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

Abstract

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.