Neodymium(III) cathodic processes in molten fluorides

The electrochemical behaviour of the Nd(III)/Nd(0) system has been investigated in several molten media and more particularly in LiF-CaF2. A preliminary study based both on thermodynamic and experimental data showed that it is not possible to observe the Nd(III)/Nd(0) system in LiF-KF and LiF-NaF melts; because the K+ and Na+ cation reduction waves hide the Nd3+ reduction wave. Then, the Nd(III)/Nd(0) system has been investigated at 810 deg. C using solutions of NdF3 in fluoride solvents without K+ and Na+ ions, such as LiF-CaF2, by cyclic voltammetry, chronopotentiometry and square wave voltammetry. Experimental results show that neodymium trifluoride is reduced in Nd(0) in a one-step process exchanging three electrons (Nd(III) + 3e- → Nd(0)). The electrode process is shown to be diffusion controlled. Nd(III) diffusion coefficient is in the range of 1.1-1.3 x 10-5 cm2 s-1 at 810 deg. C