Professor SHIMAKAWA, Y and his research group, Succeeded in Preparing “Single-crystal Thin Films of Infinite-layer Structure SrFeO2 with Square-planer Coordination of Fe2+ Ions”(Reported in 2 Sep 2008)

Professor SHIMAKAWA, Yuichi and his research group
(Laboratory of Advanced Inorganic Synthesis, Division of Synthetic Chemistry)


Succeeded in Preparing “Single-crystal Thin Films of Infinite-layer Structure SrFeO2 with Square-planer Coordination of Fe2+ Ions”
Reported at the Conference of Japan Society of Applied Physics, 2 Sep 2008

Prof Shimakawa Y(left), Mr Inoue S(center) and Mr Kawai M(right)
Mr. Satoru Inoue, Mr. Masanori Kawai, and Professor Yuichi Shimakawa in Advanced Inorganic Synthesis laboratory succeeded in preparing “single-crystal thin films of infinite-layer structure SrFeO2 with square-planer coordination of Fe2+ ions”.

Figure 1. Crystal Structure of SrFeO2.5 and SrFeO2
There are a number of oxides with transition-metal ions such as Fe, Co, and Ni.  Ionic states of the transition metals can vary in the oxides.  For instance, Magnetite (Fe3O4) contains Fe2+ and Fe3+ions, while Hematite (Fe2O3), which used to be used as a red pigment, contains only Fe3+ ions.  For strontium (Sr) and iron (Fe) containing perovskite-structure oxides, the oxygen content and Fe ionic state were considered to change between SrFeO3 and SrFeO2.5.  SrFeO3 is a simple perovskite and it contains unusually high valence 4+ iron ion, which is stabilized by a strong oxidizing atmosphere.  SrFeO2.5, on the other hand, is prepared at ambient pressure, and its brownmillerite structure consists of alternate layers of Fe3+ octahedra and tetrahedra.  However, it was not possible to produce a perovskite with Fe2+ by using any reduction techniques.  The brownmillerite SrFeO2.5 was thus historically assumed to represent the lower limit of oxygen nonstoichiometry in the perovskites.  Last year a new compound, infinite-layer structure SrFeO2, was reported in Nature to be synthesized by using a low temperature reduction with CaH2.
Immediately after this report, Mr. Inoue and Mr. Kawai started the project for thin-film growth, and they succeeded in preparing “single-crystal thin films of infinite-layer structure SrFeO2”.  A SrFeO2.5 precursor thin film was first deposited by a pulsed-laser-deposition method and the film was then reduced at low temperature with CaH2.  The resultant sample was confirmed to be a single-crystal infinite layer SrFeO2 from X-ray diffraction and absorption experiments.
With the epitaxially grown thin-film samples, we can investigate mobile behaviors of oxygen ions.  The results on high oxygen mobility will be useful for fuel-cell applications.  The study on single-crystal thin-film samples will also reveal anisotropic crystal and electronic structures of the compound.  New physical properties of the infinite-layer structure may appear by using epitaxial strain from the substrate lattice.  The present success of preparing the single-crystal thin film SrFeO2 has great impacts on not only research fields of fundamental solid state physic and chemistry but also application fields of new material synthesis with new functions.
Figure 2. X-ray diffraction of SrFeO2.5 and SrFeO2
The research was done in collaboration with research groups of Profs. Kageyama and Yoshimura at Kyoto University and of Dr. Mizumaki at SPring-8/JASRI.  The results were published in Applied Physics Letters and also presented in meetings of the Physical Society of Japan and the Japan Society of Applied Physics.