Observation of changes in the atomic and electronic structure of single-crystal YBa₂Cu₃O_6.6 accompanying bromination

To ascertain the role of bromination in the recovery of superconductivity in underdoped YBa₂Cu₃O_6+y (YBCO), we have performed polarized multiple-edge x-ray-absorption fine structure (XAFS) measurements on normal (y∼0.6) and brominated (Br/Cu∼1/30, y∼0.6) single crystals with superconducting transitions at 63 and 89 K, respectively. The brominated sample becomes strongly heterogeneous on an atomic length scale. Approximately one-third of YBCO is locally decomposed yet incorporated as a well-ordered "host" lattice as nanoscale regions. The decomposed phase consists of heavily distorted domains with an order not following that of the "host" lattice. Structurally, these domains are fragments of the YBCO lattice that are discontinued along the Cu(1)-O(1) containing planes. The local structure is consistent with the cluster expansions: Y-O(2,3)₈-Cu(2)₈-..., Ba-O₈-Cu(2)₄Cu(1)₂-..., and Cu-O₄... about the Y, Ba, and Cu sites. Interatomic distances and Debye-Waller factors for the expansions were determined from fits to Y K-, Ba L₃-, and Cu K-edge XAFS data at room temperature. Br K-edge data reveal that Br does not enter substitutionally or interstitially into the perfect YBCO lattice. However, Br does occupy the Cu(1) sites in a nanofragment of the YBCO lattice, forming Br-O(4)-Ba-Cu₂(1)Cu(2)-... nanoclusters. From polarized measurements these nanoclusters were found to be almost randomly oriented with respect to the "host" crystal, and probably are the nucleus of the decomposed phase. This heterogeneity brings about the unusual structural and electronic properties of the normal state previously reported in the literature. Implications on for diffraction, transport, and magnetization measurements are discussed.