Ion-molecular equilibria in lanthanide triiodide vapors and formation enthalpies of LnI4− and Ln2I7− ions (Ln = La, Ce, Pr, Gd, Tb, Tm, and Lu)

Ion-molecular equilibria LaI4– + LnI3 = LnI4– + LaI3, Ln2I7– = LnI3,cr + LnI4– (Ln = Ce, Pr, Gd, Tb, Tm, and Lu), and 3LaBr4– + 4LaI3 = 3LaI4– + 4LaBr3 in vapors over the LaI3-CeI3-PrI3-GdI3, LaI3-GdI3-TbI3-TmI3, and LaI3-LaBr3 mixtures and the pure lanthanide triiodides were studied in the temperature range of 700–1050 K by Knudsen effusion mass spectrometry. The equilibrium constants were measured and the reaction enthalpies were calculated by the techniques of the second and third laws of thermodynamics. The standard formation enthalpies of ions, ΔfH°(298.15 K) (in kJ mol–1), were determined to be –832 ± 21 (LaI4–), –834 ± 23 (CeI4–), –834 ± 25 (PrI4–), –812 ± 24 (GdI4–), –813 ± 25 (TbI4–), –817 ± 23 (TmI4–), –806 ± 25 (LuI4–), –1399 ± 45 (La2I7–), –1402 ± 46 (Ce2I7–), –1417 ± 47 (Pr2I7–), –1356 ± 47 (Gd2I7–), –1365 ± 47 (Tb2I7–), –1366 ± 46 (Tm2I7–), and –1322 ± 47 (Lu2I7–).