Apical Cu-O bond in YBa₂Cu₃O_7-δ superconductors by XAFS

Careful Cu K-edge X-ray absorption fine structure (XAFS) measurements on oriented powders of YBa₂Cu₃O_7-δ were made as a function of temperature, of δ, T_c, method of preparation and grain size. The XAFS data were analyzed for the Cu(1)-O(4) apical bond distance distribution. All samples indicated evidence for a double-peak in the distribution with a splitting of 0.1Å, with the clearest case for the δ ≈ 0 samples occuring for the T_c = 89K 20μm grain samples and the least clear one for the 5μm grain T_c = 93K sample. There is no significant temperature dependence of the splitting. Because the XAFS evidence for the splitting depends on a small signal, it is prudent to verify its existence by other independent measurements.