The sol-gel synthesis of perovskites by an EDTA/citrate complexing method involves nanoscale solid state reactions

摘要

Nowadays, sol-gel procedures are well established in the synthesis of complex oxides as they allow to obtain phase pure products and to control precisely their stoichiometry. This quality makes them a tool of choice for the preparation of perovskite-type oxides. To optimize the functional properties of these materials, it is essential to set accurately their possible complex stoichiornetries. However, details of the formation of the perovskite crystal remain obscure. Different stages of an ethylene-diarnine-tetraacetic acid (EDTA)/citrate-gel based synthesis process for mixed conducting (Ba_(0.5)Sr_(0.5))(Fe_(0.8)Zn_(0.2))O_(3-delta) of cubic perovskite structure are elucidated. The combination of analytical transmission electron microscopy with X-ray diffraction reveals that the perovskite-type oxide is formed already at moderate temperatures at around 700 deg C via nanoscale solid state reactions between finely-dispersed crystalline intermediates identified as a spinel and a carbonate. The reaction scheme, however, is intricate and includes stuffed tridymite structures as transient phases. The ultrafine intermixing of extremely small reactants makes EDTA/citrate-gel based procedures superior to classical solid state routes with respect to applications that demand phase purity and stoichiometry control.