Anion ordering in fluorinated La2CuO4

La2CuO4 has been fluorinated using XeF2 as a soft fluorinating agent at temperatures ranging from 200 to 400 degrees 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, c = 13.093(3) Angstrom) occurs at 250 degrees C. Samples annealed at 300 and 400 degrees C reveal the formation of a new monoclinic phase (a approximate to 17.36 Angstrom, b approximate to 5.62 Angstrom, c approximate to 10.59 Angstrom, beta approximate to 91.5 degrees; 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 formation 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. (C) 1999 Academic Press.