The Cd22+ in molten metal halides and at electrode interfaces

Abstract

The structural properties of the cadmium species formed by dissolution of cadmium metal in molten cadmium halides (CdX₂: X=Cl, Br, I) and in molten mixtures of cadmium chloride with alkali chlorides have been investigated by Raman spectroscopy. The data indicate that in these ionic solvents the predominant species present is the solvated Cd₂²⁺ subvalent cation. With increasing ionic strength of the counter anion the frequency of the diatomic cation shifts from 158 cm^-1 for the chloride melt to 183 cm^-1 for the iodide melt. Due to the overlap of the electronic absorption edge of the Cd₂²⁺ with the laser lines used to excite the spectra the Raman band intensities of the solute species are preresonance enhanced. The dissolution of cadmium metal in the cadmium chloride–alkali chloride mixtures increases with temperature and depends on the stability of the CdCl₄^2- species formed in these melts. The looser the CdCl₄^2- association the higher the solubility and the stability of the Cd₂²⁺ species. Raman spectroelectrochemical methods have been applied for studying amorphous carbon electrode surfaces during electrolysis of CdCl₂ in LiCl–KCl eutectic. A new Raman band measured at ∽158 cm^-1 during electrolysis was attributed to the subvalent Cd₂²⁺ species. It seems that these species are formed in the vicinity of the electrode by a homogeneous electrochemical reaction.