THE STRUCTURES OF ND1-XCU3O7-DELTA (X=0.05, 0.22, 0.42) PHASES REFINED FROM X-RAY SINGLE-CRYSTAL DATA(XBA2)

Crystal structures of Nd1+xBa2-xCu3O7-delta phases (x=0.05, 0.22 and 0 .42) were refined from X-ray single-crystal data (tetragonal system, s pace group P4/mmm, Z= 1, R=0.027, 0.027, 0.025 respectively). The main differences between these similar structures deal with the cation and oxygen sites in the (Ba/NdO) layer. Joint occupation of the same site by Ba and Nd ions with essentially different ionic radii results in t he splitting of that site along the c-axis and the shift of the oxygen in this layer from the ideal position at the four-fold axis to achiev e appropriate coordination for the smaller rare earth cation. These ch anges may be the structural reason for the suppression of superconduct ivity in R1+xBa2-xCu3O7-delta (R is rare earth) phases.