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Determination of morphology, dopant amount and oxygen nonstoichiometry in lanthanum manganites by SEM-EDXS technique

Sr-doped lanthanum manganites (LSMO, La1-xSrxMnO3) belong to a wide group of perovskite ceramics with promising electrical and magnetic properties and potential application in solid oxide fuel cells (SOFCs) and data storage devices. These properties can be enhanced via partial substitution of La3+ with an alkaline-earth divalent metal cation (most often Sr2+) in order to promote the formation of mixed Mn3+/Mn4+ valences and oxygen or cation vacancies. Furthermore, properties depend on the synthesis method. In our study, LSMO (x = 0, 0.1, 0.2 and 0.3) were prepared by two different methods: citrate-nitrate autocombustion (CNA) and coprecipitation, and then calcined at 1200 °C/2 h. The structural analysis was conducted by the Rietveld refinement analysis of X-ray diffractograms. However, since the Rietveld profiles are mathematical models, the obtained results were checked by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDXS). Furthermore, oxygen nonstoichiometry was determined by a direct analytical method – permanganate titration with Mohr's salt. All obtained results were in agreement showing that pure LSMO phases with targeted Sr-dopant amount were prepared. It was observed that coprecipitation has yielded samples with higher oxygen nonstoichiometry. Additionally, morphological differences were observed. CNA synthesis has yielded powders with a 'sponge-like' morphology, while powders prepared by coprecipitation were composed of large and porous spherical aggregates. The morphology was also examined on the fracture of pellets prepared by raw powder pressing and sintered at 1200 °C/ 2 h. The densification of the sintered pellets was confirmed with the presence of sub- micrometer pores. Therefore, the sintering process has resulted in good particle-to-particle contact, as well as in porosity which makes LSMO a promising SOFC material due to electronic conductivity ensured by densification and gas permeability due to the porosity.

lanthanum manganites ; morphology ; oxygen nonstoichiometry ; synthesis

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