APICAL CU-O BOND IN YBA2CU3O7-DELTA SUPERCONDUCTORS BY XAFS

Careful Cu K-edge X-ray absorption fine structure (XAFS) measurements on oriented powders of YBa2Cu3O7-delta were made as a function of temp erature, of delta, T(c), method of preparation and grain size. The XAF S data were analyzed for the Cu(I) - O(4) apical bond distance distrib ution. All samples indicated evidence for a double-peak in the distrib ution with a splitting of 0.1angstrom, with the clearest case for the delta almost-equal-to 0 samples occuring for the T(c) = 89K 20mum grai n samples and the least clear one for the 5mum grain T(c) = 93K sample . There is no significant temperature dependence of the splitting. Bec ause the XAFS evidence for the splitting depends on a small signal, it is prudent to verify its existence by other independent measurements.