Temperature dependence of the f←f hypersensitive transitions of Ho3+ and Nd3+ in molten salt solvents and the structure of the LaCl3–KCl melts

The electronic absorption spectra of the hypersensitive transitions 5G6←5I8 of Ho3+ and 4G5/2←4I9/2 of Nd3+ have been measured in molten LiCl–KCl eutectic and in LaCl3–KCl molten mixtures. The ligand field components of the above transitions in octahedral symmetry have been identified and the molar absorptivity changes with temperature and composition have been measured. The spectra are discussed in terms of the coordination geometries of the probe cations Ho3+ and Nd3+ in these molten salt solvents. The Boltzmann thermal factor appears to be responsible in part for the variation of the ‘hot’ band intensities. The data suggest that the predominant factors affecting the oscillator strength of the Ho3+ and Nd3+ hypersensitive transitions is the degree of octahedral distortions in conjunction with the overall charge asymmetry around the LnCl63− (Ln=Ho, Nd). It appears that no drastic changes occur in the coordination of the probe cations as we go from dilute in LaCl3 to pure LaCl3 melts; this suggests that in the host melt the coordination of La3+ is more likely to be six-fold. A further evidence for the six-fold coordination is given by the measured Judd–Ofelt parameters of Nd3+ in both molten NdCl3 and La(Nd)Cl3.