Physical properties of Sr2FeIrO6 and Sr1.2La0.8FeIrO6 double perovskites obtained by a new synthesis route

• Sr2−xLaxFeIrO6 (x = 0.0 and 0.8) samples were synthesized using a new heating route.
• La3+ to Sr2+ substitution induces the structural transition from I2/m → P21/n.
• The high temperature magnetic transition previously reported for x = 0.8 is probably due to impurity.
• Fe3+–Ir5+ to Fe3+–Ir4+ valence configuration changes govern the physical properties.

Previous works on Sr2-xLaxFeIrO6 double perovskite (DP) series reported a possible ferromagnetic transition at T ∼ 700 K for the x = 0.8 concentration, for which was observed the presence of spurious Fe2O3 phase. In order to prevent the formation of this impurity phase and check if this high temperature magnetic transition is intrinsic of the material, different synthesis routes became necessary. In this work, polycrystalline samples of Sr2-xLaxFeIrO6 (x = 0.0 and 0.8) have been synthesized by solid state reaction using a new heating treatment. The sample's properties were investigated by synchrotron x-ray powder diffraction (SXRD), transmission electron microscopy (TEM), magnetic susceptibility, specific heat and electrical resistivity, and compared with the previously reported results. The SXRD data revealed a structural transition induced by La to Sr substitution (I2/m↔P21/n). Moreover, it was not detected the presence of Fe2O3 on the samples obtained by the new route, which might be related to the absence of high temperature magnetic ordering. The magnetometry results indicated the emergence of Ir4+ with La doping, being corroborated by specific heat measurements which suggest Fe3+/Ir5+ and Fe3+/Ir4+ configurations for x = 0.0 and 0.8 compounds, respectively. Temperature dependent electrical resistivity measurements showed that Sr2+ to La3+ substitution leads to a decrease of electrical resistivity, possibly associated with the increase in the number of Ir valence electrons.

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