Ruddlesden-Popper Oxyfluorides La 2 Ni 1- x Cu x O 3 F 2 (0 ≤ x ≤ 1): Impact of the Ni/Cu Ratio on the Structure.

Ruddlesden-Popper oxyfluorides La 2 Ni 1- x Cu x O 3 F 2 (0 ≤ x ≤ 1) were obtained by topochemical reaction of oxide precursors La 2 Ni 1- x Cu x O 4 , 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 ( C 2/ 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 La 2 NiO 3 F 2 . The orthorhombic-to-monoclinic structural transition was found to be the result of an additional tilt component of the Jahn-Teller elongated CuO 4 F 2 octahedra. The structural transitions were additionally studied by DFT calculations, confirming the monoclinic space group symmetry. The "channel-like" anionic ordering of the endmembers La 2 NiO 3 F 2 and La 2 CuO 3 F 2 was checked by 19 F MAS NMR experiments and was found to persist throughout the entire substitution series.