**2.1 Preparation of starting materials**

The solvents LiCl (Aldrich, 99.9%), NaCl (Aldrich, 99.9%), KCl (Aldrich, 99.9%), and CsCl(Aldrich, 99.9%) were purified under vacuum in the temperatures range 293-773 K. Then the reagents were fused under dry argon atmosphere. Afterwards these reagents were purified by the operation of the direct crystallization [Shishkin & Mityaev, 1982]. The calculated amounts of prepared solvents were melted in the cell before any experiment [Korshunov et al., 1979].

Dry lanthanide trichlorides (LnCl3) were obtained by the way of well-known method [Revzin, 1967] in two steps:


The obtained lanthanide chlorides (LnCl3) were kept into glass ampoules under atmosphere of dry argon in inert glove box. Ln3+ ions were prepared by direct addition of anhydrous LnCl3 to the fused electrolytic bath.

#### **2.2 Transient electrochemical technique**

The experiments were carried out under inert argon atmosphere using a standard electrochemical quartz sealed cell using a three electrodes setup. Different transient electrochemical techniques were used such as linear sweep, cyclic, square wave and semiintegral voltammetry, as well as potentiometry at zero current. The electrochemical measurements were carried out using an Autolab PGSTAT30 potentiostat-galvanostat (Eco-Chimie) with specific GPES electrochemical software (version 4.9).

The inert working electrode was prepared using a 1mm metallic W wire (Goodfellow, 99.9%). It was immersed into the molten bath between 3 - 10 mm. The active surface area was determined after each experiment by measuring the immersion depth of the electrode. The counter electrode consisted of a 3 mm vitreous carbon rod (SU - 2000). The Cl–/Cl2 electrode is the most convenient reference electrode because it can be used for the direct thermodynamic calculations. It standard construction is the following. The quartz tube with porous membrane in the bottom and molten solvent in it has the graphite tube for chlorine gas introduction into the system. The chlorine gas is bubbling through the melt during the experiment [Smirnov, 1973].

The lanthanide concentrations were determined by taking samples from the melt which were dissolved in nitric acid solutions and then analyzed by ICP-MS.
