrochemical reduction of trivalent samarium afforded highly stable divalent samarium in a LiCl−KCl eutectic melt, suggesting that the pyrochemical process for spent nuclear fuel treatment is possibly affected by the chemical behavior of divalent as well as trivalent samarium in the molten salt. The electrochemical measurements showed the reversible redox behavior of Sm3+/2+ in the melt and similar diffusion coefficients of both Sm2+ and Sm3+. The UV−vis absorption spectrum of Sm2+ showed broad absorption bands at 380 and 530 nm in the LiCl−KCl melt, while the initial Sm3+ solution before the reduction hardly manifested its f−f transitions. DFT computation results for the [SmCl6]4− and [SmCl6]3− complexes demonstrated that the Cl− ions were more tightly bound to Sm3+ than Sm2+, and the high absorbing transitions of Sm2+ were assigned to the transitions from 4f6 to 4f55d1. In addition, laser-induced emission spectroscopy study of the solidified salt indicated that the oxidation state of Sm2+ generated in the LiCl-KCl melt remained even after cooling it down, which is potentially useful for the preparation of divalent Sm reagents for optical and organic synthesis applications.

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AUTHOR INFORMATION

Corresponding Authors

*Tel: +82 42 868 8451. Fax: +82 42 868 8148. E-mail: sebae@ kaeri.re.kr. *Tel: +82 2 3290 3128. Fax: :+82 2 3290 2121. E-mail: [email protected]. *Tel: +82 42 868 2220. Fax: +82 42 868 8148. E-mail: [email protected]. ORCID Figure 5. (a) Laser-induced emission of LiCl−KCl containing various concentrations of Sm3+ cations at 450 ° and an image capturing the emission (inset). (b) Laser-induced emission of the LiCl−KCl melt containing Sm3+ cations during application of a potential of −1.2 V. Emission obtained before (1) and during (2 and 3) the potential applicationn. (c) Laser-induced emission obtained from the LiCl-KCl salt containing Sm2+ cations at room temperature after the potential application of −1.2 V. The inset shows an image representing Sm2+ emission in the LiCl−KCl salt at room temperature.

Sang-Eun Bae: 0000-0003-2668-8950 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. F

DOI: 10.1021/acs.inorgchem.8b00909 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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ACKNOWLEDGMENTS This work was supported under the mid- and long-term nuclear research and development program through the National Research Foundation of Korea (NRF2017M2A8A5014710) funded by the Korean Ministry of Science and ICT.

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