Origin of Colossal Dielectric Permittivity in (Nb + Ga) Co-Doped TiO2 Single Crystals

(Ln + Nb) co-doped TiO2 polycrystalline ceramics possess remarkable dielectric behaviors, but their origin is still not fully understood. Compared to ceramic materials, a single crystal could keep most of the original material’s properties and eliminate interference due to grain boundaries and pores, thereby useful for exploring the origin of the material’s properties. Here, Ga3+ with a radius close to that of the Ti4+ ion was selected as an acceptor ion to study the properties. To this end, rutile single crystals and (Nb + Ga) co-doped rutile single crystals were prepared by the Verneuil method. The permittivity of the doped single crystal at frequencies from 20 Hz to 2 MHz was found to be greater than 15,000, and the dielectric loss was kept below 0.08. The latter value was much smaller than the reported values of rutile single crystal samples. Electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy indicated significantly enhanced dielectric properties, mainly attributed to the highly localized electron-pinned defect dipoles rather than the surface barrier layer capacitor effect. The electrons tightly bounded by the local electric dipole defect clusters can only range over a very small distance, which made the doped crystal have a large dielectric constant over a wide frequency range and maintain a low dielectric loss. In sum, the use of single crystals as a research object greatly provided insights into the origin and optimization of giant dielectric properties of TiO2-based materials.