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

Low temperature fluorination of Sr2 - xBaxCuO3 (x = 0.0-0.6) was carried out. The pristine samples were heat-treated with NH4F at 210 degrees 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 [P.R, Slater, P.P. Edwards, C. Greaves, I. Gameson, M.G. Francesconi, J.P. Hedges, M. Al-Mamouri, M. Slaski, Physica C 241 (1995) 151] 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 Angstrom from a broad peak of the 210 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. (C) 2000 Elsevier Science B.V. All rights reserved.