Study on the site preference of Ca in superconducting oxides Bi2Sr2 − xCaxCuO6 + δ (0.1 ≤ x ≤ 1.0)

A series of compound with the nominal composition of Bi2Sr2 − xCaxCuO6 + δ (x = 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0) were synthesized by the sol-gel method. Constituent phases and crystal structure of samples were analyzed by X-ray diffraction. It can be found that the Ca-doped Bi-2201 system was composed of Bi-2201 phase containing Ca and a small quantity of Bi16(Sr,Ca)14O38. For Bi-2201 unit cell containing Ca, chemical component and site preference of Ca atoms were characterized systematically by transmission electron microscopy. With the introduction of Ca atoms, Sr-sites have been occupied partially by Ca2 + in Bi-2201 unit cell, which leads to a decrease in the lattice parameters c and b of the Bi-2201 phase when the Ca-content x is below 0.6. Two types of new orthorhombic lattices are formed in the substitution. One is a lattice with space group Pma2 as the two nearest neighbor Sr-sites in the same Sr-O layer are occupied by Ca2 +. Its lattice parameters can be characterized as a = 5.402 Å, b = 5.313 Å and c = 24.272 Å, respectively. When two nearest Sr ions of the second neighboring Sr-O layers are replaced by Ca2 + ions, the lattice with the space group Pmn21 can be formed. Its lattice parameters are close to that of the previous. The modulation vector is lying in the a*-c* plane in the two new orthorhombic lattices (Pma2 and Pmn21). Bi/Ca-2201 lattice (with Ca) and Bi-2201 lattice (without Ca) coexist in the same Bi2Sr2 − xCaxCuO6 + δ grain, which can be described as an intergrowth structure.