INVESTIGATION OF THE MECHANISM OF CO-REDUCTION OF THE RARE-EARTH METAL IONS WITH NICKEL IONS IN AN EUTECTIC KCl–NaCl–CsCl MELT

In this work, the electrochemical behavior of dysprosium, neodymium, and lanthanum ions and their co-reduction with nickel ions on tungsten and nickel electrodes in the eutectic melt KCl–NaCl–CsCl at a temperature of 823 K have been studied. It was found that the electroreduction of Ln³⁺ ions proceed reversibly in one three electron stage up to the polarization rates of 0.2 V/s. At the combined content of lanthanide and nickel ions, the voltampere dependences show the reduction waves of nickel ions at potentials of –0.12… –0.3 V and of lanthanide ions at potentials of –2.13… –2.18 V relative to the chlorosilver reference electrode. Besides these waves on voltammetryograms there are three recovery waves in the region of potentials: –1.68… –1.77 V; –1.95… –2.0 V; –2.13… –2.18 V. The appearance of these waves is connected with the joint electroreduction of lanthanide and nickel ions on the metallic nickel previously separated on the tungsten electrode with a certain depolarization and formation of intermetallic phases of lanthanides and nickel of different composition Ln_xNi_y. The chronopotentiograms of the open circuit revealed plateaus of potential delay corresponding to dissolution of separate phases of intermetallides. Electrolysis in potentiostatic condition at potentials –1.7… –2.1 V and at a certain ratio of concentrations of lanthanide and nickel chlorides the phases of intermetallic compounds LnNi₅, Ln₂Ni₇, Ln₂Ni₃, LnNi₃, LnNi₂ were obtained. The synthesized samples of lanthanide and nickel intermetallides were characterized by X-ray phase analysis and scanning electron microscopy. The EMF values for Ln_xNi_y intermetallic compounds in two-phase coexisting states at 823 K have been measured. The relative partial moles of the Gibbs free energy and activity of lanthanide in Ln_xNi_y intermetallic compounds were calculated from the EDS values.