Mechanism of Dy3+ and Nd3+ Ions Electrochemical Coreduction with Ni2+, Co2+, and Fe3+ Ions in Chloride Melts

The present paper is devoted to the study of the processes of the mechanism of electrochemical coreduction of Dy3+ and Nd3+ ions with Ni2+, Co2+, and Fe3+ ions in the equimolar NaCl-KCl melt at 973 K and characterization of the synthesized samples. The performed voltammetry analysis of the electrochemical coreduction processes elucidated a significant difference in the values of the extraction potentials of the studied metals. This melt testifies that intermetallic compounds of Dy and Nd with Ni, Co, and Fe may be synthesized in the kinetic regime. The intermetallic phases of Dy and Nd with Ni, Co, and Fe are found to be formed along with the phases of metallic Ni, Co, and Fe either during electrolysis at the cathode current densities exceeding the limiting diffusion current of Ni2+, Co2+, and Fe3+ ions or in the potentiostatic regime at the potentials of the corresponding voltammetry curves. Therefore, the following interrelated key parameters affecting the electrochemical synthesis of Dy and Nd intermetallic compounds with Ni, Co, and Fe were determined: (i) composition of the electrolyte, i.e., concentrations of FeCl3, CoCl2, NiCl2, DyCl3, and NdCl3; (ii) cathode current density or electrolysis potential and (iii) electrolysis time. The obtained samples were characterized by micro-X-ray diffraction analysis, cyclic voltammetry, and scanning electron microscopy methods.


Introduction
In the 21st century, emerging and conventional functional materials have a great impact on the quality of our lives. We have become technology-coddled, as technologies make our lives more comfortable. We are used to microwave ovens, laptops, digital phones, and high-speed transport. All these engineering achievements are possible because of new functional and construction materials. A special place should be given to the materials based on rare-earth intermetallic and refractory compounds, such as magnets, fluorescent substances, alloys for hydrogen accumulation, etc. Magnets, based on rare-earth metals (REM)-precisely, Nd-Fe-B-are becoming increasingly used, because of their record high characteristics as opposed to other permanent magnets. First, they exhibit higher magnetic fields and technical characteristics; secondly, these magnets are cheaper and more available due to the raw materials used for their fabrication. Apart from military-industrial complex and nuclear industry being traditional consumers of the novel materials, such fields as construction industry, oil and gas industry, chemical and metallurgical industries, medicine and microbiology, thermal energetics and machine building, wood working and food industries, radio electronics, and even outdoor advertisement are highly interested in new materials development.
Typical production processes for hard magnets, such as Nd-Fe-B, SmCo 5 , and others include either thermal reduction of Co and REM oxides by Ca or alloying of the preliminary anode. A chloride-silver electrode Ag|KCl-NaCl (1:1)-AgCl (2.5 mol.%) was used as a reference electrode. A tungsten wire of 1.0 mm served as a cathode. An alundum tube dressed the tungsten wire leaving 40 mm uncovered. The area of the working electrode was calculated according to the immersion depth to the melt (10 ÷ 15 mm). The preparation and assembling of the cell were performed in the glove box mBraun Labstar 25 (MBRAUN, Garching, Germany) in pure argon atmosphere. To provide the operating temperature of 973 K, a shaft-type resistance furnace with silicon carbide heaters was used. The automatic regulation of the temperature was performed by an electronic regulator OVEN-TRM-1 with a chromel-alumel thermocouple, the accuracy of the temperature control was ±1 • C. Cyclic voltammograms and open-circuit potential measurements were obtained using an Autolab PGSTAT 30 device with IF-00 interface (Eco Chemie, Metrohm Autolab B.V., Utrecht, Netherlands). The voltammograms were processed using the software (GPES 4.9).

Methods for Analysis and Diagnostics of Cathode Deposits
To determine the phase and elemental composition of cathode deposits, an X-ray diffractometer D2 PHAZER (Bruker, Bremen, Germany); scanning electron microscope Tescan VEGA3LMH, which allows quantitative EDX chemical analysis (Tescan, Brno-Kohoutovice, Czech Republic); and X-ray fluorescent spectrometer Spektrscan MAKS GV (Spectron, St. Petersburg, Russia) were used.

Electrolyte Preparation
The KCl-NaCl equimolar melt was chosen as a background electrolyte. Chemically pure KCl and NaCl salts were preliminary dried in a vacuum oven for 10 h and then were annealed in an electric muffle for 5 h at 723 K. Dry chemically pure salts NdCl 3 , DyCl 3 , NiCl 2 , CoCl 2 , and FeCl 3 (Chemcraft, Kaliningrad, Russia) were used as sources of Nd, Dy, and iron triad metals. 3+ and Dy 3+ Ions in the KCl-NaCl Equimolar Melt at 973 K We previously studied the electroreduction of the Nd 3+ and Dy 3+ ions in the KCl-NaCl melt on the active electrode materials (Ag, Pt, and GC) using DyCl 3 and NdCl 3 obtained via dewatering of NdCl 3 ·6H 2 O and DyCl 3 ·6H 2 O by dehydrating agents NH 4 Cl and CCl 4 [26][27][28]. Experimental conditions and purity of the used reagents as well as the material of the electrode have a great impact on the structure, state, and electrochemical behavior of the Nd 3+ and Dy 3+ ions in chloride melts. In the present study, the electrochemical reductions of Nd 3+ and Dy 3+ ions with Ni 2+ , Co 2+ , and Fe 2+ ions were performed in the equimolar KCl-NaCl melt using chemically pure reagents and inert electrode materials (W) at 973 K. Figure 1 presents cyclic voltammetry dependences of the equimolar KCl-NaCl melt containing 4.0 × 10 −4 mol/cm 3 of NdCl 3 on the tungsten electrode at different polarization rates and at the temperature of 973 K. The absence of any other waves and low residual current of the voltammetry curve for the background KCl-NaCl electrolyte ( Figure 1, curve 1) up to the potentials of the alkali metals extraction testifies to the high purity of the equimolar KCl-NaCl melt and approve its implementation in the proposed tasks. The addition of NdCl 3 to the equimolar KCl-NaCl melt results in the appearance of the potential region −(1.85 ÷ 2.05) V relative to the Ag/AgCl reference electrode, which corresponds to the Nd 3+ ions reduction. The height of this reduction wave increases as the polarization rate increases. The reduction waves of Nd 3+ ions do not have a clear diffusion peak, because the Nd 3+ reduction potentials are close to the potentials of the alkali metals extraction. The increase in the polarization rate and the transition to the non-stoichiometric regimes do not result in diffusion peak formation. However, the reduction wave was noticed to shift towards the region of negative potentials and to expand. The cyclic voltammetry dependencies of the equimolar KCl-NaCl melt containing DyCl 3 ( Figure 2) have only one wave of Dy 3+ ions reduction in the potential region of −(1.9 ÷ 2.1) V and of the cathode product oxidation in the potential region of −(1.85 ÷ 2.05) V.

Electroreduction of Nd
wave was noticed to shift towards the region of negative potentials and to expand. The cyclic voltammetry dependencies of the equimolar KCl-NaCl melt containing DyCl3 ( Figure 2) have only one wave of Dy 3+ ions reduction in the potential region of −(1.9 ÷ 2.1) V and of the cathode product oxidation in the potential region of −(1.85 ÷ 2.05) V.
The analysis of the half-width of the Dy 3+ and Nd 3+ ions reduction waves according to the known diagnosis criteria [29,30] up to the polarization rates of 0.1 V/s elucidates that there are three electrons, which transfer during the cathode process. As the polarization rate increases, the half-width values of the Nd 3+ and Dy 3+ ions reduction waves increase. This testifies that the process of the Nd 3+ and Dy 3+ ions electrochemical reduction proceeds in the regime of controlled charge transfer rate.  to the known diagnosis criteria [29,30] up to the polarization rates of 0.1 V/s el that there are three electrons, which transfer during the cathode process. polarization rate increases, the half-width values of the Nd 3+ and Dy 3+ ions re waves increase. This testifies that the process of the Nd 3+ and Dy 3+ ions electroc reduction proceeds in the regime of controlled charge transfer rate.  The analysis of the half-width of the Dy 3+ and Nd 3+ ions reduction waves according to the known diagnosis criteria [29,30] up to the polarization rates of 0.1 V/s elucidates that there are three electrons, which transfer during the cathode process. As the polarization rate increases, the half-width values of the Nd 3+ and Dy 3+ ions reduction waves increase. This testifies that the process of the Nd 3+ and Dy 3+ ions electrochemical reduction proceeds in the regime of controlled charge transfer rate. Therefore, based on the structure of the chloride melts containing REM ions, the Nd 3+ and Dy 3+ ions were found to acquire the form of chloride LnCl 6 3− complexes (Ln = Dy and Nd). The reversible three-electron reaction of the electrochemical cathode reduction has the following form: 3.2. Electrochemical Reduction of the Iron Triad Metals in the KCl-NaCl Melt at 973 K The electrochemical reduction of Ni 2+ , Co 2+ , and Fe 3+ ions in ionic melts is reported in a limited number of papers, because these processes are successfully performed in water solutions. However, chloride melts are the best media for production of powders and coatings from the iron triad metals. Thus, papers [31][32][33] report on the regularities of the Co 2+ ions electrochemical reduction in the molten NaCl-CoCl 2 and KCl-NaCl-CoCl 2 systems. Sytchev et al. [32] studied the KCl-NaCl-FeCl 3 (CoCl 2 , NiCl 2 ) and KCl-NaCl-NaF(5 mol.%)-FeCl 3 (CoCl 2 , NiCl 2 ) melts by the cyclic voltammetry at 973 K. The character of the iron triad ions electrochemical reduction was determined and it was concluded that the electrode processes with the iron triad ions are diffusion-controlled. According to the research reported in [32], two electrons participate in the elementary processes of electrochemical reduction of Fe 3+ , Co 2+ , and Ni 2+ both in chloride and in chloride-fluoride melts.
To eliminate the influence of the experimental conditions and impact of different electrode materials, we performed the electrochemical reduction of the iron triad ions under conditions similar to those of Dy 3+ and Nd 3+ ions' electrochemical reduction. Figures 3 and 4 present cyclic voltammograms recorded on the tungsten cathode during the electrochemical reduction of chloride Fe 3+ and Co 2+ complexes in the equimolar KCl-NaCl melt at 973 K. A wave of the Fe 3+ and Co 2 chloride complexes reduction is observed on the cathodic branch of cyclic voltammetry curves in the regions of potentials −(0.5 ÷ 0.6) V, −(0.3 ÷ 0.4) V relative to the Ag/AgCl reference electrode. The X-ray diffraction analysis of the product of potentiostatic electrolysis obtained at the potentials of the wave termination illustrates that the cathode deposit is composed on metallic Fe and Co phases. The anodic branch of the cyclic voltammogram has one wave of metallic Co oxidation. In the case of metallic Fe oxidation, two waves are observed. If the polarization reaches −1.0 V, the voltammetry curve stages of the metallic Fe electrochemical oxidation are less noticeable. The analysis of the Fe 3+ ions reduction according to the diagnostic criteria of the chronopotentiometric method illustrates a three-electrode process of Fe 3+ ions reduction, which is controlled by diffusion:    The number of electrons (n), which transfer during the electrochemical reduction of Co 2+ ions, was calculated using the value of the wave half-width for the polarization rates less than 0.1 V/s using equation Ер/2 − Ер = 0.77RT/nF. It was found that n = 2.
Electrochemical behavior of Ni 2+ ions is analogous to that of Co 2+ ions, with the only exception that nickel ions reduce in the more positive region of potentials −(0.05 ÷ 0.1) V.

Electrochemical Coreduction of Nd and Dy Ions with Iron Triad Ions in the KCl-NaCl Melt at 973 K
According to the phase diagrams of the Dy-Ni and Nd-Ni double metallic systems [34,35] Nd 3+ and Dy 3+ ions form the following intermetallic compounds with nickel Ln2Ni17, LnNi5, Ln2Ni7, LnNi3, LnNi2, LnNi, Ln7Ni3, Ln3Ni, where Ln = Dy and Nd. Nourry et al. calculated a free Gibbs energy of intermetallic compounds formation in the double system NdxNiy and found that NdNi5 is the most stable phase (ΔG(NdNi5) = −350 kJ/mol) [36]. As the Nd 3+ concentration increases in the intermetallic compounds, the free Gibbs energy increases but its value does not exceed zero at 1073 K (ΔG(Nd3Ni) = −110 kJ/mol). Among all analyzed phases, only NdNi5, NdNi, DyNi5, and DyNi are considered as congruently melting compounds. The rest are incongruently melting compounds that The oxidation of metallic Fe proceeds in two stages: The number of electrons (n), which transfer during the electrochemical reduction of Co 2+ ions, was calculated using the value of the wave half-width for the polarization rates less than 0.1 V/s using equation E p/2 − E p = 0.77RT/nF. It was found that n = 2.
Electrochemical behavior of Ni 2+ ions is analogous to that of Co 2+ ions, with the only exception that nickel ions reduce in the more positive region of potentials −(0.05 ÷ 0.1) V.

Electrochemical Coreduction of Nd and Dy Ions with Iron Triad Ions in the KCl-NaCl Melt at 973 K
According to the phase diagrams of the Dy-Ni and Nd-Ni double metallic systems [34,35] Nd 3+ and Dy 3+ ions form the following intermetallic compounds with nickel Ln 2 Ni 17 , LnNi 5 , Ln 2 Ni 7 , LnNi 3 , LnNi 2 , LnNi, Ln 7 Ni 3 , Ln 3 Ni, where Ln = Dy and Nd. Nourry et al. calculated a free Gibbs energy of intermetallic compounds formation in the double system Nd x Ni y and found that NdNi 5 is the most stable phase (∆G(NdNi 5 ) = −350 kJ/mol) [36]. As the Nd 3+ concentration increases in the intermetallic compounds, the free Gibbs energy increases but its value does not exceed zero at 1073 K (∆G(Nd 3 Ni) = −110 kJ/mol). Among all analyzed phases, only NdNi 5 , NdNi, DyNi 5 , and DyNi are considered as congruently melting compounds. The rest are incongruently melting compounds that form according to the peritectic reaction. In the double Dy-Co system, there are two congruently melting compounds-Dy 2 Co 17 , DyCo 3 -and five incongruently melting ones-Dy 2 Co 7 , DyCo 2 , DyCo 3 , Dy 12 Co 7 , Dy 3 Co. The Nd-Co system has one congruently melting compound-Nd 2 Co 17 -and nine incongruently melting ones-NdCo 5 , Nd 2 Co 7 , Nd 5 Co 19 , NdCo 3 , NdCo 2 , Nd 2 Co 3 , Nd 2 Co 1.7 , Nd 2 Co 17 , Nd 7 Co 3 , Nd 3 Co. The phase diagram of the Dy-Fe system illustrates the formation of two congruently melting compounds Dy 2 Fe 17 and DyFe 3 and two incongruently melting compounds Dy 6 Fe 23 иDyFe 2 . The phase diagram of the Nd-Fe system demonstrates the formation of only two incongru-ently melting compounds, Nd 2 Fe 17 and Nd 5 Fe 17 [34,35]. According to these data we may estimate that the electrochemical synthesis of dysprosium and neodymium intermetallic compounds with nickel and cobalt is more feasible than those with iron.
To determine the possibility of electrochemical coreduction of Nd 3+ and Dy 3+ ions with the iron triad ions, we performed voltammetry measurements of the equimolar KCl-NaCl melt containing Nd 3+ , Dy 3+ , Fe 3+ , Co 2+ , and Ni 3+ ions. The experiments and electrochemical measurements were performed in three different ways: (i) First, measured portions of the NiCl 2 , CoCl 2 , and FeCl 3 were added to the equimolar melt and the voltammetry dependencies of the electrochemical reduction process were recorded. Then, NdCl 3 and DyCl 3 were added to the iron triad containing KCl-NaCl melt and the voltammetry dependence was recorded considering their bulk concentration in the background electrolyte. (ii) NdCl 3 and DyCl 3 were first added to the equimolar KCl-NaCl melt, the voltammetry dependence of the molten KCl-NaCl-NdCl 3 (DyCl 3 ) system was recorded; then, iron triad ions were added to the previously formed mixture and the voltammograms of electrochemical coreduction were recorded. (iii) The calculated amount of the background electrolyte compounds KCl and NaCl (1:1), NdCl 3 and DyCl 3 , as well as NiCl 2 , CoCl 2 , and FeCl 3 were mixed in a dry powder form, put into a glassy carbon crucible and melted in the three-electrode cell in argon atmosphere of a dry box with continuously increasing temperature up to 973 K.
The experiments demonstrated that the sequence of iron triad and REM chlorides addition to the equimolar mixture does not affect the behavior of the voltammetry dependences of the electrochemical coreduction. Figure 5 presents cyclic voltammograms of the process of electrochemical coreduction of NdCl 3 and NiCl 2 . The addition of NdCl 3 to the KCl-NaCl-NiCl 2 chloride melt changes the pattern of voltammetry curves and results in the appearance of several extra reduction waves apart from the Ni 2+ ions reduction waves at the potentials of −(0.0 ÷ 0.1) V (wave A). The reduction wave B was recorded at the potentials of −(0.7 ÷ 0.8) V, wave C appeared at the potentials of −(1.6 ÷ 1.7) V, wave D at the potentials of −(1.6 ÷ 1.7) V, and wave E at the potentials of −(1.9 ÷ 2.0) V (see Figure 5, curve 3). Wave E corresponds to the process of pure metallic Nd extraction (see Figure 2, curve 2). The appearance of the waves B, C, and D on the voltammogram is related to the reduction of Nd 3+ ions with depolarization onto the preliminarily extracted metallic Ni on the tungsten cathode. These waves illustrate the formation of the intermetallic Nd x Ni y phases of various compositions. Thus, according to the X-ray diffraction analysis, the cathode deposit after 5 h of potentiostatic electrolysis at the potentials of the wave B termination (−1.0 V) is mainly composed of preliminarily deposited metallic Ni phase and small amount of the Nd 2 Ni 17 intermetallic phase. The obtained X-ray diffraction pattern (Figure 6a) demonstrates that the cathode deposit at the potentials of wave C (−1.5 V) is composed of the mixture of metallic Ni and intermetallic Nd 2 Ni 17 and NdNi 5 phases and, according to the elemental analysis illustrated in Figure 6b, the electrolysis product contains mainly Ni and Nd elements. The obtained scanning electron microscopy (SEM) images illustrate microfine particles of elongated and round forms (Figure 6c). Their location at the surface of the studied sample is defined using the color mapping. The corresponding color map provided in Figure 6d testifies that intermetallic Nd x Ni y phases are present in the analyzed sample. The cathode deposit at the termination of wave D is composed of metallic Ni, intermetallic NdNi 5 and NdNi 2 phases, and traces of the Nd 2 Ni 17 phase. In addition, the more negative electrolysis potential results in higher Nd concentration in the intermetallic compounds. The anodic branch of the cyclic voltammogram also has 5 waves: A', B', C', D', and E'. The correspondence of the cathodic reduction waves and anodic electrochemical oxidation waves of the cathode cycle are verified by the voltammetry curves recorded before the reverse potentials, corresponding to the termination of each cathodic reduction wave ( Figure 5, curves 4-7).  The analogous changes in the voltammograms are observed in the equimolar KCl-NaCl melt containing DyCl 3 and NiCl 2 . Thus, the voltammetry dependence of the KCl-NaCl-DyCl 3 -NiCl 2 melt (Figure 7) illustrates five reduction waves: wave A illustrates Ni 2+ ions reduction V on the tungsten electrode at the potentials of −(0.0-0.1); waves B, C, and D are associated with the reduction of Dy 3+ ions with the depolarization of the preliminarily extracted metallic Ni on the tungsten cathode, which results in the formation of various intermetallic compounds-Dy 2 Ni 17 , DyNi 5 , and DyNi 2 ; wave E denotes the extraction of pure Dy on the intermetallic compounds. The anodic part of the cyclic voltammogram also illustrates five waves: A', B', C', D', and E'. Their correspondence to the cathodic reduction waves is verified by the voltammograms until the moment when the values of the reverse potentials become different, which corresponds to the termination of each cathodic reduction wave (Figure 7, curves 3-7). This suggestion is exemplified by the X-ray pattern of the obtained cathode deposits illustrated in Figure 8a Figure 9 illustrates the voltammograms of the electrochemical coreduction process of DyCl3 and CoCl2. The addition of CoCl2 to the molten KCl-NaCl-DyCl3 (3.0 × 10 −4 mol/cm 3 ) system results in the appearance of the cathodic part of the voltammetry dependence, apart from the DyCl6 3− ions reduction wave (Figure 9, curve 3, wave D) and other three waves. Wave A appears at the potentials of -(0.2 ÷ 0.25) V, wave B at −(1.6 ÷ 1.7) V, and wave C at −(1.8 ÷ 1.9) V. The electrochemical reduction of Со 2+ ions to metallic cobalt is illustrated by wave A. The appearance of waves B and C on the voltammograms is associated with the dysprosium ions on the preliminarily extracted metallic Co on the tungsten electrode. The formation of intermetallic DyxCoy phases of various compositions is illustrated by these waves, as in the case of the chloride KCl-NaCl-DyCl3 melt with NiCl2 concentration. The X-ray diffraction analysis of the cathode deposit obtained by the The process illustrated by wave D proceeds on the Nd(Dy)Ni 5 intermetallic compound and is accompanied with a certain depolarization. Extracted metallic Nd and Dy interact with Nd(Dy)Ni 5 and form phases with large Nd and Dy content. 2Nd(Dy) Ni 5 + 3Nd(Dy) = 5Nd(Dy) Ni 2 (10) Figure 9 illustrates the voltammograms of the electrochemical coreduction process of DyCl 3 and CoCl 2 . The addition of CoCl 2 to the molten KCl-NaCl-DyCl 3 (3.0 × 10 −4 mol/cm 3 ) system results in the appearance of the cathodic part of the voltammetry dependence, apart from the DyCl 6 3− ions reduction wave (Figure 9, curve 3, wave D) and other three waves. Wave A appears at the potentials of -(0.2 ÷ 0.25) V, wave B at −(1.6 ÷ 1.7) V, and wave C at −(1.8 ÷ 1.9) V. The electrochemical reduction of Co 2+ ions to metallic cobalt is illustrated by wave A. The appearance of waves B and C on the voltammograms is associated with the dysprosium ions on the preliminarily extracted metallic Co on the tungsten electrode. The formation of intermetallic Dy x Co y phases of various compositions is illustrated by these waves, as in the case of the chloride KCl-NaCl-DyCl 3 melt with NiCl 2 concentration. The X-ray diffraction analysis of the cathode deposit obtained by the potentiostatic electrolysis at the potentials of the wave B termination (−1.7 V) performed during 5 h illustrated that the cathode deposit is mainly composed of the metallic Co phase and small amounts of DyCo 5 intermetallic compound ( Figure 10). The electrolysis product at −1.7 V is composed of the metallic Co phase and intermetallic DyCo 5 and DyCo 2 phases. In addition, the more negative electrolysis potential results in higher Dy concentration in the intermetallic compounds. The anodic branch of the cyclic voltammogram has four waves (A', B', C', D') corresponding to the anode dissolution of metallic Co (A'), metallic Dy (D'), and intermetallic phases (B' and C'). The anodic waves correspond to the cathodic reduction waves. This fact is verified by the voltammetry curves recorded before the potentials, corresponding to the termination of each cathodic reduction wave (Figure 9, curves 3-6).
Materials 2021, 14, x FOR PEER REVIEW potentiostatic electrolysis at the potentials of the wave B termination (−1.7 V) pe during 5 h illustrated that the cathode deposit is mainly composed of the me phase and small amounts of DyCo5 intermetallic compound ( Figure 10). The ele product at −1.7 V is composed of the metallic Co phase and intermetallic DyCo5 an phases. In addition, the more negative electrolysis potential results in hig concentration in the intermetallic compounds. The anodic branch of th voltammogram has four waves (А', B', C', D') corresponding to the anode disso metallic Co (A'), metallic Dy (D'), and intermetallic phases (B' and C'). The anod correspond to the cathodic reduction waves. This fact is verified by the volta curves recorded before the potentials, corresponding to the termination of each reduction wave (Figure 9, curves 3-6).     Figure 11 demonstrates four waves on the dependencies of the molten KCl-NaCl-NdCl 3 -CoCl 2 system until the potential of the background electrolyte is reached: wave A appears at the potentials −(0.3 ÷ 0.4) V, an extended wave at the potentials −(1.9 ÷ 2.2) V, which tends to half halving on wave B at −(1.9 ÷ 2.1) V and wave C at −(2.1÷ 2.2) V ( Figure 11). Wave A denotes the electrochemical reduction of Co 2+ ions to metallic Co (see Figure 4). Waves B and C illustrate the electroreduction accompanied with depolarization of NdCl 6 3complexes on the preliminary extracted metallic cobalt with further formation intermetallic neodymium and cobalt compounds NdCo 5, NdCo 2 (Figure 12), respectively. Wave D is associated with the electrochemical extraction of metallic Nd on the previously reduced Nd x Co y intermetallic compound. The anodic branch presents the electrochemical oxidation of the cathodic waves' products. Besides, the anodic branch provides a more precise half halving of the cathodic waves B and C oxidation (waves B' and C').
precise half halving of the cathodic waves B and C oxidation (waves В' and C').
Given the obtained data, the electrochemical processes that take place during the molten KCl-NaCl-NdCl3 (DyCl3)-CoCl2 salts electrolysis are presented as follows: wave B NdCl6 3− (DyCl6 3− ) + 3e + 5 Co = Nd(Dy) Co5 + 6Cl − (11) wave C NdCl6 3− (DyCl6 3− ) + 3e + 2Co = Nd(Dy)Co2 + 6Cl − (12)    The addition of FeCl3 to the KCl-NaCl-DyCl3 melt results in the appearance of the well-reproducible wave of the Fe 3+ ions reduction at the potentials −(0.5-0.6). This process is accompanied with the extraction of the phase of metallic Fe 3+ phase (Figure 13, curves 3 and 4). reduction that may be associated with the formation of Dy x Fe y intermetallic phases is not observed, as opposed to that of the melt containing CoCl 2 and NiCl 2 . The anodic branch of the cyclic voltammogram has four waves related to the dissolution of the metallic Fe (A') and Dy (D') as well as the waves corresponding to the dissolution of the Dy x Fe y intermetallic phases (B' and C'). Figure 14 presents the X-ray pattern of the cathode deposit synthesized via electrolysis of the KCl-NaCl-DyCl 3 -FeCl 3 melt at 973 K and electrolytic cell voltage of 2.8 V.
The addition of NdCl 3 to the KCl-NaCl-FeCl 3 melt results in the formation of Nd 3+ ions at potentials of −(2.0-2.2) V ( Figure 15). The main peculiarity of the NdCl 6 3− complex's reduction in the KCl-NaCl-FeCl 3 melt is that the Nd 3+ ions extraction on the tungsten electrode coated with deposited metallic Fe is accompanied by small depolarization, caused by the formation of Nd x Fe y intermetallic compounds (Figure 16), as opposed to the similar process performed in the KCl-NaCl melt (Figure 1).
The comparative analysis of the voltammograms of the KCl-NaCl-DyCl 3 (NdCl 3 )-MeCl x (where Me = Ni, Co, Fe) melts illustrates that among the iron triad, Ni most actively interacts with metallic Dy and Nd; this interaction results in the formation of intermetallic La x Ni y compounds (La = Nd and Dy). Co is the second most active metal of the iron triad, whereas Fe is the least active one. A similar comparison of Dy and Nd illustrates that Dy interacts more actively with the iron triad than Nd. Apart from that, a significant difference in the potentials of iron triad and Dy(Nd) extraction (~1.2 ÷ 1.5 V) testifies that the electrolytic synthesis of intermetallic Ln x Me y compounds (Ln = Dy, Nd; Me = Ni, Fe, Co) is possible only in the kinetic regime. reduction that may be associated with the formation of DyxFey intermetallic phases is not observed, as opposed to that of the melt containing CoCl2 and NiCl2. The anodic branch of the cyclic voltammogram has four waves related to the dissolution of the metallic Fe (А') and Dy (D') as well as the waves corresponding to the dissolution of the DyxFey intermetallic phases (B' and C'). Figure 14 presents the X-ray pattern of the cathode deposit synthesized via electrolysis of the KCl-NaCl-DyCl3-FeCl3 melt at 973 K and electrolytic cell voltage of 2.8 V.  Figure 15). The main peculiarity of the NdCl6 3− complex's reduction in the KCl-NaCl-FeCl3 melt is that the Nd 3+ ions extraction on the tungsten electrode coated with deposited metallic Fe is accompanied by small depolarization, caused by the formation of NdxFey intermetallic compounds (Figure 16), as opposed to the similar process performed in the KCl-NaCl melt (Figure 1).

Conclusions
Based on the voltammetry studies of the electrochemical coreduction of Dy 3+ , Nd 3+ , Ni 2+ , Co 2+ , and Fe 3+ ions in the equimolar KCl-NaCl melt at 973 K, it is shown that the electrochemical synthesis of these intermetallic compounds may be performed only in the kinetic regime. It is determined that the interaction activity resulting in the formation of intermetallic compounds between metallic Dy and Nd with the iron triad decreases in a row, starting with Ni, Co, and Fe; whereas, Dy is found to be more active in the aforesaid interactions than Nd.
It is shown that depending on the electrolysis regime (galvanostatic and potentiostatic), the process of the electrochemical synthesis consists of the following subsequent stages. It is found that only metallic Ni, Co, and Fe phases are extracted on the cathode during the galvanostatic electrolysis at cathode current densities smaller than the density of the limiting diffusion current of the iron triad ions electroreduction at the potentiostatic electrolysis up to the potentials of the limiting current of nickel, cobalt, and iron ions' electrochemical reduction. The phases of intermetallic Ln x Me y compounds (Ln = Dy, Nd; Me = Ni, Fe, Co) were detected together with the metallic Ni, Fe, and Co phases on the voltammograms recorded during electrolysis at the cathode current density exceeding the limiting diffusion current of Ni 2+ , Co 2+ , and Fe 3+ ions or during potentiostatic electrolysis at the potentials of the corresponding waves.
In addition, the higher cathode current density and more negative electrolysis potentials result in the smaller concentration of iron triad metals in the cathode deposit and higher concentration of the intermetallic phases, including intermetallic phases with great REM concentration. For this reason, the process of electrochemical synthesis of intermetallic compounds Ln x Me y compounds (Ln = Dy, Nd; Me = Ni, Fe, Co) is determined by the following interrelated parameters: composition of the electrolyte (concentrations of FeCl 3 , CoCl 2 , NiCl 2 , DyCl 3 , and NdCl 3 ), cathode current density of the electrolysis potential (electrochemical cell voltage), and electrolysis time. Further, we plan to perfect these regimes and to select the optimal parameters for the electrochemical synthesis, diagnose the samples, and determine the magnetic field characteristics.