The electrochemical behavour of samarium in NaCl-KCl, KCl and CsCl melts have been studied in the temperature range 973–1173 K by different electrochemical methods. The diffusion coefficients (D) of Sm(III) and Sm(II) have been determined by linear sweep voltammetry, chronopotentiometry and chronoamperometry methods. The standard rate constants of charge transfer (k s) for the Sm(III)/Sm(II) redox couple have been calculated on the basis of cyclic voltammetry data using Nicholson’s equation. The nature of working electrode on the standard rate constants of charge transfer for the Sm(III)/Sm(II) redox couple has been studied. The formal redox potentials E * Sm(III)/Sm(II) have been obtained in alkali chloride melts from the cyclic voltammetry data.

The cyclic voltammetry method was used to determine the standard rate constants of charge transfer (ks) on a glassy carbon electrode for the Cr(III)/Cr(II) redox couple in the NaCl-KCl-CrCl3, KCl-CrCl3, and CsCl-CrCl3 melts in the temperature range of 973-1173 K. It was shown that ks grow at an increase of the temperature and decrease as sodium cations are replaced by potassium and cesium cations in the second coordination sphere of chromium complexes. Optimized geometrical structures and energies of such model particles as (M+)n[Cr(III)Cl6]3- and (M+)n[Cr(II)Cl6]4- (M - Na, K, Cs; n=1 - 6) were obtained by quantum-chemical calculations. The most stable compositions of these particles are determined. Based on the calculated reorganization energies the activation energies of charge transfer have been computed. These values change monotonously in the Na-K-Cs series in accordance with the ratio of reorganization energies: Eact (Na) < Eact (K) < Eact (Cs). In its turn, this leads to monotonous variation of the charge transfer rate constants, which is observed in experiments.

815Previously, we studied the effect of the second coordination sphere on k s in chloride melts [17,18]. The NaCl-KCl-CrCl 3 , KCl-CrCl 3 , and CsCl-CrCl 3 systems were studied. It was shown that the standard rate constants of charge transfer increased at elevated Abstract-The standard rate constants (k s ) of charge transfer on a glass carbon electrode were determined for the Cr(III)/Cr(II) redox pair in the NaCl-KCl-K 3 CrF 6 , KCl-K 3 CrF 6 , and CsCl-K 3 CrF 6 systems at 973-1173 K by cyclic voltammetry. The k s constant was found to increase at elevated temperatures and the follow ing nonmonotonic dependence of k s on the nature of the outer spheric cation was found: k s (CsCl) > k s (NaCl-KCl) > k s (KCl). On the basis of quantum chemical data for the M 3 CrF 6 + 18MCl (M = Na, K) model systems, it was shown that the complex chromium particles with four or five outer spheric sodium or potassium cations had maximum thermodynamic stability. Quantum chemical calculations were performed to interpret the experimental data on the effect of the second coordination sphere of the complexes on the standard charge transfer rate constants.

The electron transfer mechanism in a titanium containing system was investigated by electrochemical and quantum-chemical methods. The kinetics of charge transfer for the Ti(IV)/Ti(III) redox couple in the (NaCl–KCl)equimol–NaF (10 wt%)-K2TiF6 melt with addition of Ca2+ cations was studied by cyclic voltammetry. The standard rate constants of charge transfer (k s ) were calculated by Nicholson’s method. The increase values of the k s reaching the maximum at mole ratio Ca2+/Ti(IV) equals 1:1 was found. Values of activation energy for system with Ca2+ cations are considerably less than activation energies of the system without Ca2+ cations. The quantum-chemical calculations were performed using the Firefly quantum-chemical package by methods of the density functional theory. Structures with a high probability of the electron transfer from the cathode to the titanium complex were found. Using the Frontier molecular orbital method made it possible with a small amount of computer time to determine the structure of the transition state of the TiF6 2− complex. The calculated activation energy of the electron transfer was in a good agreement with experimentally determined value.

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