Ruddlesden-Popper Oxyfluorides La₂Ni_1-CuO₃F₂ (0 ≤ x ≤ 1): Impact of the Ni/Cu Ratio on the Structure

Ruddlesden-Popper oxyfluorides La₂Ni_1-CuO₃F₂ (0 ≤ x ≤ 1) were obtained by topochemical reaction of oxide precursors La₂Ni_1-CuO₄, prepared by citrate-based soft chemistry synthesis, with polyvinylidene fluoride (PVDF) as the fluorine source. Systematic changes of the crystal structure in the oxide as well as the oxyfluoride substitution series were investigated. For 0.2 ≤ x ≤ 0.9, the oxyfluorides adopt the monoclinic (C2/c) structural distortion previously solved for the x = 0.8 compound based on neutron powder diffraction data, whereas the sample with a lower Cu content of x = 0.1 crystallizes in the orthorhombic (Cccm) structure variant of La₂NiO₃F₂. The orthorhombic-to-monoclinic structural transition was found to be the result of an additional tilt component of the Jahn-Teller elongated CuO₄F₂ octahedra. The structural transitions were additionally studied by DFT calculations, confirming the monoclinic space group symmetry. The "channel-like" anionic ordering of the endmembers La₂NiO₃F₂ and La₂CuO₃F₂ was checked by ¹⁹F MAS NMR experiments and was found to persist throughout the entire substitution series.