NEUTRON-DIFFRACTION STUDY OF THE MODULATED STRUCTURE OF BI2(SR, CA)3CU2O8+Y

The modulated structure of the 80 K superconductor Bi2(Sr, Ca)3Cu2O8+y is refined from the single-crystal neutron diffraction data on a monodomain block of dimensions 1.5 × 1.0 × 0.03 mm3. The modulation is recognized as commensurate with period 4.75a, resulting in a supercell 19a × b × c (a = 5.397 (1), b = 5.401 (1), c = 30.716 (3) A ̊) and symmetry Pnaa. The essential feature of the structure is the periodic insertion of additional oxygen rows into BiO planes, which in turn causes large displacive modulation in all layers. The ideal oxygen stoichiometry is y = 4 19 {reversed tilde} 0.21. Bonds in the BiO and SrO planes form characteristic chains running along the a-axis. The coordination of the Bi3+ cations is uniform and consists of three mutually nearly perpendicular bonds to oxygens of 2.0-2.2 Å lengths. The Sr2+ cations are bonded only to eight oxygen atoms at the most. About 7% of the Sr sites, located in the vicinity of the inserted oxygen, are possibly vacant. The Ca site situated between the CuO2 planes is substituted to 30% by strontium. © 1990.