THE STRUCTURE MODULATION OF Bi-BASED HIGH-T^_C SUPERCONDUCTORS. A.A.Levin, Yu.F.Shepelev, Yu.I.Smolin, Institute of Silicate Chemistry, Russ. Ac. Sci., St.-Petersburg, Russia

The modulated structures of high-T_C superconductors Bi₄Sr₄CaCu₃O_14+x (Bi-4413, orthorhombic cell, a= 5.411(2), b= 5.417(3), c= 27.75(1) A, q= 0.213(4)b*, T_C^end = 84 K) and Bi₂Sr₂CaCu₂O_8+x (Bi2212, orthorhombic cell, a= 5.407(3), b= 5.412(3), c= 30.771(8) A, q= 0.210(2)b*, T_C^end = 75 K) are determined in both commensurate and incommensurate approximations utilizing single-crystal X-Ray diffraction data.

The modulation displacement functions of all atoms in the stuctures are obtained. It has been found that the oxygen atoms of BiO layers in both compounds have greatest modulation shifts along the b-axis (up to 1.20 A in Bi-2212 and up to 1.40 A in Bi-4413) and their modulation functions in this direction can be described by linear saw-like functitions. It is shown that the extra-oxygen atoms can be inserted in the BiO layers of the structures.

The modulation of the structures, the topology of the BiO layers and the form of the modulation displacement functions are explained by the models of shifts of the oxygen atoms in the BiO layers correlated according to antiferroelectrical law. The difference of modulation models of the Bi-2212 and Bi-4413 structures are discussed. The marked peculiarity of both structures is a rather great concentration of the vacancies in Sr positions adjacent of copper-oxygen layers. It is suggested that the cationic vacancies in the Sr sites provide the hole conductivity in the copper-oxygen layers and lead to the formation of a percolation network in the superconducting crystals.

The work of one of the authors (A.A.L.) is partly supported in 1994/1995 year by ACA/USNCCr Award for FSU Crystallographers.