Topotactic reductive fluorination of strontium cobalt oxide epitaxial thin films
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Since the discovery of oxyfluoride cuprate superconductors, many efforts have been made to search for new transition-metal oxyfluoride compounds. Recently, the topotactic fluorination reaction using polyvinylidene fluoride (PVDF) has gained attention because of the low-temperature synthesis of oxyfluorides. In this study, we report the fabrication of SrCoO x F y epitaxial thin films via topotactic fluorination of SrCoO2.5 precursor films with PVDF. X-ray diffraction analysis showed that the SrCoO x F y film, with an anion-vacant perovskite structure, was obtained by fluorination at 150 °C and that the in-plane lattice constant was completely dependent on the substrate. Energy dispersive X-ray spectrometry revealed that the chemical composition of the fluorinated film was SrCoO1.9±0.4F0.5±0.1 and X-ray photoemission spectroscopy showed that the Co ions had a mixed valence state of 2+ and 3+. This valence state was smaller than that in the SrCoO2.5 precursor film, indicating that PVDF acted as a reductive fluorinating agent for the SrCoO2.5 film. Moreover, the SrCoO1.9±0.4F0.5±0.1 film did not exhibit ferromagnetism even at 10 K, suggesting the presence of an antiferromagnetic interaction between the Co ions.
KeywordsTopotactic reaction Epitaxial thin film Oxyfluorides Transition-metal compounds Pulsed-laser deposition
We thank Prof. Kimikazu Sasa, Mr. Satoshi Ishii, Dr. Hiroshi Naramoto, and Dr. Daiichiro Sekiba of the University of Tsukuba, and Prof. Katsuyuki Fukutani of the University of Tokyo for their assistance in the NRA measurements. This work was partially supported by Nippon Sheet Glass Foundation for Materials Science and Engineering. EDS measurements were conducted in the Research Hub for Advanced Nano Characterization, at the University of Tokyo, and supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
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