Anion Ordering in Fluorinated La2CuO4

La 2 CuO 4 has been fluorinated using XeF 2 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 XeF 2 treatment. A significant structural rearrangement, associated with the formation of a tetragonal K 2 NiF 4 -type structure (I-centered lattice with cell parameters a =4.0383(6) Å, c =13.093(3) Å) occurs at 250°C. Samples annealed at 300 and 400°C reveal the formation of a new monoclinic phase ( a ≈17.36 Å, b ≈5.62 Å, c ≈10.59 Å, β ≈91.5°; SG, C 2/ 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 K 2 NiF 4 subcell in an ordered array. Several crystallites also show an incommensurate superstructure which is explained as a pseudoperiodic arrangement of shear planes.