Dielectric relaxations in fine-grained SrTiO3 ceramics with Cu and Nb co-doping

SrTi1-3x(CuxNb2x)O-3 (x = 0.05, 0.1, 0.15 and 0.2) ceramics were synthesized using a solid-state reaction method at 1400 C in air. Their structures, valence states, conduction mechanisms and dielectric properties were investigated in detail. Fine grains and vibration modes of the samples related to the doping effects were observed. A distorted pseudo-cubic structure was confirmed by XRD and Raman spectroscopy. All ceramics exhibited a colossal dielectric constant and dielectric relaxations. The dielectric constant in the low- and high-frequency ranges for x = 0.05 and 0.1 was ascribed to the contribution of the grain boundary and grain, respectively, and the electrode polarization was significant at x = 0.15 and 0.2. The dielectric relaxation peaks obeyed the T-1/4 law at low temperature for all samples, confirming the polaron relaxation process. The electric modulus analysis confirmed that low-temperature dielectric relaxation was related to the grain response, and the electric conduction exhibited the same behavior with the dielectric relaxation. The variable-range-hopping conduction indicated a highly distorted structure and the localization of carriers in SrTi1-3x(CuxNb2x)O-3, which was consistent with the XRD and Raman results. The mixed-valence structure of Cu was identified by XPS, and the polaron hopping between the mixed-valence Cu ions was supposed to be responsible for the dielectric relaxation and electric conduction.