High-resolution transmission electron microscopy study of ordered or disordered arrangements of Cu, C and N in the charge-reservoir blocks of a series of high-T-c superconductors: (Cu, C, N)Sr2Can-1CunOy (n=1-6)

A series of new oxycarbonitrate superconductors, (Cu, C,N)Sr2Can-1CunOy, (n = 1-6), recently prepared in a Sr-Ca-Cu C-N-O system under a high pressure of 5.6 GPa, are examined by electron diffraction and high-resolution trans mission electron microscopy, to reveal arrangements of Cu and C (N) in the charge-reservoir blocks (CRBs). Ordered arrangements of Cu and C (N) are ob served in the compounds with n = 1-4, and almost random arrangements for th ose with n = 5-6. In the first two series of compounds, (Cu, C, N)-1201 (n = 1) and (Cu, C, N)-1212 (n = 2), an ordering scheme of -Cu-C(N)-C(N)-C(N)- Cu-with fourfold periodicity is observed, indicating that the ratio of Cu t o C (N) in the CRB is 1 to 3. For (Cu, C, N)-1234 (n = 4), with highest T-c of 113 K in the series, the ordering scheme of Cu C(N)-Cu-C(N) Cu with two fold periodicity is observed, indicating that the ratio of Cu to C (N) in t he CRB is 1 to 1. Both types of ordering structure coexist in the n = 3 com pound, (Cu, C, N)-1223. In the last two members, (Cu, C, N)-1245 (n = 5) an d (Cu, C, N)-1256 (n = 6), no evidence of ordering was observed, suggesting that Cu, C and N are distributed almost randomly in the CRBs. Such relatio ns between the ordering scheme and n parameters are also observed locally i n the crystals which contain many intergrowth defects.