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, La1.85Sr1.15Cu2O6.25 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 Tc of 60 K in the similarly-doped calcium phase La1.6Sr0.4CaCu2O6.