Unusually large oxygen non-stoichiometry and defect thermodynamics in Sr4Mn2–xFe1+xO10–δ Ruddlesden-Popper layered oxides

Abstract

The Ruddlesden-Popper oxides Sr₄Mn_2–xFe_1+xO_10–δ (x = 0, 0.5, and 1.0) with the layered tetragonal structure (SG I4/mmm) were synthesized at the heating of organometallic precursors to 1400 °C in air. The Rietveld refinement of the X-ray diffraction patterns indicates a disordered arrangement of Mn and Fe atoms in the crystal. The results of the electron density functional-based calculations of the total energy show noticeably larger oxygen vacancy formation enthalpy for O_O2 positions in the structural rock salt layers of strontium oxide compared to the apical O_O1 and equatorial O_O3 and O_O4 positions in perovskite slabs. The coulometric titration measurements show that the Sr₄Mn_2–xFe_1+xO_10–δ phases can endure harsh reducing conditions, staying stable even when up to two oxygen atoms are taken away from the structure. Simulations of equilibrium oxygen content data reveal that oxygen exchange is coupled with electron excitation processes involving d-cations and intrinsic anion disorder. It is suggested that this disorder may be caused by unequal vacancy formation energies at crystallographically different oxygen sites. The enthalpy and entropy changes in the defect formation reactions were derived from the fitting of the experimental plots for oxygen non-stoichiometry. The obtained thermodynamic parameters were applied to determine how the concentrations of electronic and ionic defects depend on variations in temperature and partial pressure of oxygen. The consistency of the thermodynamic analysis is supported by a good coincidence of the calculated and experimental plots of the partial thermodynamic functions of labile oxygen in Sr₄Mn_2–xFe_1+xO_10–δ.