Structure of T/emph>-phase superconductors: Anomalous x-ray-diffraction study of cation ordering in LA0.9Gd0.9Sr0.2CuO4

Although it has been postulated on the basis of relative ionic sizes that the heavy cations in the newly discovered T*-phase cuprate superconductors are ordered between the two available sites in the space group P4/nmm, there has been no direct experimental evidence that such ordering actually takes place. We have carried out a detailed x-ray-diffraction study of T*-phase La0.9Gd0.9Sr0.2CO4 at four x-ray wavelengths, including one each near the absorption edge of each heavy cation in order to obtain the contrast variation necessary to gain information about cation ordering. We find the smaller Gd3+ are located in the smaller T-phase (Nd2CuO4) positions (M site) and (with less certainty) that Sr2+ are located in the larger T-phase (La2CuO4) positions (M site). The La3+ is located mostly in the M site, with the remainder in the M site, to fill the occupancies in both sites. In addition, diffraction data taken at low temperature show the crystal symmetry to be the same at both room temperature and 19 K.