SUPERCONDUCTING OXYCARBONATES HG1-XVXSR4-YBAYCU2O7-DELTA-CO3 - 3 CLOSELY-RELATED STRUCTURAL FAMILIES

Three closely related structural families of superconducting oxycarbon ates have been synthesized in the same system, for the first time, wit h the generic formulation Hg1-xVxSr4-yBayCu2O7-deltaCO3. Their XRD and ED studies evidence a pseudo tetragonal symmetry with alpha ranging f rom 3.845 to 3.922 Angstrom and c ranging from 16.240 Angstrom to 17.0 37 Angstrom. This study shows the important role of the size effect of the A cation (A = Ba, Sr) for the stabilization of these different st ructures. For 0 less than or equal to y <1.8, corresponding to the sma ller mean size of the A cations, the non collapsed [1201][S2CC] interg rowth is synthesized. Increasing the mean size of A favours the collap sed structures: the (110) collapsed structure is first formed for inte rmediate y values (1.8 < y < 2.5), whereas the (100) collapsed oxycarb onate is stabilized for 2.5 < y less than or equal to 2.65. The c para meter increases regularly with y, whereas the alpha parameter increase s but with a different evolution for each structural domain. The vanad ium content decreases as the barium content increases, from x = 0.40 f or y = O to x = 0.28 for y = 2.65. All these oxycarbonates are superco nductors with T-c ranging from 49 K (y = 2.65) to 76 K (y = 0). They e xhibit a rather unusual evolution of T-c compared to thallium and merc ury cuprates. In contrast to the latter, T-c decreases as strontium is replaced by barium.