NMR AND NQR STUDY OF ATOMIC MOTION BELOW 500 K IN YBA2CU3O7

Measurements of the intensity of the Cu-63 NQR and quadrupolar-satelli te NMR at elevated temperatures are reported for the planar Cu(2) site s in samples of nominal composition YBa2Cu3O7. It is observed that spi n-echo intensities decrease dramatically between room temperature and 500 K and that the onset of intensity loss occurs at lower temperature s in more defective samples. Measurements of the spin-echo decay rates reveal that the intensity loss is associated with loss of phase coher ence exceeding that expected from spin-spin and spin-lattice relaxatio n. By means of a simple, illustrative model, these effects can be attr ibuted to changes in the local atomic environment, presumed to be caus ed by motion of oxygen atoms in the Cu(1) layer. These motions occur o n a time scale on the order of 10 mu s at 500 K and their probability is enhanced at lower temperatures by the presence of structural defect s.