The structures of ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$Ca${\mathrm{Cu}}_2$ ${\mathrm{O}}_8$ and ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{Ca}}_2$ ${\mathrm{Cu}}_3$${\mathrm{O}}_{10}$ were refined from neutron-powder-diffraction data. The data for ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$Ca${\mathrm{Cu}}_2$ ${\mathrm{O}}_8$ were taken at 298 K, and the data for ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{Ca}}_2$ ${\mathrm{Cu}}_3$${\mathrm{O}}_{10}$ were obtained at both 150 and 13 K. The results are essentially in agreement with the structural refinements based on single-crystal x-ray-diffraction data and confirm the positional disorder of the oxygen atoms in the Tl-O planes. The low-temperature refinements for ${\mathrm{Tl}}_2$${\mathrm{Ba}}_2$${\mathrm{Ca}}_2$ ${\mathrm{Cu}}_3$${\mathrm{O}}_{10}$ indicate that the symmetry remains tetragonal down to 13 K and that there is no significant structural change or discontinuity in the cell parameters through the superconducting critical temperature near 125 K.

• Received 28 April 1988

DOI:https://doi.org/10.1103/PhysRevB.38.6624