Colossal Dielectric Behavior of Ga+Nb Co-Doped Rutile TiO₂

Stimulated by the excellent colossal permittivity (CP) behavior achieved in In+Nb co-doped rutile TiO₂, in this work we investigate the CP behavior of Ga and Nb co-doped rutile TiO₂, i.e., (Ga_0.5Nb_0.5)_<i>x</i>Ti_1–<i>x</i>O₂, where Ga³⁺ is from the same group as In³⁺ but with a much smaller ionic radius. Colossal permittivity of up to 10⁴–10⁵ with an acceptably low dielectric loss (tan δ = 0.05–0.1) over broad frequency/temperature ranges is obtained at <i>x</i> = 0.5% after systematic synthesis optimizations. Systematic structural, defect, and dielectric characterizations suggest that multiple polarization mechanisms exist in this system: defect dipoles at low temperature (∼10–40 K), polaronlike electron hopping/transport at higher temperatures, and a surface barrier layer capacitor effect. Together these mechanisms contribute to the overall dielectric properties, especially apparent observed CP. We believe that this work provides comprehensive guidance for the design of new CP materials