Structure and refinement of the composition of Bi-Sr-Ca-Cu oxide crystals

The composite structure of the red phase, which accompanies the 2212 and 2201 superconducting phases prepared upon cooling of a melt in the Bi-Sr-Ca-Cu-O system, is revealed by scanning electron microscopy and X-ray diffraction analysis. The matrix component in two crystals I and II under investigation has a monoclinic structure with space group C2/m and the unit cell parameters a = 21.832(2) and 21.8142(9) Å, b = 4.3809(4) and 4.3771(2) Å, c = 12.9430(12) and 12.9378(6) Å, β = 102.832(2)° and 102.839(1)°, and V = 1207.0(2) and 1204.45(9) ų, respectively. The composition is described by the general formula Bi₄(Ca_2?x Sr_1.5+x )O_9.5 with x = 0.13(2) for crystal I and x = 0.19(2) for crystal II. The matrix contains intergrowing clusters of the composition Cu₂O. It is demonstrated that the presence of the second component is structurally justified by the commensurability of the systems of body-centered subcells of cations in the host matrix and anion sublattices in the structure of the Cu₂O oxide.