Thermochemistry of Gaseous Ytterbium and Gadolinium Hydroxides and Oxyhydroxides

Gd₂O₃ and Yb₂O₃ are the proposed constituents of advanced coating systems in combustion environments. In such environments, they are exposed to high-temperature water vapor, which would lead to gaseous hydroxide formation. Thermodynamic parameters are reported for YbO(OH) and GdO(OH) species. We first study the MH, MO, MF, and MCl (M = Yb and Gd) species, where some experimental data exist. Structures and spectroscopic constants were calculated at the B3LYP level. For YbO(OH), the B3LYP approach is used in conjunction with an effective core potential, while for GdO(OH), it was necessary to use all-electron basis sets. Enthalpies of formation were calculated with a BD(T) approach. The enthalpies of formation, entropies, and heat capacities were added to a thermochemical database. Hydroxide and oxyhydroxide vapor pressures are calculated above pure Yb₂O₃, Gd₂O₃, and Y₂O₃ in 50% H₂O/Ar from 1000 to 3000 K. Hydroxide and oxyhydroxide vapor pressures are also calculated for potential coating compositions.