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 SrFeO2 was investigated by X-ray diffraction (XRD) at ambient and high pressure, neutron diffraction, and57 Fe 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-x Lnx )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 (Sr0.75 Ho0.25 )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.