Vibrational modes and structure of rare earth halide alkali halide binary melts - YBr3-ABr (A=Li, K, Cs) and YF3-KF

Raman spectra of the following binary molten salt systems have been measured: (a) YBr3-ABr (A = Li, K, Cs) at temperatures up to 920 degrees C and at different compositions; (b) YF3-KF at temperatures up to 1000 degrees 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 P-1 (polarized) and D-1 (depolarized) which are assigned as follows: (a) YBr63-, P-1 = 156 cm(-1), v(1)(A(1g)) and D-1 = 78 cm(-1), v(5) (F-2g); and (b) YF63-, P-1 = 445 cm(-1), v(1)(A(1g) and D-1 = 225 cm(-1), v(5)(F-2g). In molten mixtures rich in YBr3 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, 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-3v pyramidal like 'unit' arising from the D-3 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) (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 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.