The Incommensurate Modulation of the 2212 Bi-Sr-Ca-Cu-O Superconductor

The incommensurate modulation evident in the diffraction pattern of the superconductor Bi2Sr3-xCaxCu2O8+y consists of almost sinusoidally varying displacements of up to 0.4� of the Bi and Sr atoms in the a-and c-directions of the unit cell, and of up to 0.3 � of the Cu atoms in the c direction only. Thus, a newly discovered feature of the Bi2Sr3-xCaxCu2O8+y structure is sizable Cu displacement, which is related to static wave formation in the Cu-O sheets. Reported thermal parameters give evidence that similar distortions occur on cooling of the thallium-containing superconductors.