Low temperature fluorination of (Sr,Ba)₂CuO₃ using NH₄F

Low temperature fluorination of Sr_2-xBa_xCuO₃ (x = 0.0-0.6) was carried out. The pristine samples were heat-treated with NH₄F at 210 °C in flowing nitrogen gas. The major phase was drastically changed to the K₂NiF₄ type, one with poor crystallinity. The fluorinated samples exhibited Meissner signals, but the signal intensity was too low to identify the corresponding superconducting phase correctly. A maximum T_c of 68 K was detected in the fluorinated sample with x = 0.2, being different from Slater et al.'s previous result in which the fluorinated sample with x = 0.6 exhibited the maximum T_c of 64 K. The crystallite size of the K₂NiF₄ phase in the fluorinated sample with x = 0.2 was estimated to be about 220 angstroms from a broad peak of the 220 reflection in the X-ray powder diffraction (XRD) pattern. It is considered that the major phase with the K₂NiF₄ structure is responsible for the observed superconductivity and the small Meissner signals were attributed to the small grain size. In 6 months, the orthorhombicity of the K₂NiF₄ phase in the fluorinated sample with x = 0.2 increased and the observed T_c was appreciably lowered. This correspondence between the orthorhombicity and T_c also implies that the detected Meissner signals came from the K₂NiF₄ phase.