Chemical expansion in manganites

Şeren, Mehmet Hazar
Manganites are type of manganese oxides having mixed valence states with perovskite structure represented as Ln1-xAxMnO3 (Ln= rare-earth cation, A= alkaline earth cation) and they have been widely studied due to their potential applications in various important areas and their interesting properties. Numerous investigations have focused on their production, characterization of their properties and solution of problems encountered during their applications. Some of these applications are cathode and electrolyte materials for solid oxide fuel cells (SOFC), piezoelectric sensors or energy harvesting by thermoelectric materials. In some applications, materials can be in contact with highly oxidizing and/or reducing atmospheres. The possible variations in the stoichiometry of the materials under these conditions may also lead to changes in their lattice parameters. This effect is known as "chemical expansion". The component made of such a material may mulfunction when dimensional stabilities occur. For instance, dimensional changes cause residual stresses in thin film structures if the film is clamped to a rigid substrate. When these stresses are above a certain limit, some defects like cracks or hillocks are formed and the film loses its function. Therefore it is of cardinal importance to observe these dimensional variations and understand the governing mechanims. The aim of this study is to explore chemical expansion coefficients of La0.5Ca0.5MnO3-δ (LCMO) and La1-xSrxMnO3-δ (LSMO) (x= 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1) for the first time in literature. In addition, effect of Sr dopant on chemical expansion coefficient is another aim of this study. The chemical expansion in the structure during O loss was determined by combined study of dilatometry, in-situ X-ray diffraction and thermogravimetric analysis. Microstructural and chemical analysis were performed using room temperature X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. With increasing Sr dopant, chemical expansion coefficient and δ values have decreasing trend whereas the absence of La in the structure leads to increase in chemical expansion. Therefore, the lowest chemical expansion coefficient is observed in LSMO9 samples. Slope changes are observed in TGA curves and the reason for this can be explained with the effect of Jahn-Teller distortion due to change in Mn valence state.