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Nb-SrTiO 3 perovskite ceramics, without and with the addition of 3 mol % MnO, were synthesized by an organic gel-assisted process, sintered under argon/hydrogen, and annealed under oxygen. For both series, colossal permittivity (>10 4 ), low loss tangent (<0.1), and high resistivity (10 9 Ω cm) at 1 kHz were obtained. Furthermore, the dielectric measurements exhibit good frequency stability regardless of the grain size of the ceramics. Indeed, the Nb-SrTiO 3 samples show a small grain size (2 μm), and the addition of MnO leads to a strong increase in grain size up to about 40 μm. The mechanism responsible for the colossal permittivity and low dielectric loss is discussed on the basis of analyses of impedance spectroscopy and EPR data. EPR measurements reveal the existence of oxygen vacancies and different oxidation states of Mn in the grains. The grain boundaries and grains resistance were deduced from a fit of an RC element model to the impedance spectroscopy data. From all of these results, it was shown that the grains are semiconducting and the grain boundaries are insulating. The correlation between permittivity as a function of grain size and grain boundary capacity validates the barrier layer model between the grains and grain boundaries. Thus, the electrical behavior of the grain boundaries is one of the key factors to obtain ceramic capacitors useful for industrial applications.

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