Investigation of Native Point Defects and Nonstoichiometry Mechanisms of Two Yttrium Silicates by First‐Principles Calculations

Abstract

First‐principles method is used to study the native point defects in Y2SiO5 and Y2Si2O7 silicates. The calculated defect formulation energies show similar native point defect behaviors in Y2SiO5 and Y2Si2O7: the oxygen Frenkel defect is predominant; and it is followed by the cation antisite and Schottky defects. The possible chemical potential range of each constituent is further considered in the calculation of defect formation energy. Oxygen interstitial (Oi) and oxygen vacancy (VO) are the predominant native point defects under O‐rich and O‐poor condition, respectively. In addition, the mechanisms of accommodating composition deviations from stoichiometric Y2SiO5 and Y2Si2O7 are investigated. For Y2SiO5, Y2Si2O7 impurity may appear, together with the defects of SiY antisite, Oi interstitial, and/or VY vacancy when SiO2 is excess; while YSi antisite appears together with Yi interstitial and/or VO vacancy in Y2SiO5 when Y2O3 is excess. For Y2Si2O7, the main process is the formation of SiY antisite accompanied by Oi interstitial and/or VY vacancy when SiO2 is excess; but Y2SiO5 impurity forms, together with YSi antisite, VO vacancy, and/or Yi interstitial in Y2Si2O7 when Y2O3 is excess. We expect that the results are useful to control of processing conditions and further to optimization of performance of the two silicates.

Composition‐dependent intrinsic defect structures in pyroclore RE2B2O7 (RE = La, Nd, Gd; B = Sn, Hf, Zr)

“…Furthermore, the mono‐vacancy structure is built by removing one atom from its regular lattice site. The 48 f is the preferred oxygen vacancy position as it has lower formation energy than 8 b , being the same as previous report …”

“…The good agreements ensure the reliability of our first‐principles calculations. Furthermore, the calculated E g is smaller than the experimental values, which is the typical deviation of DFT calculations …”

“…In RE 2 B 2 O 7 , there is only one configuration for the cation vacancies and cation antisites, but the interstitials and oxygen vacancies are not. For modeling interstitials, one additional atom is put on the possible interstitial position (both 32 e and 8 a ) in the lattice . After completing geometry optimization, the interstitial configuration of each constitute ( RE , B or O) with the lowest total energy is selected as the stable interstitial defect.…”

“…Furthermore, the calculated E g is smaller than the experimental values, 31,[41][42][43][44][45][46][47] which is the typical deviation of DFT calculations. 48 In RE 2 B 2 O 7 , there is only one configuration for the cation vacancies and cation antisites, but the interstitials and oxygen vacancies are not. For modeling interstitials, one additional atom is put on the possible interstitial position (both 32e and 8a) in the lattice.…”

Failure Mechanism Associated with the Thermally Grown Silica Scale in Environmental Barrier Coated C/SiC Composites

“…(2) The Microstructure and Strength Evolution of the Y 2 Si 2 O 7 -BSAS-Coated C/SiC Composites In order to determine the failure behavior of other EBCs system rather than BSAS-coated C/SiC composites in water vapor, we coated Y 2 Si 2 O 7 -BSAS on the C/SiC composites as EBC. The selection of Y 2 Si 2 O 7 is because it has a lower oxygen diffusion coefficient than BSAS, 31 and the matched CTE (3.9 AE 0.4 9 10 À6 /°C) for the C/SiC composites. 32 It is expected that there is a different silica growth rate.…”