Electrochemical and spectrophotometric study on neodymium ions in molten alkali chloride mixtures

The thermodynamic stability of Nd(III) complex in various molten alkali chlorides at 923 K was studied by electrochemical analysis. The standard Gibbs free energy change of the Nd3+ formation, ΔG3/0∘, decreased by adding MCl (M = Na, K, Rb, or Cs) into LiCl. The Nd(III) complex was more stable in an alkali chloride mixture with larger averaged cationic radius. This means that the stability is controlled by the polarizing power of solvent cations. The electronic absorption spectrum of the hypersensitive f–f transition of Nd3+ was investigated to know the change in its coordination environments. The oscillator strength of the 4G5/2, 2G7/2 ← 4I9/2 transition and the degree of the energy splitting in electronic energy levels were estimated. The results suggested that the octahedral symmetry of the NdCl63− complex was more distorted in the melt with higher LiCl content, and the distortion was depressed by decreasing the polarizing power of solvent cations. The local structure around Nd3+ was significantly correlated with the thermodynamic stability of the Nd(III) complex in molten alkali chlorides.

Research highlights
► The thermodynamic stability of the NdCl63− complex and the change in its coordination environments for various molten alkali chlorides at 923 K was studied by electrochemical and absorption spectrophotometric analyses, respectively. A correlation between the macroscopic and microscopic properties and the melt composition was found. The stability decreased by adding MCl (M = Na, K, Rb, or Cs) into LiCl.