HIGH-PRESSURE AND HIGH OXYGEN-PRESSURE SYNTHESES OF OXIDE SUPERCONDUCTORS

We report high-pressure and high-oxygen pressure syntheses of various oxide superconductors. Several new oxycarbonate superconductors were discovered in the Ca-Ba-Cu-C-O system at 5 GPa. They are (Cu0.5C0.5)Ba2Can-1CunO2n+3 (n=3; Tc=67 K, n=4; Tc=117 K) and (Cu0.5C0.5)(2)Ba3Can-1CunO2n+5 (n=4,5; Tc similar to 113 K). The former series of compounds are essentially isomorphous to TI (or Hg)-12(n-1)n, n=3,4, respectively. On the contrary, no compounds corresponding to the latter have been known. An ''infinite-layer'' sample was prepared at 6 GPa by adding CaCO3 and SrCO3. HRTEM observation for this sample suggested that it included defect layers, (C,Cu)O-y which may be responsible for similar to 100 K superconductivity. Finally, Ga- and Al-based 1212 phases prepared under high pressure are discussed.