A new copper oxide superconductor containing carbon

Abstract

SUPERCONDUCTIVITY in the high-transition-temperature (high-T_c) copper oxide superconductors seems to arise from layers of copper–oxygen squares, pyramids or octahedra. Recently the compound Sr₂CuO₂CO₃ was found to contain layers of CuO₆ octahedra¹ (Fig. 1), suggesting that it might be made superconducting by appropriate doping. But the presence of carbonate as an impurity is known to degrade the superconducting properties of materials such as ${\text{YBa}}_{\text{2}}{\text{Cu}}_{\text{3}}{\text{O}}_{\text{7}-{\delta }^{\prime }}$ and ${\text{YBa}}_{\text{2}}{\text{Cu}}_{\text{4}}{\text{O}}_{\text{8}}$ (refs 2, 3), much effort having been made to minimize residual carbon in these compounds by optimizing processing methods⁴. It is thus of some interest to see whether Sr₂CuO₂CO₃ can be made superconducting despite the carbonate ions incorporated in the structure. Here we report the synthesis, at 50 atm oxygen partial pressure, of superconducting ${\text{(}{\text{Ba}}_x{\text{Sr}}_{\text{1-}x}\text{)}}_{\text{2}}{\text{Cu}}_{\text{1}+y}{\text{O}}_{\text{2}+\text{2}y+\delta }{\text{(}{\text{CO}}_{\text{3}}\text{)}}_{\text{1}-y}$ (0.4≤x≤0.65, y≈O.1), with T_c (onset) up to ∼40 K and zero resistance at up to ∼26K. The crystal structure contains CuO₂ sheets alternating with ${\text{(}{\text{Ba}}_x{\text{Sr}}_{\text{1-}x}\text{)}}_{\text{2}}{\text{Cu}}_y{\text{O}}_{\text{2}y+\delta }{\text{(}{\text{CO}}_{\text{3}}\text{)}}_{\text{1}-y}$, slabs, which serve as charge reservoir layers: substitution of ∼10% copper for carbon in the slabs introduces holes into the CuO₂ sheets, making the compound superconducting.