Oxygen ordering and superconductivity in La(Ba_2−xLa_x)Cu₃O_7+δ

Abstract

The observation of superconductivity above 90 K in the Y–Ba–Cu–O system^1,2 has prompted a search for other structures that display high-temperature superconductivity. The discovery of superconductivity above 70 K in La_3−xBa_3+xCu₆O_l4+y³ raises some questions concerning its relationship to the YBa₂Cu₃O_7−δ structure and the origins of the high-temperature superconductivity. The original X-ray diffraction study of La₃Ba₃Cu₆O_14+y, indicated a tetragonal structure^4,5, on the basis of which it has been proposed that La_3−xBa_3+xCu₆O_14+y is a new class of high-temperature superconductor which, in contrast to YBa₂Cu₃O_7−δ, contains no Cu–O chains³. More recently, a neutron diffraction study of non-superconducting La₃Ba₃Cu₆O_14+y⁶has indicated that the correct structure of this compound is simply the tetragonal YBa₂Cu₃O_7−δ structure with partial substitution of lanthanum on the barium site^7–12. In the present neutron powder diffraction study of a series of La(Ba_2−xLa_x)Cu₃O_7+δ compounds, we show that the superconducting phase is a similarly disordered isomorph of the orthorhom-bic YBa₂Cu₃O_7−δ structure^13–19. The region of solid solubility for this compound extends from x = 0.5 (La:Ba:Cu ratios of 3:3:6) to x≈ 0.25 and does not include the stoichiometric compound x = 0 (LaBa₂Cu₃O_7−δ). The presence of Cu–O chains, and not just a formal copper charge state of ≥2, is vital for the occurrence of superconductivity.