Theoretical investigation of formation and diffusion mechanisms for point defects in ytterbium and lutetium silicates

Understanding the mechanism of point defects and the oxygen vacancy diffusion in rare earth (RE) silicates is important to evaluate their performance as environmental barrier coatings. In this work, the defect formation energies are calculated and the similar predominant native point defect type, that is, O Frenkel defect, in RE2SiO5 and RE2Si2O7 (RE = Yb and Lu) is uncovered. Furthermore, the nonstoichiometry related defects and phase composition in RE2SiO5 and RE2Si2O7 is also studied. For RE2SiO5, SiRE center dot+Oi '' or SiRE center dot+VRE ''' is the predominate defect complex accompanied with the RE2Si2O7 separation under SiO2-rich condition, while RESi '+VO center dot center dot or RESi '+REi center dot center dot center dot is favorable under RE2O3-rich condition. In the case of RE2Si2O7, SiRE center dot+VRE ''' or SiRE center dot+Oi '' is the favorable defect complex under SiO2-rich condition while RESi '+VO center dot center dot or RESi '+REi center dot center dot center dot dominates the defects, accompanied with the formation of RE2SiO5, under RE2O3-rich case. Finally, the high oxygen migration barriers are revealed for both RE2SiO5 and RE2Si2O7, indicating the low oxygen permeability in these silicates. These results will not only guide the future performance optimization of RE2SiO5 and RE2Si2O7 through tailoring their composition, but also suggest their potential application as oxidation protective coatings.