Electrochemistry Of Rare-earth Metals And Synthesis Their Borides In Halide Melts

Abstract:

Electrochemical behaviour of rare-earth metal trichlorides (RECl3), where RE-Nd, Sm, Eu, Yb was studied by different electrochemical methods in NaCl-KCl, KCl and CsCl melts. The diffusion coefficients (D) for the oxidation states RE3+ and RE2+ were determined by linear sweep voltammetry, chronoamperometry and chronopotentiometry methods showing that D decreases with increase in the oxidation state of RE, while the activation energy for diffusion increases. On the basis of the data obtained in the temperature range 973-1123 K it is possible to conclude that DNd(III,II)>DSm(III,II)>DEu(III,II)>DYb(III,II) in chloride melts. The standard rate constants of charge transfer (ks) for the redox couples Nd(III)/Nd(II), Sm(III)/Sm(II), Eu(III)/Eu(II) and Yb(III)/Yb(II) were calculated on the basis of cyclic voltammetry. The values of constants testify that the redox processes proceed quasi-reversibly, mostly under diffusion control. It was found that ksNd >ksSm>ksEu >ksYb and the standard rate constants increase when going from NaCl-KCl to CsCl melt. According to the theory of elementary charge transfer, the smaller and stronger bond complexes require higher rearrangement energy, and in consequence the electroreduction proceeds with slower rate. The complex size and metal-ligand bond length decreases from NaCl-KCl to KCl and CsCl. Therefore a decrease of the charge transfer constants would have been expected that contradicts data for the redox reactions RE(III) + e- = RE(II) (1) However, the previous approach is not valid for all electrochemical systems. Our experimental results allow to suggest that for the redox reaction (1) the limiting stage is not rearrangement of complexes, but electron transfer over outer sphere cations. In this case the explanation of the larger values observed for the standard rate constants in CsCl melt is the larger polarizability of caesium. The formal redox potentials of E*Nd(III)/Nd(II), E*Sm(III)/Sm(II), E*Eu(III)/Eu(II), E*Yb(III)/Yb(II) were determined from cyclic voltammetry, chronopotentiometry and steady-state voltammetry data. Some thermodynamic properties for dilute solutions of rare-earth metal chlorides such as relative partial molar enthalpy of mixing and activity coefficients were calculated. In present work the study of mechanism and kinetics of boron and RE joint electrodischarge in chloro-fluoride molten systems is reported too. The optimum regime of neodymium, europium and gadolinium boride manufacture has been worked out on the base of voltammetric experiments. It is necessary to note that europium and gadolinium compounds were synthesized as monophase products (EuB6 or GdB6), while co-deposition NdB4 and NdB6 was found for neodymium.

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