Visualizing the Atomic Configuration of Carbonate Groups  in a (Cu,C)Ba₂ Ca₃ Cu₄ O_11+δ Superconductor

Carbonates (CO₃ ^2- ) have always been known as impurities to degrade the superconductivity in cuprate high-Tc superconductors. Herein, the atomic arrangement of carbonates is directly visualized in (Cu,C)Ba₂ Ca₃ Cu₄ O_11+δ via integrated differential phase contrast (iDPC) combined with state-of-the-art scanning transmission electron microscopy. The carbon atoms replace Cu atoms in the charge-reservoir layers, contributing to the formation of carbonates through strong orbital hybridization with the surrounding oxygen atoms. Using first-principles calculations, the spatial configuration of the carbonate groups is confirmed and their influence on the local crystal lattice and electronic states is further investigated. The carbonates not only accommodate distortions by improving the flatness of the outer CuO₂ layers but also reduce the density of states at the Fermi level. These two factors play competitive roles to affect the superconductivity. This study provides direct evidence of the presence of CO₃ ^2- groups and gains an insight into the underlying mechanism of superconductivity in oxycarbonate superconductors.