Suppression of modulations in fluorinated Bi-2201 phases

Bi2Sr1.6La0.4CuO6.33 and Bi2Sr1.2La0.8CuO6.46 are fluorinated using XeF2 as a ffuorinating agent and studied by X-ray powder diffraction and high-reso lution electron microscopy. The highly fluorinated Bi-2201 phase undergoes a structural transformation that is accompanied by the formation of an I-ce ntered tetragonal unit cell, the suppression of the root 2a(p) x root 2a(p) superstructure, the disappearance of the (BiO)(2) modulation, and an elong ation of the c parameter of about 1.65-1.77 Angstrom. The modification of t he Bi2O2 block into a Bi2O2-deltaF2+delta block with filled tetrahedral int erstitial positions is responsible for the observed structural changes and eliminates the mismatch between the (BiO) rock salt layers and the perovski te (CuO2) layers. The structure of the fluorinated material is compared wit h the structures of other intercalated Bi-based cuprates and phases composi ng Bi-2(O,F)(4) blocks in the structure. (C) 2001 Academic Press.