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)

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