Correlating changes in structure and dynamical properties in LnX3 (Ln = Y, Ho, Dy, Gd, Nd, La and X = Cl, Br) ionic melts

Raman spectroscopy was used to probe the vibrational dynamics of LnX3 molten salts for a variety of lanthanides (Ln = La, Nd, Gd, Dy, Ho, Y) and halides (X = Br, Cl). Analysis of the line profiles has been performed by employing a time-correlation function with an analytical counterpart in the frequency domain, which permits fitting of specific vibrational features in an effort to elucidate the structure and obtain dynamical information in parallel. It is established using 2-D correlation methodology that all lanthanide halides exhibit similar structural properties and analogous vibrational relaxation and frequency modulation times. The structure of the lanthanide halide melts studied in this work is composed of distorted octahedra that constitute a loose network as reported before. The rigidity of the network is related to the splitting of the P1 and P2 bands, attributed to the ν1(A1g) and ν5(F2g) fundamental octahedral modes, and increases with increasing distortion of the octahedra in the sequence La → Y and Cl → Br. A variety of picosecond dynamics models have been tested and found that only the Rothschild approach complies well with the experimental data assuming that the environmental modulation is described by a stretched exponential decay. The experimental results are discussed in terms of theoretical models providing insights in the inter-molecular coupling mechanisms.