AC electrical properties of TiO2 and Magnéli phases, TinO2n − 1

Abstract

This paper presents a comprehensive impedance spectroscopy comparison of the AC properties of dense stoichiometric TiO2 and conductive TinO2n−1 Magnéli phases over a broad temperature range (up to 1000°C for TiO2 and 375°C for TinO2n−1). The frequency dependent conductivity and permittivity of both materials is explained in terms of “universal” power law behaviour. A deviation from the law, with a giant relative permittivity which is largely independent of frequency from 0.1Hz to 100–200kHz is observed in the case of TinO2n−1, due to the presence of residual TiO2 generating an Internal Barrier Layer Capacitor (IBLC) effect. The real–imaginary impedance plots are interpreted using an RC model and allow separation of the contribution of the grain bulk and the grain boundaries to the total resistivity of the material. In the case of the TinO2n−1 based materials this confirms that the IBLC effect is generated by insulating grain boundaries. The conduction mechanism in both TiO2 and TinO2n−1 appears to be dominated by electronic conductivities, activated mainly through shallow donor levels up to 200°C and over the entire band gap, which is narrower for TinO2n−1, above 200°C. A deeper understanding of the AC properties of Magnéli phases of Ti at different temperatures aids in the optimisation of electrical properties for a variety of sensor and electrical applications.