Ionic radius regulation on defect structure and electrical properties of flash-sintered TiO₂-based ceramics

TiO₂-based ceramics with high permittivity and excellent nonlinear current density-electric field (J-E) characteristics are essential for miniaturizing electronic devices. However, the intrinsic correlations among ionic radius, defect evolution, and the synergistic optimization of dielectric and varistor properties in flash-sintered TiO₂-based ceramics remain unclear. Herein, (A0.5Sb0.5)0.01Ti0.99O₂ (A = La, Sm, Tm; LSTO, SSTO, TSTO) ceramics were prepared via flash sintering, establishing an ionic radius engineering strategy. All dopants form a single-phase rutile solid solution. Larger rare-earth ions induce greater lattice distortion, promoting the formation of Ti³⁺ (14.04% for LSTO, 11.65% for TSTO) and oxygen vacancies (9.50% for LSTO, 7.66% for TSTO). Among all compositions, SSTO (Sm³⁺, moderate ionic radius) exhibits the best comprehensive properties: high relative density (95.26%), colossal permittivity (ε=1.18×10⁴), moderate dielectric loss (tan δ = 0.572) at room temperature (RT) and 1 kHz, superior dielectric temperature stability, and excellent varistor performance (nonlinear coefficient α=2.5, leakage current IL = 479 μA, and threshold field VT=55.9 V/mm). Mechanistic analysis reveals that the ionic radius synergistically modulates lattice distortion and defect chemistry (Ti³⁺/oxygen vacancies). Moderate lattice distortion in SSTO forms a stable defect structure with optimized electron localization, enabling the simultaneous enhancement of dielectric and varistor properties. This work provides a fundamental ionic radius engineering principle for advanced ceramic materials.