NMR study of ¹⁹F resonance above T_c in fluorinated YBa₂Cu₃O_x

The ¹⁹F NMR spectra were obtained at 282.365 MHz, for a series of YBa₂Cu₃O_xF_γ samples at a number of different temperatures, where γ = 0.1, 0.2 when x = 7; γ = 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 superconductor, the partially oxygenated superconductor, and the antiferromagnetic non-superconductor when the material is undoped. The spectra were obtained using a phase cycled echo. By measuring the echoes using a series of different times between pulses, we were able to resolve the spectra into two separate lines, representing two different F sites where the occupancy of one is much larger than the other, their ratio depending on the concentrations. The major site is attributed to the chain sites along a or b or both and the minor site is tentatively assigned to the apical position between the chain copper and the plain copper. The frequency shift of the major component is independent of both the oxygen and the fluorine concentration.