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

Abstract

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 777–946K; lnp(Gd2I6)=−(38.07±0.42)×103/T+(24.49±0.47) for 843–946K; lnp(TmI3)=−(31.84±0.25)×103/T+(21.88±0.29) for 760–965K; and lnp(Tm2I6)=−(38.23±0.27)×103/T+(25.71±0.43) for 799–965K. 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 [kJmol−1] at 298.15K 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 [kJmol−1] of the gaseous species at 298.15K are −341±9 (GdI3), −912±16 (Gd2I6), −1506±50 (Gd3I9), −344±5 (TmI3), −909±7 (Tm2I6), and −1506±50 (Tm3I9).