Probing the structure of GdCl3-KCl melt mixtures by electronic absorption spectroscopy of the hypersensitive f←f transitions of Ho3+ and by Raman spectroscopy

The structure of molten mixtures of GdCl3-KCl at different compositions and temperatures has been investigated by f <-- f electronic absorption and Raman spectroscopy. The systematics of the Raman spectra indicate that in melt mixtures rich in KCl the predominant species are GdCl63- octahedra, while at higher GdCl3 mole fraction edge bridging of the octahedra occurs. At all compositions including the pure GdCl3 melt the gadolinium coordination is presumably six-fold. The electronic absorption spectra of Ho3+ in molten LiCl, CsCl and LiCl-KCl eutectic have been measured. The data suggest that the predominant factor affecting the intensities of the Ho3+ hypersensitive transitions is the degree of octahedral distortions in conjunction with the overall symmetry around the HoCl63-. Changes of the electron donating ability of the Cl- ligands by the polarizing power of the alkali metal counter cations do not affect drastically the f <-- f intensities. Electronic absorption spectral measurements of Ho3+ centers in different GdCl3-KCl molten mixtures show that the hypersensitive band intensities increase with increasing GdCl3 mole fraction. These changes are attributed to increasing octahedral distortion imposed on the HoCl63- octahedra by the host melt. Furthermore, the spectra indicate that there are no drastic coordination changes occurring as we go from the mixtures dilute in GdCl3 to the pure GdCl3 melt, and support the Raman data view that the coordination of Gd3+ is six-fold at all compositions.