Electrochemical formation and thermodynamic properties of Tb–Bi intermetallic compounds in eutectic LiCl–KCl

Abstract

The electrochemical reactions of Tb(III) were investigated on a W electrode, Bi pool electrode and Bi film electrode in eutectic LiCl–KCl by transient electrochemical techniques. The exchange current densities of the Tb(III)/Tb(0) redox couple were determined on W and Bi film electrodes at different temperatures by the linear polarization method. On both Bi electrodes, the redox potential of Tb(III)/Tb couple was observed at less negative potential values than that on the W electrode, which indicated underpotential deposition of Tb occurring on the both Bi electrodes. The result of cyclic voltammetry performed on the Bi pool electrode suggested that the electrochemical reaction of Tb(III) to Tb_{in liquid Bi} was a quasi-reversible and diffusion-controlled process. From the cyclic voltammogram and square wave voltammogram of Tb(III) obtained on the Bi film electrode, three reduction signals corresponded to the formation of Tb–Bi intermetallic compounds. The thermodynamic data, such as the activities of Tb in Tb–Bi alloys and the standard Gibbs free energies of formation for different Tb–Bi intermetallic compounds, were estimated using open circuit chronopotentiometry in the temperature range from 673 to 873 K. Moreover, the electrochemical preparation of Tb–Bi alloys was conducted in LiCl–KCl–TbCl₃ melts on a liquid Bi electrode by galvanostatic and potentiostatic electrolysis, respectively. The Tb–Bi alloys were characterized by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). XRD results showed that the Tb–Bi alloys were composed of the TbBi phase and TbBi, TbBi_3/4 and TbBi_3/5 phases, respectively.