NEW HIGH-T-C SUPERCONDUCTORS (GEZCU1-Z)SR2CAN-1-XYXCUNOY (N=4,6) PREPARED AT HIGH-PRESSURE

Recently a new superconductor (Ge2Cu1-z)Sr2Ca2-xYxCu3Oy having the hig hest T-c of 90 K was synthesized under high pressure [A.T. Matveev et al., Physica C 262 (1996) 279]. This compound is thought to be the n = 3 member of a homologous series (GezCu1-z)Sr2Can-1-xYxCunOy. This pap er reports two new members of the series with n = 4 and n = 6 which we re prepared at 6 GPa. The former phase showed superconductivity below 89 K while the latter below 78 K. They have simple tetragonal cells wi th a = 3.8411(2), c = 17.733(2) Angstrom for n = 4 and a = 3.8496(2), c = 24.183(2) Angstrom for n = 6. HRTEM observations for them indicate d M-12(n - 1)n-type structures with Ge-Cu mixed atoms at the M site.