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) YBr ₃ –ABr (A = Li, K, Cs) at temperatures up to 920 °C and at different compositions; (b) YF ₃ –KF at temperatures up to 1000 °C and compositions up to 50% YF ₃ . The spectral changes occurring upon melting of Cs ₂ NaYBr ₆ , YBr ₃ and K ₃ YF ₆ crystalline compounds were also measured. The data indicate that, in mixtures rich in alkali halide, YX ₆ ³⁻ (X = F, Br) octahedra are the predominant species giving rise to two main bands P ₁ (polarized) and D ₁ (depolarized) which are assigned as follows: (a) YBr ₆ ^3- , P ₁ = 156 cm ⁻¹ , ν ₁ (A _1g ) and D ₁ = 78 cm ⁻¹ , ν ₅ (F _2g ); and (b) YF ₆ ³⁻ , P ₁ = 445 cm ⁻¹ , ν ₁ (A _1g ) and D ₁ = 225 cm ⁻¹ , ν ₅ (F _2g ). In molten mixtures rich in YBr ₃ in addition to the P ₁ and D ₁ bands a new depolarized D ₂ (ca. 210 cm ⁻¹ ) and a strong new polarized P ₂ band appear in the spectra. The P ₂ band shifts from ca. 200 cm ⁻¹ to ca. 250 cm ⁻¹ with increasing YBr ₃ content. The presence of these four bands and their polarization characteristics suggest that the predominant vibrational modes in the YBr ₃ -rich mixtures are due to a close C _3ν pyramidal like ‘unit’ arising from the D ₃ distortions of the YBr ₆ ³⁻ octahedra bound by edges in the melt. This behaviour, as well as the spectral changes upon melting YBr ₃ , supports the view that the structure of pure molten YBr ₃ consists of edge-sharing distorted octahedra. The molten fluoride mixtures YF ₃ –KF at composition X _YF3 > 0.25 also show four bands, two depolarized at ca. 240 cm ⁻¹ (D ₁ ), ca. 370 cm ⁻¹ (D ₂ ) and two overlapping polarized bands at ca. 440 cm ⁻¹ (P ₁ ), ca. 460 cm ⁻¹ (P ₂ ). Finally, the trends of the YX ₃ –KX spectra on going from the bromide to chloride to fluoride melts suggest that pure molten YF ₃ is likely to possess a loose ‘network’ structure of edge-bridged distorted octahedra as in the case of molten YCl ₃ and YBr ₃ .