Stabilization and Study of SrFe1-xMnxO2 Oxides with Infinite-Layer Structure

A series of layered oxides of nominal composition SrFe1-xMnxO2 (x = 0, 0.1, 0.2, 0.3) have been prepared by the reduction of three-dimensional perovskites SrFe1-xMnxO3-delta with CaH2 under mild temperature conditions of 583 K for 2 days. The samples with x = 0, 0.1, and 0.2 exhibit an infinite-layer crystal structure where all of the apical O atoms have been selectively removed upon reduction. A selected sample (x = 0.2) has been studied by neutron powder diffraction (NPD) and X-ray absorption spectroscopy. Both techniques indicate that Fe and Mn adopt a divalent oxidation state, although Fe2+ ions are under tensile stress whereas Mn2+ ions undergo compressive stress in the structure. The unit-cell parameters progressively evolve from a = 3.9932(4) angstrom and c = 3.4790(4) angstrom for x = 0 to a = 4.00861(15) angstrom and c = 3.46769(16) angstrom for x = 0.2; the cell volume presents an expansion across the series from V = 55.47(1) to 55.722(4) angstrom(3) for x = 0 and 0.2, respectively, because of the larger effective ionic radius of Mn2+ versus Fe2+ in four-fold coordination. Attempts to prepare Mn-rich compositions beyond x = 0.2 were unsuccessful. For SrFe0.8Mn0.2O2, the magnetic properties indicate a strong magnetic coupling between Fe2+ and Mn2+ magnetic moments, with an antiferromagnetic temperature T-N above room temperature, between 453 and 523 K, according to temperature-dependent NPD data. The NPD data include Bragg reflections of magnetic origin, accounted for with a propagation vector k=(1/2,1/2,1/2). A G-type antiferromagnetic structure was modeled with magnetic moments at the Fe/Mn position. The refined ordered magnetic moment at this position is 1.71(3)mu(B)/f.u. at 295 K. This is an extraordinary example where Mn2+ and Fe2+ ions are stabilized in a square-planar oxygen coordination within an infinite-layer structure. The layered SrFe1-xMnxO2 oxides are kinetically stable at room temperature, but in air at similar to 170 degrees C, they reoxidize and form the perovskites SrFe1-xMnxO3-delta. A cubic phase is obtained upon reoxidation of the layered compound, whereas the starting precursor SrFeO2.875 (Sr8Fe8O23) was a tetragonal superstructure of perovskite.