Structure ofYBa2Cu3O7−δversus temperature by x-ray-absorption spectroscopy

We present extended x-ray-absorption fine-structure data on $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$ 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 ${\sigma }^2$. The distances show little temperature dependence. The ${\sigma }^2\text{'}\mathrm{s}$ 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 $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$. We find that a peak at ∼9002 eV in the XANES, the origin of which has been the subject of conflicting opinions in ${\mathrm{La}}_{2-x}{\mathrm{Sr}}_x\mathrm{Cu}{\mathrm{O}}_{4-y}$, is associated with interference effects in Cu(II) complexes and its presence is not evidence for Cu(III) in $\mathrm{Y}{\mathrm{Ba}}_2{\mathrm{Cu}}_3{\mathrm{O}}_{7-\delta }$.