Superlattice Structure of Bi2Sr2GdCu2O8+y

An approximate superstructure of Bi2Sr2GdCu2O8+y was determined by single-crystal X-ray diffraction. The crystal is isomorphic with those of both Bi2Sr2YCu2O8+y and superconducting Bi2Sr2CaCu2O8+y ; the structure is modulated along the fundamental b 0 axis. The true modulation period is 8b 0, but can be regarded as 4b 0 for a good approximation. The approximate structure is orthorhombic, space group I m c b; a=5.4514(9), b=4b 0=21.948(3) and c=30.221(4) Å; Z=16. The final R value was 0.041 for 1924 unique reflections collected up to 2θ=70° with AgK α radiation. The modulation is displacive and sinusoidal in the (100) plane.