ANION ORDERING IN FLUORINATED La2CuO4

摘要

La2CuO4 has been fluorinated using XeF2 as a soft fluorinating agent at 200-400 C. The structure of the fluorinated samples was studied by a combination of XRD, electron diffraction (ED), 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) angstroms, c=13.093(3) angstroms) occurs at 250 C. Samples annealed at 300 and 400 C reveal the formation of a new monoclinic phase (a approx equal to 17.36 angstroms, b approx equal to 5.62 angstroms, c approx equal to 10.59 angstroms, beta approx equal 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 F atoms with removal of the neighboring O, 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. 19 refs.