Large-size-mismatch co-dopants for colossal permittivity rutile TiO₂ ceramics with temperature stability

Colossal permittivity (CP) in donor-accepter co-doped rutile TiO₂ has attracted significant interest. Here, the CP behavior of (Ta + La) co-doped rutile TiO₂ ceramics were studied, where the ionic radii of Ta⁵⁺ and La³⁺ are much larger than that of Ti⁴⁺. The ceramics with an extremely low doping exhibit colossal dielectric permittivity (∼2.6 × 10⁴) with an acceptable low dielectric loss (<0.07) in the frequency range from 40 to 10⁶ Hz. The CP properties obtained in (Ta + La) co-doped TiO₂ ceramics show excellent temperature stability over a wide temperature range of 20–400 °C. The X-ray diffraction analysis and the density functional theory calculation illustrates that the La₂³⁺V_o^••Ti₂³⁺ and Ta₂⁵⁺Ti³⁺Ti⁴⁺ defect complexes with the lowest energy are responsible for the enhanced dielectric properties. Moreover, the defect complex formed by large-size trivalent substitutions and oxygen vacancy is very stable, and assists in improving temperature stability of the dielectric properties of co-doped rutile TiO₂ ceramics.