NANOSTRUCTURAL TRANSITIONS RELATED TO FLEXIBLE SHEARING PHENOMENA IN THE SUPERCONDUCTORS HG1-XVXSR4-YBAYCU2O7-DELTA-CO3

The HREM study of the superconducting copper oxycarbonates Hg1-xVxSr4- yBayCu2O7-deltaCO3 (0.28 less than or equal to x less than or equal to 0.40; 0 less than or equal to y less than or equal to 2.65) shows the extraordinary nanostructural flexibility of these materials. Starting from the [S2CC] [1201] intergrowth for y = 0, coexistence of (100) an d (110) CSP, in the form of isolated defects, is evidenced for 0 < y < 1.8 and curved crystallographic shear boundaries are observed, for y = 1.8. The (110) collapsed structure is obtained for 1.8 < y < 2.5, wh ereas twinned (110) CSP and mixed (110)/(110) CSP appear around y = 2. 5, the (100) collapsed structure being synthesized for 2.50 < y less t han or equal to 2.65. Beyond y = 2.65, phasoids corresponding to the c oherent intergrowth of the ''1201'' cuprate, S2CC oxycarbonate, and (1 00) collapsed phase are evidenced. (C) 1996 Academic Press, Inc.