Single-Crystal Thin Films of SrFeO 2 and LaNiO 2 with Infinite-Layer Structures
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1148-PP07-08
Single-Crystal Thin Films of SrFeO2 and LaNiO2 with Infinite-Layer Structures Yuichi Shimakawa1, Satoru Inoue1, Masanori Kawai1, Noriya Ichikawa1, Masaichiro Mizumaki2, and Naomi Kawamura2 1 Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan 2 Japan Synchrotron Radiation Research Institute/SPring-8, Sayo, Hyogo 679-5198, Japan ABSTRACT Infinite-layer-structure epitaxial thin films of SrFeO2 and LaNiO2 respectively were prepared by low-temperature reduction with CaH2 from brownmillerite SrFeO2.5 and perovskite LaNiO3 epitaxial thin films grown on single-crystal substrates. The reduction process, removing oxygen ions from the perovskite-structure framework and causing rearrangements of oxygen ions, topotactically transforms the initial compounds to the c-axis oriented infinite-layerstructure epitaxial thin films. Consequently, the oxidation state of transition-metal ions in the film changed in wide ranges. INTRODUCTION Transition metal ions form oxygen-deficient perovskite oxides ABO3–δ and some of them show special ordered arrangement of oxygen defects (Fig. 1). SrFeO3–δ, for example, shows oxygen nonstoichiomery from a perovskite SrFeO3 (δ = 0) with unusual high oxidation Fe4+ to a brownmillerite SrFeO2.5 (δ = 0.5), whose structure consists of alternate layers of Fe3+O6 octahedra and Fe3+O4 tetrahedra [1,2]. It was, however, not possible to produce SrFeO2 (δ = 1.0), a perovskite with Fe2+ ions, by using H2/Ar gas or electrochemical reduction techniques [3] because the perovskite structure was not conserved under those reducing conditions in thermodynamically controlled solid-state reaction processes [4]. A low-temperature reduction
SrFeO3 (LaNiO3)
SrFeO2.5 SrFeO2 (LaNiO2)
LaNiO2.5
Fig.1 Crystal structures of SrFeO3,SrFeO2.5, LaNiO2.5, and SrFeO2.
with CaH2 provides further a low oxidation state in Fe and makes the sample SrFeO2 with the Fe2+O2 infinite-layer structure [5]. Similarly, a pseudo-cubic LaNiO3 perovskite changes to an infinite-layer structure LaNiO2 with planer-square coordinated Ni+ ions thorough LaNiO2.5, which consists of alternate units of NiO6 octahedra and NiO4 square planes with oxygen vacancies ordering along the [110] direction [6-8]. This infinite-layer compound has attracted much interest because the squareplanar-coordinated Ni+ (d9) is isoelectronic with the Cu2+ in high-TC superconductors. In this study, we applied the low-temperature reduction processes with CaH2 to epitaxial thin films of SrFeO2.5 and LaNiO3 and prepared single-crystal epitaxial thin films of SrFeO2 and LaNiO2 both with the infinite-layer structures. The oxidation states of transition-metal ions could be controlled through topotactic transformations within the perovskite-structural framework. Changes in transport properties by changing the oxygen content in LaNiO3–δ are also reported. EXPERIMENTS
SrFeO2.5(0 0 9/2)
SrFeO2(0 0 4)
SrFeO2.5(0 0 4)
SrFeO2.5(0 0 7/2)
KTaO3(0 0 4)
KTaO3(0 0 4)
SrFeO2.5(0 0 3)
SrFeO2(0 0 3)
KTaO3 (0 0 3)
SrFeO2.5(0 0 5/2) KTaO3(0 0 3)
SrFeO
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