Phase identification of the new 126 K Ba-Ca-Cu-O superconductor

New superconducting oxides that have a composition of Ba₂Ca₂Cu₃(Ca_xCu_y)O _8+δ with 0.4 ≥ x ≥ 0.9 and 0.7 ≥ y ≥ 0, and show an optimum T_c ∼ 126 K were identified. Structural analysis, based on X-ray diffraction, indicated a common tetragonal unit cell among these oxides, which has lattice constants a = 3.85 Å and c = 28.2 Å with an I/4mmm symmetry. The cell consists of a rock-salt (BaO)₂ block sandwiched between two perovskite Ca₂Cu₃O₆ blocks with interstitial Ca⁺²_x and Cu⁺²_y located, e.g. at the 4d site between the two BaO planes. In other words, a so-called 0223 cell with interstitial Ca_xCu_y. The oxides differ from one another by the amounts of the inserted cations, i.e. the values of x and y. The presence of these interstitial cations offers a new way to adjust the carrier concentration. These oxides are unstable against the incorporation of H₂O and CO₂ in open air. The decayed products (Ca,Cu,H,CO₂)₂Ba₂Ca₂Cu ₃O_8+δ have an expanded lattice constant c ∼ 33.8 Å and a lower T_c ∼ 90 K. A structure model for the decayed phases was also proposed.