Physical properties of Sr₂FeIrO₆ and Sr_1.2La_0.8FeIrO₆ double perovskites obtained by a new synthesis route

Previous works on Sr_2-xLa_xFeIrO₆ 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 Sr_2-xLa_xFeIrO₆ (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. - • Sr_2−xLa_xFeIrO₆ (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.