${}^{63,65}\mathrm{Cu}$ nuclear quadrupole resonance (NQR), ${}^{63,65}\mathrm{Cu}$ and ${}^{79,81}\mathrm{Br}$ nuclear magnetic resonance (NMR), and Br K-edge x-ray-absorption fine-structure (XAFS) measurement techniques have been used to study the local structures of Cu and Br in well-characterized samples of deoxygenated and brominated ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$ (YBCO). The combined results provide a detailed picture of the role of bromine in reoxygenating the YBCO structure and an explanation for the partial restoration of superconductivity in the YBCO system. Characterization of the powder samples, with particle sizes of 1, 20, 30, and 40 μm, included x-ray diffraction, thermal gravimetric analysis, energy dispersive x-ray analysis, and magnetometry. From the XAFS and NMR results, it is concluded that, upon bromination at 260 °C, Br does not enter the YBCO lattice either substitutionally or interstitially. Instead, there is compelling evidence for the formation of nanoscale ${\mathrm{BaBr}}_2$ precipitates which result from the local destruction of the ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_y$ phase. Furthermore, on the basis of the NQR and NMR experiments, it is concluded that through this decomposition into an inhomogeneous material, oxygen is liberated which repopulates the nearby O(4) chain sites, thus restoring superconductivity.

• Received 5 April 1999

DOI:https://doi.org/10.1103/PhysRevB.60.10489