Local structural changes in lithium-doped YBa2Cu3Oy

The structure of orthorhombic and tetragonal samples of lithium (Li)-doped YBa2Cu3Oy has been determined by neutron powder diffraction measurements. T he experiment shows that when the samples are synthesized in oxygen, Li sub stitutes for the copper of the CuO2 planes. When the samples are synthesize d in air, a small fraction (approximate to 20%) of the Li substitutes for t he copper of the Cu-O chains. Moreover, the best fits to the diffraction sp ectra, as well as the best overall understanding of a number of experimenta l observations, are obtained by assuming that each substitution of a Li for a Cu entails the loss of the neighbouring apical oxygen. This feature seem s to be retained during the annealing in argon at 750 degrees C, which tran sforms the crystallographic structure from orthorhombic to tetragonal. (C) 2000 Elsevier Science B.V. All rights reserved.