Oxyfluoride Sr2Cu(O,F)y thin films were prepared by a sol-gel method using Sr(O-iC3H7)2 and CuF2 as starting materials and trifluoroacetic acid as an additional fluorine source. The sols were coated on SrTiO3 (STO) or LaAlO3 (LAO) single crystal substrates and then heated at 800°C under a condition controlled by the concentration of O2 gas. For the films on STO, Sr2CuO3 was formed first and changed to the K2NiF4-type Sr2Cu(O,F)y phase by post-fluorination using a methanol solution of NH4F. In the case of using LAO substrates, the Sr2Cu(O,F)y phase was directly formed by one time heating at 800°C. A direct formation of the Sr2Cu(O,F)y phase has never been reported before. The presence of fluorine in the films was confirmed both by chemical analysis and XPS spectroscopy. The discrepancy in the crystallization behavior of the films on STO and on LAO substrates is explained by the formation of SrF2 impurities owing to the reaction between fluorine in the film and strontium in the STO substrate. The Sr2Cu(O,F)y thin films on STO exhibited the superconducting transition at 45 K in a SQUID measurement.
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