6 - Electrochemical properties of complex pyrochlores

Abstract

Special features of the pyrochlore structure A2B2O6O′ with two types of both cationic and anionic sites and a large range of cation radii ratios, where this structural type is preserved, provide for a multitude of compositions as well as a wide spectrum of properties and possible fields of application. In recent decades, oxide compounds with the pyrochlore-type structure have been intensely studied to search for new materials suitable for electrochemical use. Electrical properties and electrochemical behavior of oxide pyrochlore compounds depend on the composition, electronic structure, cation valence, and the defectiveness of cation and anion sublattices. This chapter presents the electrochemical behavior of complex oxide pyrochlores, the heightened interest in them being associated with their potential use as the basis for materials for electrochemical detectors, sensors, membranes, solid electrolytes, electrode material for solid oxide fuel cells (SOFCs), and electrochemical catalysis. The conductivity of binary oxide pyrochlores varies over a wide range depending on their composition, from dielectric to metallic. Electronic and ionic (oxygen, proton, cation) conductivity depends on the composition, temperature, and external atmosphere. Often, several types of conductivity show themselves in various temperature ranges. By doping into both A and B cation sites, both electronic and ionic components of conductivity may be regulated. A group of complex pyrochlores may be set off with a high doping level, where cationic sites are occupied by cations with different valence or with variable valence in comparable ratios. The control of the defect structure by varying the composition offers the prospect of essentially varying the electrochemical properties of compounds. The main factors determining the electrochemical properties of complex oxide pyrochlores and the trends resulting from investigations are given in this chapter. Topics explored include general conductivity aspects, structural disordering, defects, oxygen migration pathways, and regulation of the electrochemical behavior of oxide pyrochlores through doping.