Colossal permittivity and dielectric relaxations in Tl plus Nb co-doped TiO2 ceramics

A series of Tl + Nb co-doped TiO2 ceramics ((Tl0.5Nb0.5)(x%)Ti1-x%O2 0.5 <= x <= 10.0) were prepared by a solid-state reaction method under N-2 atmosphere. The evolution of their microstructures, and dielectric properties were systematically studied. The co-doped ceramics exhibited a tetragonal rutile structure wherein the Nb and Tl elements were homogeneously distributed. The cell volumes, grain size, and permittivity increased with doping x, whereas the impedance values of the grain and grain boundary decreased with an increasing x. The optimum dielectric performance (epsilon(r) > 10(4), tan delta < 0.05) in the range of 10-10(6) Hz was obtained for x = 1.5 with a corresponding grain boundary active energy of 0.86 eV. Four types of dielectric relaxation were observed at different temperature ranges: 10-30 K, 30-200 K, 200-350 K and 350-475 K; those dielectric relaxtions were respectively caused by electron-pinned defect-dipoles, electron hopping, oxygen vacancy hopping, and Maxwell-Wagner polarization. The colossal permittivity is primarily a result of the electron-pinned defect-dipole polarization.