Temperature-dependent ion-channeling experiments with He ions were performed on Y-Ba-Cu-O single crystals and c-axis or a-axis oriented Y(Eu)-Ba-Cu-O thin films. Angular yield curves (scans) were measured above and below ${\mathit{T}}_{\mathit{c}}$ in the different sublattices. For both, [001] and [100] crystal directions anomalous changes in the scans with respect to their halfwidths ${\mathrm{\Psi }}_{1/2}$ and minimum yield values ${\mathit{X}}_{\min }$ were observed in the oxygen sublattice by cooling below ${\mathit{T}}_{\mathit{c}}$. In [001] direction also ${\mathrm{\Psi }}_{1/2}$ in the scan from Cu revealed an anomalous increase at ${\mathit{T}}_{\mathit{c}}$. No special effect was observed in the Y(Eu)-Ba sublattice. The observations are discussed in terms of structural distortions above ${\mathit{T}}_{\mathit{c}}$ which are removed upon cooling into the superconducting state. The values of the displacement amplitudes were determined in Monte Carlo simulation calculation fitting the experimental data. The main effects are displacements of the apical oxygen atoms of 3.5 pm and of 1.5 pm in the remaining oxygen sublattice.

• Received 16 August 1993

DOI:https://doi.org/10.1103/PhysRevB.48.16168