The novel high temperature superconducting Ba-Ca-Cu-O system

Two new high temperature superconducting homologous series of the Ba-Ca-Cu- O system have been synthesized under high pressure: Ba2Can-1CunO2n+2 [02(n - 1)n] interstitially doped with Ca or Ca and Cu, and CaBa2Can-1CunO2n+3 [C a-12(n - 1)n] with or without partial substitution of Ca between the BaO-la yers by Cu. 02(n - 1)n with an I4-symmetry displays a T-c above 126 K and d egrades to a lower T-c compound when exposed to air, whereas Ca-12(n - 1)n with a P4-symmetry shows a T-c only up to 107 K but is stable in air. They form a good system for studying the effects of local symmetry on supercondu ctivity and instability. (C) 1998 Elsevier Science Ltd. All rights reserved .