Although it has been postulated on the basis of relative ionic sizes that the heavy cations in the newly discovered $T^{\mathrm{*}}$-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 $T^{\mathrm{*}}$-phase ${\mathrm{La}}_{0.9}$${\mathrm{Gd}}_{0.9}$${\mathrm{Sr}}_{0.2}$${\mathrm{CO}}_4$ at four x-ray wavelengths, including one each near the absorption edge of each heavy cation in order to obtain the contrast variation necessary to gain information about cation ordering. We find the smaller ${\mathrm{Gd}}^{3+}$ are located in the smaller T’-phase (${\mathrm{Nd}}_2$${\mathrm{CuO}}_4$) positions (M’ site) and (with less certainty) that ${\mathrm{Sr}}^{2+}$ are located in the larger T-phase (${\mathrm{La}}_2$${\mathrm{CuO}}_4$) positions (M site). The ${\mathrm{La}}^{3+}$ 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.

• Received 14 August 1989

DOI:https://doi.org/10.1103/PhysRevB.41.1889