NEUTRON-DIFFRACTION AND ION-CHANNELING INVESTIGATIONS OF ATOMIC DISPLACEMENTS IN YBA2CU3O7-DELTA BETWEEN 10-K AND 300-K

Neutron powder diffraction was used to investigate the structural and vibrational properties of a single-phase ceramic specimen of YBa2Cu3O7-delta in the temperature range from 10 to 300 K. Measurements were taken at closely-spaced (10 K) intervals between 50 and 150 K, and the data were refined to a precision equal to or exceeding that achieved in previous studies. No clear evidence is found for an anomalous change in any of the structural parameters, including the orthorhombicity parameter, 2(b-a)/(a+b), in the vicinity of T(c). The vibrational properties obtained from the neutron diffraction data are compared with previous results from ion-channeling in single-crystals of YBa2Cu3O7-delta; these latter results revealed a clear anomaly at T(c) in displacements of the Cu(1), Cu(2) and O(4) atoms in the a-b plane. The average vibrational properties of the lattice atoms determined from the two techniques agree quite well, showing an overall decrease in the apparent vibrational amplitude from 300 to 100 K, followed by a further drop as the temperature is lowered through T(c). The evidence for an abrupt change (approximately 0.015 angstrom) in displacement amplitude at T(c) is clearly much stronger in the channeling results; the magnitude of any anomaly in the diffraction results approaches the statistical uncertainty. The absence of a clear abrupt change in the neutron results at T(c) suggests that the ion channeling anomaly is not due to an overall reduction in average vibration amplitude, but arises instead from a strongly correlated sequence of Cu and O(4) displacements appearing in the superconducting state.