The parent structure of the layered high-temperature superconductors

Abstract

Oxide superconductors in the system Tl₂Ba₂Ca_n–1Cu_nO_4+2n (ref. 1) have transition temperatures (T_c) above 100 K, increasing with n. So far, stacking sequences up to n = 3 have been found in small crystals, and sequences with n > 3 have been seen in electron microscopy studies¹. For large n, the stoichiometry of Tl₂Ba₂Ca_n–1Cu_nO_4+2n approaches CaCuO₂, a structure expected to consist only of CuO₂ planes separated by Ca atoms. By analogy with Tl₂Ba₂Ca_n–1Cu_nO_4+2n, the unit cell of this hypothetical phase is expected to be tetragonal with a = 3.86 Å. Such a compound is not known in the Ca–Cu–O system, but Roth² recently reported that small amounts of Sr on the Ca site can stabilize this simple structure. Here we report the growth of small single crystals of this phase, with composition (Ca_0.86Sr_0.14)CuO₂, and their characterization by single-crystal X-ray diffraction. The crystals are tetragonal with space group P4/mmm, and the structure contains planar [CuO₂]_∞ layers separated by Ca and Sr atoms. The structure is a simple defect perovskite with ordered oxygen vacancies and can be regarded as the n = ∞ parent of the A₂B₂Ca_n–1Cu_nO_4+2n (A = Bi, Tl; B = Sr, Ba) superconductors.