New high- Tc superconductors (Ge zCu 1− z)Sr 2Ca n−1− xY xCu nO y ( n = 4, 6) prepared at high pressure

Abstract

Recently a new superconductor (Ge z Cu 1− z )Sr 2Ca 2− x Y x Cu 3O y having the highest 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 (Ge z Cu 1− z )Sr 2Ca n−1− x Y x Cu n O y . This paper reports two new members of the series with n = 4 and n = 6 which were prepared at 6 GPa. The former phase showed superconductivity below 89 K while the latter below 78 K. They have simple tetragonal cells with a = 3.8411(2), c = 17.733(2) A ̊ for n = 4 and a = 3.8496(2), c = 24.183(2) A ̊ for n = 6 . HRTEM observations for them indicated M-12( n − 1) n-type structures with GeCu mixed atoms at the M site.