Vibrational modes and structure of rare earth halide–alkali halide binary melts

YBr₃–ABr (A = Li, K, Cs) and YF₃–KF

Abstract

Raman spectra of the following binary molten salt systems have been measured: (a) YBr₃–ABr (A = Li, K, Cs) at temperatures up to 920 °C and at different compositions; (b) YF₃–KF at temperatures up to 1000 °C and compositions up to 50% YF₃. The spectral changes occurring upon melting of Cs₂NaYBr₆, YBr₃ and K₃YF₆ crystalline compounds were also measured. The data indicate that, in mixtures rich in alkali halide, YX₆³⁻ (X = F, Br) octahedra are the predominant species giving rise to two main bands P₁ (polarized) and D₁ (depolarized) which are assigned as follows: (a) YBr₆^3-, P₁ = 156 cm⁻¹, ν₁(A_1g) and D₁ = 78 cm⁻¹, ν₅ (F_2g); and (b) YF₆³⁻, P₁ = 445 cm⁻¹, ν₁(A_1g) and D₁ = 225 cm⁻¹, ν₅(F_2g). In molten mixtures rich in YBr₃ in addition to the P₁ and D₁ bands a new depolarized D₂ (ca. 210 cm⁻¹) and a strong new polarized P₂ band appear in the spectra. The P₂ band shifts from ca. 200 cm⁻¹ to ca. 250 cm⁻¹ with increasing YBr₃ content. The presence of these four bands and their polarization characteristics suggest that the predominant vibrational modes in the YBr₃-rich mixtures are due to a close C_3ν pyramidal like ‘unit’ arising from the D₃ distortions of the YBr₆³⁻ octahedra bound by edges in the melt. This behaviour, as well as the spectral changes upon melting YBr₃, supports the view that the structure of pure molten YBr₃ consists of edge-sharing distorted octahedra. The molten fluoride mixtures YF₃–KF at composition X_YF3 > 0.25 also show four bands, two depolarized at ca. 240 cm⁻¹ (D₁), ca. 370 cm⁻¹ (D₂) and two overlapping polarized bands at ca. 440 cm⁻¹ (P₁), ca. 460 cm⁻¹ (P₂). Finally, the trends of the YX₃–KX spectra on going from the bromide to chloride to fluoride melts suggest that pure molten YF₃ is likely to possess a loose ‘network’ structure of edge-bridged distorted octahedra as in the case of molten YCl₃ and YBr₃.