Molecular and ionic sublimation of the lanthanide triiodides. Thermochemical properties of LnI3 and Ln2I6 molecules and LnI4– and Ln2I7– ions

Abstract

The results of systematic investigations of the molecular and ionic sublimation of lanthanide triiodides LnI3 (Ln = La–Lu) by Knudsen effusion mass spectrometry are summarized. The LnI3 monomer and Ln2I6 dimer molecules as well as the associated LnI4– and Ln2I7– ions are the main constituents of saturated vapor. The total fraction of ions is negligible compared to that of neutral species. The partial vapor pressures of the molecules and the equilibrium constants of ion-molecular reactions are determined. On the basis of a joint analysis of all vaporization thermodynamics data obtained in the period from 1952 to 2020, the sublimation enthalpies ΔsHo(298.15 K) of LnI3 in the form of monomer and dimer molecules are recommended. The standard thermochemical properties such as the formation enthalpy and atomization/dissociation enthalpy of the molecules and ions are obtained. The variation of these properties along the lanthanide series is discussed.