Thermochemistry of Gaseous Ytterbium and Gadolinium Hydroxides and Oxyhydroxides.

Gd ₂ O ₃ and Yb ₂ O ₃ 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 _n (OH) _m and GdO _n (OH) _m 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 _n (OH) _m , the B3LYP approach is used in conjunction with an effective core potential, while for GdO _n (OH) _m , 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 Yb ₂ O ₃ , Gd ₂ O ₃ , and Y ₂ O ₃ in 50% H ₂ O/Ar from 1000 to 3000 K. Hydroxide and oxyhydroxide vapor pressures are also calculated for potential coating compositions.