Calcium substitution in the Y-Ba-Cu-O system at elevated P(O2) results in superconducting compounds with novel and enhanced properties: tetragonal CaxY(1-x)Ba2Cu3Oy with Tc=86 K for x=0.2 prepared at P(O2)=16 bars, and CaxY(1-x)Ba2Cu4Oy (1:2:4) with Tc=89 K for x=0.1 prepared at 50200 bars. Calcium substitution shifts the phase-stability boundary between 1:2:3, 2:4:7, and 1:2:4 phases to higher P(O2), and stabilizes a tetragonal 1:2:3 structure at moderately elevated P(O2) without substantial decrease in Tc. The Tc 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. © 1990 The American Physical Society.
