Sublimation enthalpies of gadolinium and thulium triiodides and formation enthalpies of the molecules LnI3, Ln2I6, and Ln3I9 (Ln = Gd, Tm)

- Sublimation of GdI3 and TmI3 was studied by Knudsen effusion mass spectrometry.
- Monomer, dimer, and trimer molecules were registered in vapor.
- Partial vapor pressures of the molecules were determined.
- Sublimation and formation enthalpies were evaluated.

The sublimation of solid gadolinium and thulium triiodides was studied by Knudsen effusion mass spectrometry. The partial vapor pressures [atm] of the monomer and dimer molecules were determined as: lnp(GdI3) = −(32.31 ± 0.18) × 103/T + (21.38 ± 0.21) for 777-946 K; lnp(Gd2I6) = −(38.07 ± 0.42) × 103/T + (24.49 ± 0.47) for 843-946 K; lnp(TmI3) = −(31.84 ± 0.25) × 103/T + (21.88 ± 0.29) for 760-965 K; and lnp(Tm2I6) = −(38.23 ± 0.27) × 103/T + (25.71 ± 0.43) for 799-965 K. In addition, the trimers Gd3I9 and Tm3I9 were detected in very small fractions. On the basis of a joint analysis of all available data in literature, the sublimation enthalpies [kJ mol−1] at 298.15 K are recommended as 283 ± 9 (GdI3), 336 ± 16 (Gd2I6), 366 ± 50 (Gd3I9), 276 ± 3 (TmI3), 330 ± 6 (Tm2I6), and 354 ± 50 (Tm3I9). The standard formation enthalpies [kJ mol−1] of the gaseous species at 298.15 K are −341 ± 9 (GdI3), −912 ± 16 (Gd2I6), −1506 ± 50 (Gd3I9), −344 ± 5 (TmI3), −909 ± 7 (Tm2I6), and −1506 ± 50 (Tm3I9).

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