Yttrium silicates (Y-Si-O oxides), including Y 2 Si 2 O 7 , Y 2 SiO 5 , and Y 4·67 (SiO 4 ) 3 O apatite, have attracted wide attentions from material scientists and engineers, because of their extensive polymorphisms and important roles as grain boundary phases in improving the high-temperature mechanical/thermal properties of Si 3 N 4 and SiC ceramics. Recent interest in these materials has been renewed by their potential applications as hightemperature structural ceramics, oxidation protective coatings, and environmental barrier coatings (EBCs).