Impact of lanthanoid substitution on the structural and physical properties of an infinite-layer iron oxide

Abstract

The effect of lanthanoid (Ln = Nd, Sm, Ho) substitution on the structural and physical properties of the infinite-layer iron oxide SrFeO2 was investigated by X-ray diffraction (XRD) at ambient and high pressure, neutron diffraction, and 57Fe Mössbauer spectroscopy. Ln for Sr substituted samples up to ∼30% were synthesized by topochemical reduction using CaH2. While the introduction of the smaller Ln3+ ion reduces the a axis as expected, we found an unusual expansion of the c axis as well as the volume. Rietveld refinements along with pair distribution function analysis revealed the incorporation of oxygen atoms between FeO2 layers with a charge-compensated composition of (Sr1–xLnx)FeO2+x/2, which accounts for the failed electron doping to the FeO2 layer. The incorporated partial apical oxygen or the pyramidal coordination induces incoherent buckling of the FeO2 sheet, leading to a significant reduction of the Néel temperature. High-pressure XRD experiments for (Sr0.75Ho0.25)FeO2.125 suggest a possible stabilization of an intermediate spin state in comparison with SrFeO2, revealing a certain contribution of the in-plane Fe–O distance to the pressure-induced transition. © 2016 American Chemical Society