STRUCTURAL MODULATION IN BI₂SR₂CA_0.4Y_0.6CU₂O_8+d

A.I. Beskrovnyi¹ and Z.Jirák²

¹Joint Institute for Nuclear Research, 141980 Dubna, Moscow Region, Russia
²Institute of Physics of ASCR, Cukrovarnická 10, 16200 Prague 6, Czech Republic

Characteristic feature of the Bi based layered cuprates is a structural modulation with period which varies between 4a and 5a (a~5.4 A) depending on the actual composition. It appears that the modulation arises due to incorporation of an extra oxygen into the BiO plane. These additional atoms help to release an interlayer stress and enable a better bonding of the Bi³⁺ cations. As shown for the superconducting cuprates Bi₂Sr_3-xCa_xCu₂O_8.2 and Bi₂Sr_2-xCa_xCuO_6.2 and some other related compounds, the bismuth-oxygen bonds form Bi-O-Bi- chains (or ribbons) which run along the a-direction.

The present neutron diffraction study has been performed on an yttrium doped compound Bi₂Sr₂Ca_0.4Y_0.6Cu₂O_8+d. The experiment revealed a modulation with the 4.5a period within which two extra oxygen atoms are added into the BiO plane. The overall oxygen stoichiometry thus achieves to d~0.44. In addition to the basic 4.5a modulation we have observed diffuse diffraction spots corresponding to a doubled period of 9.0a. The detailed analysis of the diffraction picture enabled us to conclude that in Bi₂Sr₂Ca_0.4Y_0.6Cu₂O_8+d the Bi-O-Bi- chains are segmented to length of 4.5a and these segments alternate between b=0 and 1/2. In this way, instead of infinite ribbons a tweed arrangement of Bi₉O₁₁ chains is formed in the BiO planes.