Thermodynamic study of the rare earth vapour complexes: ScAl2Cl9 and YAl2Cl9

Abstract

In all previous publications dealing with LnAl_nCl_{3n+ 3}(Ln = rare earth elements), the LnAl₃Cl₁₂ complex is established to be the main vapour complex in the temperature range 500–900 K. Here we present results from quenching experiments which suggest that the LnAl₂Cl₉ complex instead of LnAl₃Cl₁₂ is the predominant vapour complex for Ln = Sc and Ln = Y in the ranges 588–806 K and 0.02–0.22 MPa. This observation would indicate that the stoichiometry of the LnAl_nCl_{3n+ 3} vapour complex depends mainly on the rare earth ionic structure and not on the rare earth ionic radius. Thermodynamic functions of the reaction LnCl₃(s)+ Al₂Cl₆(g)= LnAl₂Cl₉(g) derived from the measurements are: ΔH^°₂₉₈= 9.6 ± 3 kJ mol^–1, ΔS^°₂₉₈=–7.1 ± 4 J mol^–1 K^–1 for ScAl₂Cl₉ and ΔH^°₂₉₈= 20.1 ± 3 kJ mol^–1, ΔS^°₂₉₈=-4.8 ± 4 J mol^–1 K^–1 for YAl₂Cl₉.