CATIONIC SUBSTITUTION IN MERCURY-BASED CUPRATES - GENERATION OF 5 SUPERCONDUCTORS

Five mercury-based superconductors, Hg(1-x)M(x)Ba(2)CuO(4+delta) with M=Mn, Nb, Hg(1-x)M(x)Ba(2)CaCu(2)O(6+delta) with M=Mn, Ge and Hg1-xNbx Ba2Ca2Cu3O8+delta, have been synthesized. A significant decrease of th e c parameter of the tetragonal cells of the Ge and Mn phases with res pect to the unsubstituted mercury cuprates is observed for the Ge and Mn phases, whereas the Nb phases exhibit a 'c' parameter close to that of the unsubstituted cuprates, in agreement with the relative sizes o f Mn-IV, Ge-IV and Nb-V. EDS analyses confirm that Mn, Ge and Nb have entered into the matrix of the cuprate in a homogeneous way, Magnetic studies of the as-synthesized phases and of the samples after argon an d oxygen annealing show sharp transitions except for the 1201-Nb phase . The critical temperatures, T-c (midpoint), of these phases after opt imisation are close to those of the unsubstituted mercury cuprates: 91 K for the 1201-Mn phase, 123-124 K for the 1212 Mn and Ge phases and 128 K for the 1223 Nb phase. The investigation of the Al-substituted m ercury cuprates is also discussed.