Ionic field strength links local coordination chemistry to CMAS resistance in medium entropy rare earth silicates

Medium entropy rare earth (RE) silicates represent a promising frontier for environmental barrier coatings, yet the interplay between chemical complexity, phase formation, and calcium–magnesium–aluminosilicate (CMAS) resistance remains poorly understood. This study elucidates how medium entropy substitution at the RE site modulates [REO n ] coordination polyhedra to dictate the phase formation and corrosion behavior of monosilicates (RE 2 SiO 5 ), disilicates (RE 2 Si 2 O 7 ), and oxyapatite silicates (RE 9.33 (SiO 4 ) 6 O 2 ). We find that X2-RE 2 SiO 5 formation is highly sensitive to local [REO n ] configurations, where increased cation size mismatch and larger average radii destabilize the framework, promoting the formation of secondary disilicate and vacancy rich oxyapatite phases. In contrast, the RE 2 Si 2 O 7 system exhibits superior structural adaptability, accommodating broad multi cation substitutions across monoclinic and orthorhombic polymorphs. CMAS corrosion tests at 1300 ºC reveal that X2 dominated compositions facilitate rapid formation of thin, dense Ca apatite barriers that effectively arrest melt infiltration. Conversely, formulations rich in RE 9.33 (SiO 4 ) 6 O 2 provide poor protection, where the vacancy-rich oxyapatite framework is associated with enhanced Ca-mediated reaction and deep melt penetration. To unify these findings, this study identifies ionic field strength (IFS), derived from the local chemistry of [REO n ] coordination polyhedra, as a key descriptor linking medium entropy substitution, phase formation, and CMAS corrosion behavior. Higher IFS, associated with smaller RE ions and lower coordination numbers, is found to promote increased lattice rigidity and self-protecting corrosion kinetics. By establishing IFS as a structure-based metric for rational rare earth selection, this work provides a mechanistic foundation for the accelerated design of multi component silicates for high temperature environmental barrier coatings.