Crystal Structure and Superconductivity of the Mo-Stabilized Sr-Based YSr₂Cu_2.7Mo_0.3O_7-δ Compound

The crystal structure of the Mo-stabilized Sr-based superconductor with nominal composition YSr₂Cu_2.7Mo_0.3O_7-δ has been analyzed by X-ray powder diffraction (XRD), electron diffraction, and high-resolution electron microscope techniques. The material crystallizes in the space group P4/mmm, with lattice parameters a = 3.8138(2) and c = 11.5204(8) Å. In the structure, the Mo ions are exclusively substituted for copper and are located mainly on the planar coordinated [Cu(2)] sites (∼24%), with only a small proportion on the chain [Cu(1)] sites (∼6%) in the parent YBa₂Cu₃O₇ (123-type) structure. A disordering of the hole reservoir layer [Mo(1)/Cu(1)]-O(3) due to the displacement of the oxygen ions [O(3)] from their ideal positions of (0, 0.5, 0) to (0.13, 0.5, 0) was found in the XRD refinement, but neither a superlattice nor any form of intergrowth along the a* and c* directions was observed by electron diffraction and high-resolution electron microscopy. The chemical substitution of copper by molybdenum renders the YSr₂Cu₃O₇ compound superconducting, with T_c up to 45 K.