The Ho(III) as structural probe for high temperature ionic liquids: RCl3 (R = rare earth)

The electronic absorption spectra of the f ← f transitions of Ho3+ as a probe cation doped in solid and liquid YCl3 and TbCl3 have been measured from room temperature to above the melting point of both compounds. In the same temperature range Raman spectroscopy has been utilized to characterize all phases present. Emphasis is given on the Ho3+ ligand field spectra of the 5G6 ← 5I8 hypersensitive transition and their similarities with the known spectra of holmium chloro-elpasolite. Smooth spectral changes with nearly invariant molar absorptivities (between 30 and 40 L mol−1 cm−1) are observed upon melting YCl3 and α-TbCl3 which indicate that most probably the Ho3+ is in 6-fold distorted octahedral coordination in both compounds and both phases. Drastic spectral changes are observed alongside the α-TbCl3 to β-TbCl3 phase transition where the coordination is altered from six to eight and the molar absorptivity quadruples. The spectra of liquid Y(Ho)Cl3 and Tb(Ho)Cl3 are correlated to the spectra of liquid La(Ho)Cl3 and Gd(Ho)Cl3 and are attributed to the ligand field states of Ho3+ in distorted octahedral geometries. In conjunction with the molar volumes of all RCl3 (R = rare earth) liquids which are invariant and independent of R and with Raman spectroscopic measurements for the same systems, a common possible structure of the rare earth chloride liquids is deduced and proposed.