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 Ra man spectroscopy. The systematics of the Raman spectra indicate that in mel t mixtures rich in KCl the predominant species are GdCl63- octahedra, while at higher GdCl3 mole fraction edge bridging of the octahedra occurs. At al l compositions including the pure GdCl3 melt the gadolinium coordination is presumably six-fold. The electronic absorption spectra of Ho3+ in molten L iCl, CsCl and LiCl-KCl eutectic have been measured. The data suggest that t he 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 abil ity of the Cl- ligands by the polarizing power of the alkali metal counter cations do not affect drastically the f <-- f intensities. Electronic absor ption spectral measurements of Ho3+ centers in different GdCl3-KCl molten m ixtures show that the hypersensitive band intensities increase with increas ing GdCl3 mole fraction. These changes are attributed to increasing octahed ral distortion imposed on the HoCl63- octahedra by the host melt. Furthermo re, the spectra indicate that there are no drastic coordination changes occ urring as we go from the mixtures dilute in GdCl3 to the pure GdCl3 melt, a nd support the Raman data view that the coordination of Gd3+ is six-fold at all compositions.