A new family of copper oxide superconductors Sr_n+1Cu_nO_2n+1+δ stabilized at high pressure

Abstract

THE search for new copper oxide superconductors¹ has been pursued mainly by exploring a range of chemical compositions (counter-cations and oxygen content) and reaction temperatures. We have explored the effects of an additional variable, the reaction pressure, and have thereby synthesized some new compounds, including (Ca_1-ySr_y)_1-xCuO₂ which shows superconductivity at 110 K (refs 2, 3). Here we report the synthesis of a new family of high-pressure phases with the formula Sr_n+1Cu_nO_2n+1+δ (n = 1, 2, 3,...) extending from Sr₂CuO_3.1, (n = 1) to SrCuO₂ (n = ∞ )⁴. Sr₂CuO_3.1 and Sr₃Cu₂O₅+δ′ (n=2) are superconductors with transition temperatures (T_c ) of 70 and 100K, respectively. The metal sublattice of Sr₂CuO_3.1, is of the tetragonal K₂NiF₄ (or Nd₂CuO₄) type with lattice constants a = 3.764 Å and c-12.548 A, and iodometric measurements suggest that nearly half of the oxygen atoms are missing from the counter-layers to Sr₂O_1.1. Homogeneous pyramidal or the oxygen atoms are missing from the counter-layers to Sr₂O_1.1. Homogeneous pyramidal or octahedral coordination, which has generally been believed to be indispensable to attain T_cs of ∼100 K, is thus absent. The n = 2 and 3 members seem to correspond to the high-pressure phases in the Ca–Sr–Cu–O system recently reported by Adachi et al.⁵.