Static and dynamic displacements in R${\mathrm{Ba}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (R=Y, Ho; δ=0.05, 0.5): A neutron-diffraction study on single crystals

Mean-square displacements in superconducting R${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{7\mathrm{-}\mathrm{\delta }}$ (R=Y, Ho) were studied as function of temperature by neutron diffraction on single crystals. From a comparison of the results with the pure dynamical displacements calculated from the phonon dispersion relations of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_7$ we estimate the static contributions to the displacements. In particular we find static contributions to the displacements parallel to the (a,b) plane of the chain and the apical oxygen atoms. There is no evidence of an anomalous displacement of the apical oxygen in the c direction, in contrast to extended x-ray-absorption fine-structure studies suggesting a double-well potential. We also report on a structural anomaly near ${\mathit{T}}_{\mathit{c}}$, which is observed in the intensity of specific Bragg reflections.