Suppression of Modulations in Fluorinated Bi-2201 Phases

Bi2Sr1.6La0.4CuO6.33 and Bi2Sr1.2La0.8CuO6.46 are fluorinated using XeF2 as a fluorinating agent and studied by X-ray powder diffraction and high-resolution electron microscopy. The highly fluorinated Bi-2201 phase undergoes a structural transformation that is accompanied by the formation of an I-centered tetragonal unit cell, the suppression of the √2ap×√2ap superstructure, the disappearance of the (BiO)2 modulation, and an elongation of the c parameter of about 1.65–1.77 A. The modification of the Bi2O2 block into a Bi2O2−δF2+δ block with filled tetrahedral interstitial positions is responsible for the observed structural changes and eliminates the mismatch between the (BiO) rock salt layers and the perovskite (CuO2) layers. The structure of the fluorinated material is compared with the structures of other intercalated Bi-based cuprates and phases composing Bi2(O,F)4 blocks in the structure.