Structural features of a substituted strontium ferrite Sr$_{1-x}$La$_{x}$FeO$_{3-\delta}$: I. Sr$_{2}$LaFe$_{3}$O$_{9-\delta}$

The structure and the valence states of iron in substituted strontium ferrite Sr$_{2/3}$La$_{1/3}$FeO$_{3-\delta}$ or Sr$_{2}$LaFe$_{3}$O$_{9-\delta}$ at various condition synthesis and heat treatment have been studied using X-ray diffraction and the Mössbauer spectroscopy. A series of vacuum annealing (10$^{-3}$ Torr, 400–650$^\circ$C) allows the observation of the evolution of the structure from the rhombohedral (for the Sr$_{2}$LaFe$_{3}$O$_{9}$) to the orthorhombic (Sr$_{2}$LaFe$_{3}$O$_{8}$) via intermediate multiphase states and also the redistribution of valence iron states.