Influence of average radii of RE3+ ions on phase structures and thermal expansion coefficients of high-entropy pyrosilicates

High-entropy pyrosilicate element selection is relatively blind, and the thermal expansion coefficient (CTE) of traditional beta-type pyrosilicate is not adjustable, making it difficult to meet the requirements of various types of ceramic matrix composites (CMCs). The following study aimed to develop a universal rule for high-entropy pyrosilicate element selection and to achieve directional control of the thermal expansion coefficient of high-entropy pyrosilicate. The current study investigates a high-entropy design method for obtaining pyrosilicates with stable beta-phase and gamma-phase by introducing various rare-earth (RE) cations. The solid-phase method was used to create 12 different types of high-entropy pyrosilicates with 4-6 components. The high-entropy pyrosilicates gradually transformed from beta-phase to gamma-phase with an increase in the average radius of RE3+ ions ((r) over bar( RE3+)). The nine pyrosilicates with a small (r) over bar (RE3+) preserve beta-phase or gamma-phase stability at room temperature to the maximum of 1400 degrees C. The intrinsic relationship between the thermal expansion coefficient, phase structure, and RE-O bond length has also been found. This study provides the theoretical background for designing high-entropy pyrosilicates from the perspective of (r) over bar (RE3+). The theoretical guidance makes it easier to synthesize high-entropy pyrosilicates with stable beta-phase or gamma-phase for the use in environmental barrier coatings (EBCs). The thermal expansion coefficient of gamma.-type high-entropy pyrosilicate can be altered through component design to match various types of CMCs.