Effect of Tl doping on the phase stability of Hg 2Ba 2(Y,Ca) 1Cu 2O 8−δ superconductors prepared by a high-pressure technique

Abstract

(Hg, Tl) based cuprates with the nominal compositions of (Hg 1− x Tl x ) 2Ba 2Y 1− y Ca y Cu 2O 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 the Hg-2212 phase for various Tl contents was examined by X-ray powder diffraction and energy-dispersive X-ray spectroscopy. The substitution of Ca 2+ for the Y 3+ 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 Tl doped samples is probably explained by the compensation of excess negative charges in the (Hg 1− x Tl x ) 2O 2−δ charge reservoir. The fact that the Y free Hg-2212 phase is obtained by Tl doping suggests the possibility of synthesizing a Hg-22( n − 1) n( n = 3, 4, …) homologous series.