Investigation of the phase transition to the Ruddlesden–Popper phase in La- or Nb-doped Sr2Fe1.5Mo0.5O6−δ double perovskites and the impact of lanthanum or niobium doping†

Sr₂Fe_1.5Mo_0.5O_6−δ (SFM) is a well-known representative of the double perovskite family, recognized for its remarkable properties, such as good conductivity in air and hydrogen. However, this material can undergo a phase transition under reductive atmospheres, which might be a challenge for its practical use. Herein, we focus on the impact of lanthanum or niobium dopants, which would not only stabilize the structure during the reduction but also have a beneficial impact on the properties of the material, e.g., electrical conductivity. As a result, lanthanum doping (LSFM – La_0.3Sr_1.7Fe_1.5Mo_0.5O_6−δ) was found to be the most stable and the lowest amount of a new Ruddlesden–Popper phase was formed during the reduction. Moreover, the aliovalent La-doping resulted in increased electrical conductivities in both air and hydrogen compared to those of pristine SFM. Niobium doping resulted in a behavior similar to that of SFM with only slight stabilization, but the exsolution process in this material was found to be more intense. In situ studies during oxidation allowed us to retrieve the original structure at 700 °C. Ex situ XAS analyses enabled us to focus on the electronic state, which in most cases was restored almost to the original state after the re-oxidation process. This showed that not only the crystallographic structure but also the local atomic structure were re-established. The use of wavelet transform on the Fe K-edge allowed us to differentiate contributions from the Fe–Fe and Fe–Mo bonds in LSFM.