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 .