Preparation and properties of superconducting Y(Ba1-xSrx)2Cu4O8.

We have prepared single-phase or near-single-phase Y(Ba{sub 1{minus}{ital x}}Sr{sub {ital x}}){sub 2}Cu{sub 4}O{sub 8} (0{le}{ital x}{le}0.4) samples by means of a high-oxygen-pressure technique using an O{sub 2} hot isostatic pressing. All the lattice constants, {ital a}, {ital b}, and {ital c}, of Y(Ba{sub 1{minus}{ital x}}Sr{sub {ital x}}){sub 2}Cu{sub 4}O{sub 8} monotonically decreased with increasing Sr content. The superconducting transition temperatures of these samples were nearly constant, being around 80 K, while the {ital T}{sub {ital c}}'s of Y(Ba{sub 1{minus}{ital x}}Sr{sub {ital x}}){sub 2}Cu{sub 3}O{sub {ital z}} ({ital z}{similar to}7) samples decreased with increasing Sr content. When Y in Y(Ba{sub 0.8}Sr{sub 0.2}){sub 2}Cu{sub 4}O{sub 8} was partially substituted by Ca, the superconducting critical temperature increased to near 90 K.