Crystal structure and superconductivity in the Hg-containing Ba- and Sr-based cuprates

We have synthesized the Hg-containing Barium-based cuprate of the HgBa₂Ca₂Cu₃O_8+δ (1223) compound with T_c up to 135 K without post-annealing by directly mixing from mono-oxide powders of Hg, Ba, Ca, and Cu as starting materials. This is the n = 3 phase of the homologous series HgBa₂Ca_n-1Cu_nO_2n+2+δ. The isotopic strontium-based cuprates, corresponding to the formula HgSr₂Ca_n-1Cu_nO_2n+2+δ have not been synthesized up to now. However, the stabilization of the strontium cuprates with n = 1 and 2 structures can be performed by introducing a second element (e.g. Pb) besides Hg in the hole reservoir layers. Moreover, the chemical doping of Y³⁺ into the Ca²⁺ sites and La³⁺ into the Sr²⁺ sites in the strontium cuprates of the n = 1 and n = 2 compounds, respectively, is necessary to induce superconductivity. Bulk superconductivity with T_c up to 35 K and 90 K has been achieved in (Pb_0.5Hg_0.5)(Sr_1.2La_0.8) CuO_4+δ (n = 1) and (Pb_0.5Hg_0.5)Sr₂(Ca_0.7Y_0.3)Cu₂O_6+δ (n = 1), respectively. The crystal structures of the newly discovered Hg-containing phases have been investigated by selected area electron diffraction (SAED). The appearance of superlattice spots with a commensurate modulation in the SAED patterns was observed in the Hg-containing Sr-based cuprates. However, such a phenomenon was not found in the HgBa₂Ca₂Cu₃O_{8_δ} phase.