Superlattice Modulation in Nd sub 2 CuO sub 4 and the Electron-Doped Systems Nd sub (2-x) Ce sub x CuO sub 4 and Nd sub 2 CuO sub (4-x) F sub x

We have observed a periodic superlattice modulation in Nd sub 2 CuO sub 4 and in the electron doped Nd sub (2-x) Ce sub x CuO sub 4 and Nd sub 2 CuO sub (4-x) F sub x systems by electron diffraction and microscopy. The superlattice appears identical in all the systems studied; it was found in both superconducting and nonsuperconducting samples, but most of the samples examined were not homogeneous with respect to the superlattice. The superlattice modulation is characterized by a wave vector q = (1/4,1/4,0) parallel to the Cu-O plane; it exists in four possible variants with its c-axis parallel to the four equivalent [111] zone axes of the underlying tetragonal lattice.