Several complex oxides of cubic perovskite-type are excellent mixed ionic-electronic conductors (MIEC). Interest in these materials increased in the last years as they allow the supply or removal of oxygen to or from reaction mixtures with unrivalled selectivity and high oxygen fluxes. Today, the synthesis of highly-doped double perovskites is widely done via EDTA/citrate complexing methods (or derivates of them) as they allow to obtain phase pure products and to control precisely their stoichiometry. However, details of the formation of the perovskite crystal are little known. The synthesis process for cubic perovskites of (Ba,Sr)(Zn,Fe)O_3-d-type and (Ba,Sr)(Co,Fe)O_3-d-type have been studied in very detail by analytical transmission electron microscopy in a field-emission instrument. The methods applied are diffraction contrast (bright-field and dark-field), high-resolution transmission electron microscopy (HRTEM), elastically-filtered selected area electron diffraction (SAED), energy-filtered transmission electron microscopy (EFTEM), scanning transmission electron microscopy (STEM) in bright-field (BF) and high-angle-annular dark-field (HAADF) contrast, energy-dispersive X-ray spectroscopy (EDXS), and electron energy-loss spectroscopy (EELS) with special emphasize on energy-loss near-edge structures (ELNES). It turns out that the perovskite-type oxides are formed already at moderate temperatures at around 700 °C via nanoscale solid state reactions between finely-dispersed crystalline intermediates identified as a spinel and a carbonate. The analytical TEM study gives a clear picture of reaction pathways and mechanisms as it includes orientation relationships between intermediates and products.

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