Effect of defect dipoles on the colossal dielectric behaviors of TiO2-x ceramics

Abstract

Considerable efforts have been made to explore high-dielectric constant ceramics for their promising applications as high-energy density capacitors and fine metal-oxide-semiconductor field effect transistors. Here, we investigated the colossal dielectric behaviors of TiO2-x ceramics influenced by defect dipoles introduced by oxygen vacancies and Ti3+. As temperature decreases from room temperature, three thermally activated relaxations were observed. A room temperature relaxation (R3) was attributed to Maxwell-Wagner relaxation due to the electrode/sample contact. A low-temperature Maxwell-Wagner relaxation (R1) featuring nearly constant loss behavior was associated with frozen carriers. The middle-temperature relaxation (R2) was verified to be the polaron relaxation composed of two segment contributions: polaron hopping and polaron reorienting. The defect dipoles gradually changed from a disordered state into an ordered state in external electric field when temperature falls and reaches a frozen-in state below 217 K. The ordered phase will turn into disordered again when temperature increases to 217 K due to thermal perturbation corresponding to a phase transition. These results indicate that the colossal dielectric constant properties of TiO2 system are attributed to defect dipole clusters. This work exhibits a way to identify the dipole effect. Moreover, we found that the dielectric properties of TiO2-x ceramics hardly depend on temperature and frequency. This implies that TiO2-x ceramics is a kind of promising colossal dielectric materials which can work at various temperatures.