Electrical properties and defect structures of praseodymium-cerium oxide solid solutions

Abstract

A defect chemistry model consistent with observed trends in the pO2 and temperature dependence of electrical conductivity in praseodymium cerium oxide (PCO) was developed. Four point DC conductivity measurements were made from 1 atm to 1018 atm p02 over isotherms ranging from 600-1 000ʻC in materials containing 0-20% Pr. A pO02-dependent ionic conductivity was observed at high pO2 values in compositions containing 0.5% and 1% Pr. This behavior was attributed to oxidation of Pr3+ to Pr4+ under oxidizing conditions, thereby decreasing the concentration of acceptor dopants in the PCO material. In compositions containing 10% and 20% Pr, an electron hopping conductivity was observed at high pO02 values. This contribution was strongest at low temperatures and was attributed to the formation of a praseodymium impurity band within the CeO2 band gap. Defect association significantly altered the predicted pO2 dependence of the impurity band conductivity, especially at low temperatures. The temperature dependences of the thermodynamic parameters governing defect formation and transport in PCO were determined. The reduction enthalpy of cerium was significantly decreased with additions of Pr from approximately 4.7 eV (the value in pure CeO2) to 3.4 eV in 20% PCO. The energy between the Pr impurity band and the CeO2 conduction band was approximately 0.95 eV for 10% and 20% PCO samples. The measured trap depth was significantly higher (approximately 1.6 eV) in 0.5% and 1% PCO. The migration enthalpy for impurity band hopping conductivity was approximately 0.55 eV, slightly higher than the hopping enthalpy for intrinsic carriers in CeO2 (0.4 eV).

(cont.) The oxygen ion migration enthalpy measured for most samples was approximately 0.6- 0.7 eV, in agreement with values determined for other rare-earth doped systems. At 20% Pr, the total migration energy increased to approximately 0.9 eV. This increase was attributed to an association energy at high doping levels. Coulometric titration and points to the possible existence of uncharged oxygen vacancies, particularly at low temperatures. During the course of these experiments, it became evident that the mechanical stability of PCO needs to be addressed if the material is to be used in real applications. Oxygen uptake/evolution during reduction/oxidation cycles appears to result in development of significant stresses and cracking. While the material may be useful in powder form, this cracking issue must be addressed if it is to be used in bulk or thin film form.