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)Sr₂Ca_n-1Cu_nO_y (n = 1–6)

Abstract

A series of new oxycarbonitrate superconductors, (Cu, C, N)Sr₂Ca_n-1Cu_nO_y, (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 transmission electron microscopy, to reveal arrangements of Cu and C (N) in the charge -reservoir blocks (CRBs). Ordered arrangements of Cu and C (N) are observed in the compounds with n = 1–4, and almost random arrangements for those 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 to 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 twofold periodicity is observed, indicating that the ratio of Cu to C (N) in the CRB is 1 to 1. Both types of ordering structure coexist in the n = 3 compound, (Cu, C, N)-1223. In the last two members, (Cu, C, N)-1245 (n = 5) and (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 relations between the ordering scheme and n parameters are also observed locally in the crystals which contain many intergrowth defects.