Impedance and Mott-Schottky Analysis of a Pr 0.15 Ce 0.85 O 2-x Solid Solution
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Impedance and Mott-Schottky Analysis of a Pr0.15Ce0.85O2-x Solid Solution R. Bouchet, P. Knauth, T. Stefanik*, H. L. Tuller* MADIREL, Université de Provence-CNRS (UMR 6121), Centre Saint-Jérôme, F-13397 Marseille Cedex 20, France *Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA-02139, USA
ABSTRACT The semiconductor properties of a praseodymium-cerium oxide solid solution with composition Pr0.15Ce0.85O2-x (PCO) were investigated by d.c. current-voltage and bias-dependent impedance measurements in aqueous solution. The solution data were compared with impedance values of dry cells in air. A Mott-Schottky analysis of the PCO-solution interface capacitance showed p-type semiconductivity, a flat-band potential Efb = (2.0 ± 0.1) V/NHE and an ionized acceptor density NA = 3 1017 cm-3. Using these data, an electron hole mobility µh ≈ 10-5 cm2 V-1 s-1 was calculated pointing to a small polaron conduction mechanism with a hopping energy (Eh = 0.4 eV).
INTRODUCTION Mixed ionic-electronic conducting (MIEC) electroceramics can be designed for various applications, including oxygen separation membranes, fuel cell electrodes, and automotive catalysts. Their function in 3-way automotive catalysts is the storage and release of oxygen during transients from reducing (fuel-rich) to oxidizing (fuel-lean) environments.1 Praseodymium-cerium oxides (Pr,Ce)O2-y (PCO) are materials with a high oxygen storage capacity, given the ready reduction-induced valence change of the praseodymium cations (Pr4+/Pr3+) even at elevated oxygen partial pressures.2 The oxygen deficiency of several PCO solid solutions has been investigated using coulometric titration and was found to be extensive in agreement with the above expectation.3,4,5 Measurements of the temperature and oxygen partial pressure dependence of the conductivity of PCO demonstrate a p(O2)-dependent conductivity at high p(O2) which is predominantly ionic for Pr levels up to ~10 mol%. However, a minority electronic component becomes increasingly important as Pr levels increase due to the apparent formation of a Pr impurity band.6 The relationship between Pr concentration in (Pr,Ce)O2-y and the effective electronic carrier density therefore becomes of interest. This work is a part of a program for characterizing the defect and transport properties of PCO solid solutions.3,4,5,6,7,8 Here, the semiconductor properties of polycrystalline ceramics of composition Pr0.15Ce0.85O2-y are investigated utilizing a Mott-Schottky analysis of the PCO/aqueous electrolyte junction. This analysis is capable of yielding the type and density of majority ionized dopants (acceptors or donors) and the flat-band potential of the semiconductor.9 The bias-dependent impedance measurements in aqueous solution were complemented with a study of the temperature dependence of the complex impedance of the PCO specimen both in aqueous solution (290-360 K) and in air (290-420 K) to confirm the activated nature of electronic transport in PCO at these reduced temp
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