Structure of YBa2Cu3O/sub 7-//sub δ/ versus temperature by x-ray-absorption spectroscopy

We present extended x-ray-absorption fine-structure data on YBa2Cu3O/sub 7-//sub δ/ taken at four temperatures: 80, 112, 170, and 297 K. Spectra from both the Y edge and Cu edge are analyzed for the mean distances to near neighbors and the mean-square relative displacements, σ2. The distances show little temperature dependence. The σ2's are unusual because they vary more strongly at low temperature (80--110 K) than at high temperature (170--300 K). This could indicate some softening of the bond strengths near 170 K which may entail a subtle structural change. From a study of the Cu K edge we conclude that the x-ray-absorption near-edge structure (XANES) does not provide evidence for the existence of Cu(III) in YBa2Cu3O/sub 7-//sub δ/. We find that a peak at ∼9002 eV in the XANES, the origin of which has been the subject of conflicting opinions in La/sub 2-//sub x/Sr/sub x/CuO/sub 4-//sub y/, is associated with interference effects in Cu(II) complexes and its presence is not evidence for Cu(III) in YBa2Cu3O/sub 7-//sub δ/