Can halogen atoms be inserted into the YBCO system?

We have measured the ¹⁹F and ³⁵Cl NMR spectra of fluorine and chlorine doped YBCO. We show that the positions and widths of the resonance lines do not match any of the most probable decomposition products of the doped YBCO. In addition, we show that the temperature dependence of the ¹⁹F linewidth follows the evolution of the vortex lattice in the superconducting phase. The ³⁵Cl NMR spectra are consistent with the fact that the Cl occupies the chain or apical oxygen site. ESR results show an increase in the ESR signal above a certain halogen concentration, which depends on the oxygen content. This is interpreted as an appearance of either a very small amount (of the order of 0.1%) of a new paramagnetic phase or a decrease in the grain size due to halogenation.