Cationic substitution in mercury-based cuprates: generation of five superconductors

Abstract

Five mercury-based superconductors, Hg_1–xM_xBa₂CuO_4+δ with MMn, Nb, Hg_1–xM_xBa₂CaCu₂O_6+δ with M = Mn, Ge and Hg_1–xNb_xBa₂Ca₂Cu₃O_8+δ, have been synthesized. A significant decrease of the c parameter of the tetragonal cells of the Ge and Mn phases with respect 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 of 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 and oxygen annealing show sharp transitions except for the 1201-Nb phase. The critical temperatures, T_c(midpoint), of these phases after optimisation 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 AI-substituted mercury cuprates is also discussed.