Journal of Solid State Chemistry 156, 445 } 451 (2001) doi:10.1006/jssc.2000.9020, available online at http://www.idealibrary.com on Suppression of Modulations in Fluorinated Bi-2201 Phases J. Hadermann, N. R. Khasanova, and G. Van Tendeloo EMAT, University of Antwerp (RUCA), Groenenborgerlaan 171, B-2020 Antwerp, Belgium and A. M. Abakumov, M. G. Rozova, A. M. Alekseeva, and E. V. Antipov Department of Chemistry, Moscow State University, 119899 Moscow, Russia Received August 16, 2000; in revised form October 12, 2000; accepted October 27, 2000 Bi 2 Sr 1.6 La 0.4 CuO 6.33 and Bi 2 Sr 1.2 La 0.8 CuO 6.46 are 6uorinated using XeF 2 as a 6uorinating agent and studied by X-ray powder di4raction and high-resolution electron microscopy. The highly 6uorinated Bi-2201 phase undergoes a structural transformation that is accompanied by the formation of an I-centered tetragonal unit cell, the suppression of the 2a p 2a p superstructure, the disappearance of the (BiO) 2 modulation, and an elongation of the c parameter of about 1.65+1.77 A  . The modi5cation of the Bi 2 O 2 block into a Bi 2 O 2 F 2 block with 5lled tetrahedral interstitial positions is responsible for the observed structural changes and eliminates the mismatch between the (BiO) rock salt layers and the perovskite (CuO 2 ) layers. The structure of the 6uorinated material is compared with the structures of other intercalated Bi-based cuprates and phases composing Bi 2 (O,F) 4 blocks in the structure.  2001 Academic Press Key Words: 6uorination; Bi-2201; Bi 2 Sr 1.6 La 0.4 CuO 6.33x F 2x ; Bi 2 Sr 1.2 La 0.8 CuO 6.46 ; Bi 2 Sr 1.6 La 0.4 CuO 6.33 . 1. INTRODUCTION Among the di!erent layered cuprates the bismuth cuprates, Bi  Sr  Ca  Cu  O  , exhibit a particular behavior. These oxides, which consist of an intergrowth structure, di!er from the corresponding thallium bilayer cuprates by the presence of a strong modulation. The in- commensurate modulation is believed to be caused by a mismatch between the (BiO) rock salt type layer and the perovskite (CuO  ) layers. The mismatch is reduced by the insertion of extra oxygen in the (BiO) layer and by the cooperative displacements of the Bi and O atoms from their ideal positions within this plane which enables the appropriate Bi coordination to occur (1, 2). However, this rearrangement of the Bi  O  block a!ects the rest of the structure through the corrugation of the adjacent layers and leads to the appearance of strong satellite re#ections observed by di!erent di!raction methods. The inserted oxygen provides holes to the conduction band contributing thereby to the superconductivity. For compounds of a given cation composition the amount of extra oxygen, the  value, can be varied in a limited range depending on the synthesis conditions (3, 4). These parameters appear to be crucial for the phase stability and for the super- conducting properties of the Bi-2201 phase. Monophasic samples with a partial substitution of Bi for Sr, Bi  Sr  CuO  are not superconducting, but supercon- ductivity can be induced by rare earth doping on the Sr site. The parabolic-like compositional dependence of ¹  with a maximum of 33 K at x"0.4 was observed in a Bi  Sr  La  CuO  solid solution (4). On further La doping ¹  decreases and for Bi  Sr  La  CuO  the superconductivity disappears. The suppression of supercon- ductivity at a higher La content was assigned to the reduc- tion of the Cu oxidation state upon heterovalent substitution, and attempts to oxidize further by oxygen treatments were unsuccessful. Fluorination was successfully explored for oxidizing di!erent cuprates, and several new superconducting com- pounds were prepared (5, 6). In contrast to the anion exchange of F for O, which reduces the copper oxida- tion state, #uorine intercalation on the interstitial site leads to the oxidation of the conducting (CuO  ) planes and, in several cases, to structural transformations (5, 6, 17). It is of interest to examine the possibility of further oxidation of the Bi-based cuprates by #uorination and to study its e!ect on ¹  . Considering the structural simplicity of the Bi-2201 phase and the strong e!ect of slight hole doping on its properties (since all holes correspond to one (CuO  ) plane) the #uorination of the Bi-2201 phase was attempted, and two compounds, the superconducting Bi  Sr  La  CuO  and the non superconducting Bi  Sr  La  CuO  , were selected for this purpose. 445 0022-4596/01 $35.00 Copyright  2001 by Academic Press All rights of reproduction in any form reserved.