ORDERING OF CA AND SR ATOMS IN (CA0.5SR0.5) (CU0.75BI0.25)O-2 SINGLE-CRYSTALS OF CACUO2-TYPE STRUCTURE

By X-ray single crystal analysis, the ordering of Ca and Sr atoms has been revealed in the compound (Ca0.5Sr0.5)(Cu0.75Bi0.25)O-2, which has a CaCuO2-type structure with lattice constants a = 5.465(1) and c = 3 .2143(6) Angstrom, space group P4/mmm. The atom positions and temperat ure parameters have been determined from 131 independent reflections a nd taking into account absorption and twinning effects, with a final R -factor value of 0.0376. The chemical and physical conditions necessar y to form the CaCuO2-type structure in both the ordered and disordered states have been considered. The distance between the CuO2 networks h as been chosen as a crystal-chemical parameter which characterizes the stability of this structure type. This distance, which is equivalent to the lattice constant c, depends on the pressure during crystal grow th, and is dictated by the size of the large cation. The CaCuO2-type s tructure is related to that of CuO type. Two layered elements have bee n distinguished in CaCuO2, which is the main parent structure for cupr ates containing large cations. These elements are bounded by planes on which the large cations lie and which are either normal or parallel t o the fourfold axis.