Oxygen-defect structure of non-superconducting La_1.85Sr_1.15Cu₂O_6.25: excess oxygen in the interlayer site

The structure of the metallic, but nonsuperconducting, double-layer phase, La_1.85Sr_1.15Cu₂O_6.25 has been refined from neutron powder diffraction data; space group 14/mmm, a=3.8530(1) A ̊, c=20.0833 (3) A ̊. Sr²⁺ ions are ordered predominantly into the nine-coordinated site in the T-type layer of the structure. Occupation of the interlayer site by the relatively large La³⁺/Sr²⁺ ions allows intercalation of the excess oxygen at the (0, 0, case1 2) site between the CuO₂ layers. In addition, vacancies are introduced in the O(1) sites of the CuO₂ planes. This partial destruction of the two-dimensional nature of the structure may be responsible for the lack of superconductivity in this phase, in contrast to the recent observation of a T_c of 60 K in the similarly-doped calcium phase La_1.6Sr_0.4CaCu₂O₆.