Cuprate Oxyfluorides La₂Cu_0.8Ni_0.2O₃F₂ and La₂CuO₃F₂ with "Channel-like" Anion Ordering

Highly fluorinated cuprate Ruddlesden-Popper oxyfluorides La₂Cu_0.8Ni_0.2O₃F₂ and La₂CuO₃F₂ were obtained by topochemical reaction between poly(vinylidene fluoride) (PVDF) and the corresponding oxides La₂Cu_0.8Ni_0.2O₄ and La₂CuO₄ prepared by citrate-based soft chemistry synthesis. The crystal structures of both oxyfluorides were investigated by powder diffraction techniques. The structure of La₂Cu_0.8Ni_0.2O₃F₂ was solved based on combined neutron and X-ray powder diffraction. It crystallizes in a new monoclinic distorted version [C2/c a = 13.1880(3) Å, b = 5.7244(1) Å, c = 5.6007(1) Å, and β = 90.85(1)°] of the anionic ordered structure lately reported for La₂NiO₃F₂. For La₂CuO₃F₂, an even less symmetrical triclinic structure was derived from X-ray powder diffraction data [P1̅ a = 5.6180(5) Å, b = 5.7316(6) Å, c = 7.1978(9) Å, α = 113.32(1)°, β = 90.89(9)°, and γ = 90.16(11)°]. For both compounds, an additional tilt component of the partially Jahn-Teller elongated (Cu,Ni)O₄F₂ octahedra was found as the origin for the lowered symmetry. The formation reaction of La₂CuO₃F₂ was studied by in situ XRD measurements. In these investigations, two new reaction intermediates were identified. The magnetic properties of both oxyfluorides La₂Cu_0.8Ni_0.2O₃F₂ and La₂CuO₃F₂ were characterized by field- and temperature-dependent measurements. An antiferromagnetic ordering with T_N = 240 K was found for La₂Cu_0.8Ni_0.2O₃F₂. In La₂CuO₃F₂, additional weak ferrimagnetism was observed, resulting in a pronounced hysteresis but a weak saturation moment, which was attributed to result from a canted antiferromagnetic spin arrangement.