Orthorhombic and tetragonal structures in the La(Ba2-yLay)Cu3O7-x system
The crystal structures present in a series of La(Ba2-yLay)Cu3O7-x samples with y=0.5, 0.375, 0.25, and 0.125 have been investigated by Rietveld refinements of X-ray and neutron powder diffraction data. For y=0.5 and x=-0.20, the structure is tetragonal, P4/mmm, similar to YBa2Cu3O6, but with 60(2)% occupancy of the O1 (and, equivalently, O5) sites. At all compositions barium populates only the Ba site. For y<0.5, samples contain both orthorhombic (O) and closely related tetragonal (T) components, in relative amounts that depend sensitively on synthesis conditions. The present X-ray and neutron diffraction data are consistent with tetragonal symmetry being adopted for oxygen contents x<0.0 For x>∼0.0, both O and T phases can occur, although there is apparently a significant barrier to converting such O components to more oxygen rich T materials with x<∼0.0 by low-temperature oxygen annealing. Thus, oxygen pressure annealing of a typical material with y=0.125 (which significantly improves the superconducting behavior) increases slightly the orthorhombic distortion of the O component, but little affects the relative amount of the T component. The tendency towards O and/or T character is apparently determined primarily by the temperature at which the compound is initially formed, possibly as a result of differences in the La/Ba distribution in the Ba planes. A comparison between the structural results and Meissner effect data from these same samples, suggests that optimal bulk high temperature superconductivity in this system is associated with an orthorhombic distortion. The average structural details of these various materials are discussed and compared with those of the YBa2Cu3O7-x system. © 1989.
Newsam, JM; Jacobson, AJ; Goshorn, DP; Lewandowski, JT; Mitzi, DB; Kapitulnik, A; Xie, D; Yelon, WB
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