Spectrophotometric study of the gaseous species over molten HoCl3, LiCl-HoCl3 and CsCl-HoCl3 at temperature up to 1500K

Electronic absorption spectroscopy is used in the temperature range 1180–1480 K, to study the vapor species over molten HoCl3, and 1:1 molten mixtures of MCl: HoCl3 (M = Li, Cs). Emphasis is given on the f ← f hypersensitive transition of Ho(III) and the effect of the chloride coordination geometries on the splitting of the most intense hypersensitive transition 5G6 ← 5I8. The absorbance measurements permit the determination of the enthalpies of vaporization of the species formed over the melts studied: ΔHvap.(HoCl3) = 240 ± 9 kJ mol−1, ΔHvap.(LiHoCl4) = 193 ± 4 kJ mol−1 and ΔHvap.(CsHoCl4) = 202 ± 8 kJ mol−1. The volatility enhancement of Ho(III) due to the formation of the MHoCl4 vapor species is calculated at different temperatures. A three-fold and a four-fold coordination of Ho(III) is anticipated for the HoCl3 (g) and the MHoCl4 (g) species, respectively. The molar absorptivities (ε) and oscillator strengths (f) increase as the coordination decreases from the ‘‘octahedral’’ HoCl63− (ε = 9 L mol−1 cm−1; f = 15 × 106) to the distorted tetrahedral HoCl4− (ε = 84 L mol−1 cm−1; f = 102 × 10−6) to the trigonal HoCl3 (ε = 105 L mol−1 cm−1; f = 110 × 10−6). The vapor spectra support the view for a trigonal (most likely C3v) geometry for HoCl3(g) and a C2v symmetry for the MHoCl4(g) (M = Li, Cs) with the Ho(III) in a distorted HoCl4 tetrahedral symmetry. At high kT values, due to the higher mobility of Li+, the LiHoCl4(g) structure has a nearly D4h like symmetry with the Ho(III) possessing a pseudo-like inversion center. [Display omitted] •Elucidation of chloride coordination geometries effect on 5G6←5I8 hypersensitive transition.•Enthalpies of vaporization of the species formed over the melts were determined.•Estimation of Ho(III) volatility enhancement due to MHoCl4 (M = Li, Cs) vapor species.•Coordination, symmetry and ligand polarizability affect hypersensitive transition.