Effect of Tl doping on the phase stability of Hg₂Ba₂(Y,Ca)₁Cu₂O _8-δ superconductors prepared by a high-pressure technique

(Hg, T1) based cuprates with the nominal compositions of (Hg_1-xT1_x)₂Ba₂Y_1-yCa _yCu₂O_8-δ (0 ≦ x ≦ 1.0 and 0 ≦ y ≦ 1.0) were prepared under a high pressure and high temperature. The solubility of Ca ions into Ae Hg-2212 phase for various T1 contents was examined by X-ray powder diffraction and energy-dispersive X-ray spectroscopy. The substitution of Ca²⁺ for the Y³⁺ site for the samples with x = 0.3 and 0.5 is realized over a full range of 0 ≦ y ≦ 1.0, while the solubility range is limited for the samples with x ≦ 0.15 and x ≧ 0.75. The samples with x = 0.3 exhibited a maximum T_c of 84 K, which was about 30 K higher than the maximum T_c for the samples with x = 0. The significant extension of Ca solubility for the T1 doped samples is probably explained by the compensation of excess negative charges in the (Hg_1-xT1_x)₂O_2-δ charge reservoir. The fact that the Y free Hg-2212 phase is obtained by T1 doping suggests the possibility of synthesizing a Hg-22(n - 1)n (n = 3, 4, ⋯) homologous series.