Sm 3 + substitution-mediated oxygen vacancy clustering: Theoretical analysis and experimental verification for suppressing low-temperature contraction of La 2 Ce 2 O 7 ceramics

Lanthanum cerate (La2Ce2O7, LC) is a promising thermal barrier coating (TBC) candidate with superior thermophysical properties over yttria-stabilized zirconia (YSZ), but its practical application is hindered by low-temperature thermal expansion coefficient (TEC) contraction. Previous studies primarily focused on regulating oxygen vacancy concentration, while neglecting the influences of vacancy distribution. Herein, we employ Sm3+ isovalent substitution for La3+ to maintain a constant vacancy concentration and isolate the vacancy distribution effects. The optimal composition (La0.8Sm0.2)2Ce2O7 significantly suppresses low-temperature contraction, reducing the linear shrinkage rate by ~88.5% and increasing the TEC by ~12.56% (to 12.37 × 10–6 K–1) compared with LC (10.99 × 10–6 K–1). Combining density functional theory (DFT) calculations and HR-TEM/AC-STEM characterization, we directly reveal Sm3+-induced oxygen vacancy clustering in LC-based ceramics. The underlying mechanism involves: (1) Randomly distributed free vacancies induce contraction via vacancy-phonon coupling and local symmetry breaking; (2) Sm3+ substitution introduces dislocations whose stress fields, together with Sm3+’s higher ionic potential, drive vacancy clustering; (3) Clustering reduces mobile vacancies and restores lattice order, thereby suppressing contraction. This work confirms that LC’s low-temperature contraction is co-regulated by vacancy concentration and distribution, complementing existing concentration-modulation strategies.