Journal of Solid State Chemistry 142, 440 — 450 (1999) Article ID jssc.1998.8064, available online at http://www.idealibrary.com on Anion Ordering in Fluorinated La 2 CuO 4 A. M. Abakumov,*- J. Hadermann,* G. Van Tendeloo,* R. V. Shpanchenko,- P. N. Oleinikov,- and E. V. Antipov- *EMAT, University of Antwerp (RUCA), Groenenborgerlaan 171, B-2020 Antwerp, Belgium; -Department of Chemistry, Moscow State University, 119899 Moscow, Russia Received May 27, 1998; in revised form October 11, 1998; accepted October 12, 1998 La2CuO4 has been fluorinated using XeF2 as a soft fluorinating agent at temperatures ranging from 200 to 400°C. The structure of the fluorinated samples was studied by a combination of X-ray diffraction (XRD), electron diffraction (ED), high-resolution electron microscopy (HREM), and microanalysis. XRD shows the sequential changes with increasing temperature of the XeF2 treatment. A significant structural rearrangement, associated with the formation of a tetragonal K2NiF4-type structure (I- centered lattice with cell parameters a  4.0383(6) A s , c  13.093(3) A s ) occurs at 250°C. Samples annealed at 300 and 400°C reveal the formation of a new monoclinic phase (a+17.36 A s , b+5.62 A s , c+10.59 A s , +91.5°; SG, C2/m). The structural model, deduced from ED and HREM, involves the filling of part of the interstitial positions by fluorine atoms with removal of the neighboring oxygen, resulting in the forma- tion of a fluorite-like arrangement of the La(O, F) layers. The fluorite-type and NaCl-type slabs alternate along the [110] direction of the K2NiF4 subcell in an ordered array. Several crystallites also show an incommensurate superstructure which is explained as a pseudoperiodic arrangement of shear planes.  1999 Academic Press INTRODUCTION Stoichiometric La  CuO  exhibits a semiconducting be- havior. The hole concentration, which is necessary for the appearance of superconductivity, may be achieved either by heterovalent cationic substitution on the A-sublattice (1) or by the insertion of additional anions into the interstitial positions between the double LaO layers using a large variety of different techniques: high oxygen pressure anneal- ing (2, 3), electrochemical oxidation, oxidation by a KMnO  solution (4, 5), and fluorination (6—8). La  CuO  treated by diluted or pure F  gas reveals a superconducting transition with ¹  +35—40 K (6). The formation of a super- conducting oxyfluoride was observed only for a low fluor- ination level, which is associated with a low temperature of the fluorine treatment (150—200°C). Earlier investigations were mainly devoted to the structural aspects of fluorine accommodation for this superconducting oxyfluoride. It was found that incorporation of a small amount of fluorine into the La  CuO  structure (up to a La  Cu(O, F)  com- position) is accompanied by an enhancement of the orthor- hombic distortion and an increase of the c parameter of the unit cell (7). Neutron powder structure refinement as well as EXAFS studies revealed that no significant structural trans- formation occurs during the fluorination; fluorine atoms are located at interstitial positions, tetrahedrally coordinated by La atoms and placed between two successive LaO layers (8). This behavior is similar to that previously found for oxygen-doped La  CuO  samples (9). An increase of the fluorination temperature leads to a structural rearrangement. The formation of a new non- superconducting tetragonal phase with cell parameters a"5.7(1) A s and c"13.1(1) A s was observed by X-ray dif- fraction when La  CuO  was treated by F  at 230°C (10). Electron diffraction revealed the presence of a superstruc- ture indexed on the basis of a monoclinic unit cell (11). An incommensurate superstructure connected with the forma- tion of shear planes was also found in limited regions of the sample. The origin of this ordering was not determined, however, and the structure of the compound formed at a high level of fluorination is still unclear. Recently we showed that XeF  is a soft and effective fluorinating agent and may be applied for the preparation of different superconducting oxyfluorides (12—15). In this pa- per we report the results of the fluorination of La  CuO  by XeF  at different synthesis conditions and the structural characterization of the materials by a combination of X-ray diffraction, electron diffraction, and high resolution electron microscopy. EXPERIMENTAL La  CuO  for further fluorination was prepared by a rou- tine ceramic technique using La  O  and CuO as initial reagents. Stoichiometric amounts of these oxides were inti- mately mixed, pressed into pellets, and heated in air at 440 0022-4596/99 $30.00 Copyright  1999 by Academic Press All rights of reproduction in any form reserved.