ION CHANNELING ALONG THE [301] AND [331] AXES IN YBA2CU3O7-X

In order to characterize more fully the phonon anomaly observed previously across T, in YBa2Cu3O7-x and (Bi1.7Pb0.3)Sr2Ca,Cu2O(x), ion channeling was performed along the [301] and [331] directions in YBa2Cu3O7-x. Because twinning in these crystals occurs via interchange of tbe 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 [3011 and [331] directions show a splitting of the channeling dip by 0.96-degrees and 1.92-degrees, 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 new results reveal that the c-axis component of the anomalous atomic displacements that occur across T(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 T(c).