Vibrational modes and structures of lanthanide halide–alkali halide binary melts LnBr3–KBr (Ln=La, Nd, Gd) and NdCl3–ACl (A=Li, Na, K, Cs)

Abstract

Raman spectra of the following rare earth halide–alkali halide binary molten salt systems have been measured: LnBr3–KBr (Ln=La, Nd, Gd) and NdCl3–ACl (A=Li, Na, K, Cs). The complete composition range has been studied at temperatures up to 850°C. The spectral changes occurring upon melting the elpasolite compounds Cs2NaLnBr6 (Ln=La, Nd, Gd) and Cs2NaNdCl6 and the pure crystalline solids LnBr3 (Ln=La, Nd, Gd) and NdCl3 were also measured. The data indicate that the behavior of these melt mixtures is similar to those of the YX3–KX (X=F, Cl, Br) binaries studied before. In molten mixtures rich in alkali halide with lanthanide halide mole fractions less than 0.25, the predominant species are the LnX63- octahedra giving rise to two main bands P1 (polarized) and D1 (depolarized) which are assigned to the ν1(A1g) and ν5(F2g) octahedral modes. In molten mixtures rich in LnX3 the spectra are characterized by the P1 and D1 bands plus two new bands D2 (depolarized) and P2 (polarized). The P2 band shifts continuously to higher frequencies with increasing LnX3 content. These four bands are attributed to the D3 distortions of the LnX63- octahedra bound by edges in the melt. The room temperature Raman spectra of the LnX3 solid compounds were characterized by bands due to the vibrational modes of the different crystalline structures: hexagonal for LaBr3 and NdCl3, orthorhombic for NdBr3 and rhombohedral for GdBr3 having the Ln3+ coordination number (CN) or 9, 8 and 6, respectively. With increasing temperature the spectra of the GdBr3 solid are dominated by six Raman bands which are assigned to the vibrational modes of a triple layer of ions consisting of distorted octahedra GdBr63- (CN=6) which share edges with neighboring octahedra. Upon melting, the molar volume of GdBr3 does not change much and the spectra are characterized by the above-mentioned P1, P2, D1 and D2 bands and can be correlated to the triple layer modes of the solid. The high temperature spectra of the hexagonal LaBr3, NdCl3 and the orthorhombic NdBr3 show that the structure and CN remain the same up to melting. However, upon melting, the compact orthorhombic (CN=8) and hexagonal (CN=9) forms increase drastically their molar volume and give spectra similar to those of molten GdBr3, YBr3 and YCl3 where the Ln3+ is in a six-fold coordination (CN=6). It appears that the structures of all the LnX3 melts are similar and independent of the structure of the solids. The frequency changes upon melting the LnX3 solids, the presence and assignment of the P1, P2, D1 and D2 bands in the spectra, the continuous shift of the P2 band with composition in the LnX3–AX binaries and the correlation of the high temperature modes of the rhombohedral LnX3 solid (CN=6) to the liquid suggest that the loose network structure proposed for the LnX3 melts is more likely to arise from ‘triple layer’ like structures composed of distorted octahedra. The rigidity of the network is related to the splitting of the P1 and P2 band and increases with increasing distortion of the octahedra in the sequence La–Y; F–Br. Fast interchange of ions leads to short lifetimes for the octahedra and weak intralayer interactions.