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Two powder samples of electrochemically oxidized La2Cu04+z (nominally 5=0.08 and 0.12) and one single crystal (5=0.1), with superconducting critical temperatures of 32, 42, and 40 K, respectively, were studied using neutron diffraction. All samples appear to be single phase, both at room temperature and at low temperature (10 -18 K), as evidenced by sharp Bragg peaks, indicating that these samples have compositions beyond the phase-separated region of the phase diagram. A detailed analysis of the Bragg rejections demonstrated that the basic crystallographic structure of all samples has Fmmm symmetry, with the excess oxygen located between adjacent LaO layers. However, a number of low-intensity peaks in the powder data suggested the existence of a very large superstructure. The satellites could be clearly identified in the single-crystal data, allowing the propagation vectors of the modulation to be determined. Rietveld refinements of the average structure, based on the main Bragg peaks, are presented here for samples prepared with this technique.

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