Low dielectric loss induced by coupling effects of donor-acceptor ions in (Nb+Al) co-doped rutile TiO₂ colossal permittivity ceramics

Colossal dielectric properties in donor-acceptor co-doped rutile TiO₂ ceramics have attracted increased interest in recent years. However, dielectric loss (tan δ) of (Nb+Al) co-doped TiO₂ ceramics are quite scattered in the previous studies. Here, we show the dielectric properties of (Nb+Al) co-doped rutile TiO₂ ceramics are sensitive to sintering and ageing technologies, which is related to the coupling behavior of acceptor-donor ions. Both permittivity and dielectric loss of as-sintered (Nb+Al) co-doped TiO₂ ceramics are very large, and increase with the sintering time increasing. After ageing treatment at 900 °C, the dielectric loss of co-doped ceramics reduces from 139 to 0.15 in a wide frequency range. Based on the X-ray diffraction and complex impedance analyses, it can conclude that Nb⁵⁺-Al³⁺ pair formation during ageing treatment reduces the concentration of oxygen vacancy, resulting in the decreases of the conductivity and dielectric loss. The changes in dielectric response of (Nb+Al) co-doped TiO₂ ceramics clearly suggest that ageing process is an effective way for the Nb⁵⁺ and Al³⁺ ions to form the Nb⁵⁺-Al³⁺ pairs. We believe that the coupling effect of donor-acceptor ions is a practical and feasible route to reduce the dielectric loss of (Nb+Al) co-doped TiO₂ ceramics for high-energy-density storage applications.