Origin of giant dielectric constant in Ta+Gd co-doped TiO2 single crystals by optical traveling floating zone method

The co-doped TiO₂ polycrystalline ceramics have dramatic dielectric behavior (>10⁴), but its source is still not completely clarified. Compared to ceramic materials, single crystals can maintain most of the original properties of the material and eliminate the grain boundary and pore interferences, thus facilitating the exploration of the source of the dielectric properties. Here, we chose Gd³⁺ with a moderate ionic radius as the acceptor ion and Ta⁵⁺ as the donor, preparing (Gd_0.5Ta_0.5)_0.01Ti_0.99O₂ single crystals by the optical traveling floating zone method to investigate giant dielectric properties. It was found that a dielectric constant (ε′ ∼1.5 × 10⁴), and a dielectric loss (tanδ ∼ 0.07) were achieved simultaneously in (Gd_0.5Ta_0.5)_0.01Ti_0.99O₂ single crystals at 10⁵Hz. Electrochemical impedance spectroscopy and XPS analyses indicate that the high dielectric properties are mainly attributed to electrons pinning defective dipoles clusters. In addition, dielectric relaxation behavior under DC bias suggests that electrode effects also affect the dielectric constant. This study provides insights for the origin of the large dielectric constant and the growth of single crystals in TiO₂-based materials.