Calcium-substituted Y-Ba-Cu-O superconductors with enhancedTcsynthesized at elevated oxygen pressures

Abstract

Calcium substitution in the Y-Ba-Cu-O system at elevated P(${\mathrm{O}}_2$) results in superconducting compounds with novel and enhanced properties: tetragonal ${\mathrm{Ca}}_{\mathit{x}}$ ${\mathrm{Y}}_{\left(1\mathrm{}\mathrm{-}\mathit{x}\right)}$ ${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{\mathit{y}}$ with ${\mathit{T}}_{\mathit{c}}$=86 K for x=0.2 prepared at P(${\mathrm{O}}_2$)=16 bars, and ${\mathrm{Ca}}_{\mathit{x}}$ ${\mathrm{Y}}_{\left(1\mathrm{}\mathrm{-}\mathit{x}\right)}$ ${\mathrm{Ba}}_2$ ${\mathrm{Cu}}_4$ ${\mathrm{O}}_{\mathit{y}}$ (1:2:4) with ${\mathit{T}}_{\mathit{c}}$=89 K for x=0.1 prepared at 50–200 bars. Calcium substitution shifts the phase-stability boundary between 1:2:3, 2:4:7, and 1:2:4 phases to higher P(${\mathrm{O}}_2$), and stabilizes a tetragonal 1:2:3 structure at moderately elevated P(${\mathrm{O}}_2$) without substantial decrease in ${\mathit{T}}_{\mathit{c}}$. The ${\mathit{T}}_{\mathit{c}}$ of 1:2:4 increases with Ca substitution up to x=0.1, and then decreases gradually for larger x, possibly because of excessive hole concentration.