HIGH-PRESSURE SYNTHESIS OF Y1-XCAXSR2GACU2O7-PLUS-OR-MINUS-DELTA (0-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-1.0)

High-pressure synthesis at 6 GPa was performed for a series of compounds, Y1-xCaxSr2GaCu2O7+/-delta with 0 less-than-or-equal-to x less-than-or-equal-to 1.O. The solubility limit extended up to x=1.0 at 6 GPa in contrast to x almost-equal-to 0. 3 at ambient pressure and the Y-free compound, CaSr2GaCu2O7+/-delta, was prepared successfully. Superconductivity appeared in samples with x greater-than-or-equal-to 0.4 which showed metallic conductivity above T(c). T(c) increased up to 49 K at x=0.6 with increasing x, while T(c) decreased to 15 K at x=0.8, probably owing to overdoping of carriers. However, T(c) of the Y-free sample was as high as 70 K in spite of the fact that it seems to have a larger carrier density. Possible reasons for this ''abnormal'' result are discussed.