Systematics and Anomalies in Rare Earth/Aluminum Bromide Vapor Complexes:  Thermodynamic Properties of the Vapor Complexes LnAl₃Br₁₂ from Ln = Sc to Ln = Lu

Systematics and anomalies in the rare earth/aluminum bromide vapor complexes have been investigated by the phase equilibrium-quenching experiments. The measurements suggest that the LnAl₃Br₁₂ complexes are the predominant vapor complexes for the 16 rare earth elements Ln = Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu in the temperature range 601−833 K and pressure range 0.01−0.22 MPa, which is different from the rare earth/aluminum chloride systems, where the predominant vapor complexes are LnAl₃Cl₁₂ from Ln = La to Ln = Lu, but LnAl₂Cl₉ for Ln = Sc and Y are roughly in the same ranges, which indicates the importance of the halogen anion radius on the rare earth vapor complex formation. In the temperature and pressure ranges, gaseous Al₂Br₆ and AlBr₃ are dominant species and the molar fraction of LnAl₃Br₁₂ is normally less than 0.01. Thermodynamic functions of the reactions LnBr₃(s) + (3/2)Al₂Br₆(g) = LnAl₃Br₁₂(g) were calculated from the measurements for the 16 rare earth elements and then smoothly interpolated for the radioelement Ln = Pm. The standard molar enthalpies and standard molar entropies show significant Gd divergences from LaAl₃Br₁₂ to LuAl₃Br₁₂ when plotted as functions of the rare earth atomic number. They also suggest nearly linear manner for ScAl₃Br₁₂, LuAl₃Br₁₂, YAl₃Br₁₂, and LaAl₃Br₁₂ when plotted as functions of the rare earth ionic radius.