Ion channeling along the [301] and [331] axes in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$

In order to characterize more fully the phonon anomaly observed previously across ${\mathit{T}}_{\mathit{c}}$ in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$ and (${\mathrm{Bi}}_{1.7}$${\mathrm{Pb}}_{0.3}$)${\mathrm{Sr}}_2$${\mathrm{Ca}}_1$${\mathrm{Cu}}_2$${\mathrm{O}}_{\mathit{x}}$, ion channeling was performed along the [301] and [331] directions in ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{7\mathrm{-}\mathit{x}}$. Because twinning in these crystals occurs via interchange of the a and b crystal axes, twins produce no effect on channeling along the [001] axis. However, twinning is a major factor along all other directions. In particular, axial scans along [301] and [331] directions show a splitting of the channeling dip by 0.96° and 1.92°, respectively. In areas of the specimen in which one twin variant dominates, a single dip is shown to occur along the appropriate [331] axis, but the dip remains split along all [301] directions. Detwinned crystals produced reasonably good single dips along both directions. The observed splitting of the channeling dips is accounted for using a geometrical analysis based upon the known difference in the a and b lattice dimensions. These results reveal that the c-axis component of the anomalous atomic displacements that occur across ${\mathit{T}}_{\mathit{c}}$ is at most only slightly larger than the component in the a-b plane. The [301] channeling scans also show unambiguously that the Cu atoms participate in the anomalous displacements observed at ${\mathit{T}}_{\mathit{c}}$.