High-resolution electron microscope analysis of new type of superconductors in a BaCaCuCO Oxycarbonate system

Abstract

A new homologous series of oxycarbonate superconductors with ideal compositions (Cu 0.5C 0.5) mBa m+1Ca n-1Cu nO y, where m=1 & 2; n=3, 4 (and 5 for m=2), are prepared in a BaCaCuCO system under high pressure of around 5 Gpa, and their structures are revealed by HRTEM. Both (Cu,C)-1223 ( T c =67 K) and 1234 ( T c =117 K) consist of BaO[Cu,C]OBaO blocks sandwiched by Ca n-1(CuO 2) n infinite-layer like blocks. Structures of (Cu,C)-2334 ( T c =113 K) and 4679 (2334+2345), on the other hand, consists of thicker blocks of BaO[Cu,C]OBaO, also sandwiched by the Ca n-1(CuO 2) n type blocks. Superstructures with dual periodicity a=2 a t due to ordering of Cu and C within the [Cu,C]O layers are commonly observed. Anti-phase relations of Cu and C positions along the c-direction are observed in (Cu,C)-1223, 1234 and 2323 phases with c=2 c t , while in-phase relations are observed in (Cu,C)-2334 and 4679 (2334+2345) phases with c= c t .