Anion ordering in fluorinated La2CuO4

La2CuO4 has been fluorinated using XeF2 as a soft fluorinating agent at tem peratures ranging from 200 to 400 degrees C, The structure of the fluorinat ed samples was studied by a combination of X-ray diffraction (XRD), electro n diffraction (ED), high-resolution electron microscopy (HREM), and microan alysis. XRD shows the sequential changes with increasing temperature of the XeF2 treatment. A significant structural rearrangement, associated with th e 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 degree s C. Samples annealed at 300 and 400 degrees C reveal the formation of a ne w 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 f illing of part of the interstitial positions by fluorine atoms with removal of the neighboring oxygen, resulting in the formation of a fluorite-like a rrangement of the La(O, F) layers. The fluorite-type and NaCl-type slabs al ternate 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 Academi c Press.