Impact of Lanthanoid Substitution on the Structural and Physical Properties of an Infinite-Layer Iron Oxide.

The effect of lanthanoid (Ln = Nd, Sm, Ho) substitution on the structural and physical properties of the infinite-layer iron oxide SrFeO was investigated by X-ray diffraction (XRD) at ambient and high pressure, neutron diffraction, and Fe Mössbauer spectroscopy. Ln for Sr substituted samples up to ∼30% were synthesized by topochemical reduction using CaH. While the introduction of the smaller Ln 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 FeO layers with a charge-compensated composition of (SrLn)FeO, which accounts for the failed electron doping to the FeO layer. The incorporated partial apical oxygen or the pyramidal coordination induces incoherent buckling of the FeO sheet, leading to a significant reduction of the Néel temperature. High-pressure XRD experiments for (SrHo)FeO suggest a possible stabilization of an intermediate spin state in comparison with SrFeO, revealing a certain contribution of the in-plane Fe-O distance to the pressure-induced transition.