Thermodynamic and electrochemical properties of holmium and HoxAly intermetallic compounds in the LiCl-KCl eutectic

This work presents a study of electrochemistry of holmium in the LiCl-KCl eutectic by applying a series of electrochemical techniques on a tungsten electrode at the temperature range of 684-809 K. The reduction of Ho(III) cations in the LiCl-KCl eutectic is a one step process with the transfer of three electrons. The diffusion coefficients of the Ho(III) is temperature dependent and can be expressed aslnDHo(III)=−446lT−1−5.498. The activation energy for the diffusion of Ho(III) in the LiCl-KCl eutectic was also calculated to be 37.09 kJ/mol. The apparent standard potential of Ho(III) can be described asEHo(III)/Hoθ,(vs.Cl2/Cl−1)=−3.450+5.789×10−4T/V and the apparent Gibbs free energy for the formation of HoCl3 in the LiCl-KCl eutectic can be expressed as: ΔGHoCl3θ,(vs.Cl2/Cl−1)=−998.62+0.1676T (kJ/mol). The thermodynamic properties of six AlyHox intermetallic compounds were determined by open circuit chronopotentiometry (OCP) at the temperature of 723 and 773 K. AlyHox intermetallic compound samples were prepared by potentiostatic electrolysis in the LiCl-KCl-AlCl3-Ho2O3 and LiCl-KCl-HoCl3 melts on an aluminium electrode at 773 K, respectively. Scanning Electron Microscopy coupled with Energy Dispersive Spectroscopy (SEM-EDS) and X-ray diffraction (XRD) characterizations identified the formation of three intermetallic compounds (Al17Ho2, Al3Ho and Al2Ho). As far as we know, this is the first time that intermetallic compound Al17Ho2 was obtained by an electrochemical method.