7 - Ionic conductivity in materials with a pyrochlore structure

Abstract

Pyrochlores (A2B2O7) are a relatively underexplored class of material, unlike perovskite. Researchers started to explore the physical and chemical properties of pyrochlores extensively beginning in the last decade of the 20th century. Different properties could be realized in pyrochlores by suitably varying the cations present in them and pyrochlore materials are considered promising in a wide range of applications. This chapter discusses ionic conductivity in materials with pyrochlore structure. It begins with a brief analysis of pyrochlore structure, with a focus on conductivity and potential applications. It then covers pyrochlore synthesis, impedance instrumentation and measurement, electrical conductivity, and titanate, zirconate, hafnate/stannate, niobate/tantalate, and molybdate pyrochlores from the point of view of their conductivity. The book also discusses the conductivity of irradiated samples and pyrochlore cationic conductivity. In general, it is found that the zirconate pyrochlores exhibit superior ionic conductivity. Lower valent substitution at A- or B-sites helps to increase the number of oxygen vacancies, which in turn leads to improvement in ionic conductivity. Sometimes an increase in the number of mobile species causes an increase in activation energy, which results in poor conductivity. Therefore, an optimum degree of ordering is essential to have a greater number of mobile species as well as minimum activation energy. It is also important to have higher density of the materials to realize higher ionic conductivity, and oxygen partial pressure has a strong influence on the electronic conductivity of materials with pyrochlore structure. The chapter’s topic is important because materials with purely ionic conductivity are used as electrolytes, whereas the materials with mixed ionic-electronic conductors (MIECs) are useful as electrode materials in solid oxide fuel cells. Interestingly, the conductivity of molydate pyrochlores is found to be in the metallic range. Some pyrochlores like pristine and doped Sm2Sn2O7 and Sm1.9Ca0.1ScMO6.95 (M=Nb, Ta) not only show oxide ion conductivity but also exhibit proton conductivity. In some cation-deficient pyrochlore structures, cation migration has been reported. The chapter concludes with a summary, highlighting areas requiring further study.