NMR-STUDY OF F-19 RESONANCE ABOVE T-C IN FLUORINATED YBA2CU3OX

The F-19 NMR spectra were obtained at 282.365 MHz, for a series of YBa 2Cu3OxFy samples at a number of different temperatures, where y = 0.1, 0.2 when x = 7; y = 0.10, 0.15, 0.29, 0.38 when x = 6.7; and y = 0.6, 0.98, 1.50 when x = 6. These values of x span the fully oxygenated su perconductor, the partially oxygenated superconductor, and the antifer romagnetic non-superconductor when the material is undoped. The spectr a were obtained using a phase cycled echo. By measuring the echoes usi ng a series of different times between pulses, we were able to resolve the spectra into two separate lines, representing two different F sit es where the occupancy of one is much larger than the other, their rat io depending on the concentrations. The major site is attributed to th e chain sites along a or b or both and the minor site is tentatively a ssigned to the apical position between the chain copper and the plain copper. The frequency shift of the major component is independent of b oth the oxygen and the fluorine concentration. (C) 1998 Elsevier Scien ce B.V. All rights reserved.