Colossal permittivity and impedance analysis of niobium and aluminum co-doped TiO₂ ceramics

Niobium and aluminum co-doped TiO₂ ceramics, i.e., (Nb_0.5Al_0.5)_xTi_1-xO₂ (x = 0, 0.01, 0.05, 0.1, 0.15, abbreviated as NAT100x) were synthesized via a solid-state reaction route. As could be seen from X-ray diffraction (XRD) patterns, all samples were in pure rutile phases. A colossal permittivity (CP) over 10⁵ with relatively low dielectric loss could be noted in the NAT10 ceramic. Additionally, the high dielectric permittivity (>10⁵) could be maintained over a wide temperature range of -100-250 °C and frequency range of 10² to 10⁶ Hz, respectively. This indicated that the dielectric property of NAT10 possessed superior temperature-dependent and frequency-dependent stability. Analysis of impedance spectroscopy and current density vs. electric field plots revealed that the CP was attributed to an internal barrier layer capacitance effect.