Ruddlesden–Popper Oxyfluorides La₂Ni_1–xCu_xO₃F₂ (0 ≤ x ≤ 1): Impact of the Ni/Cu Ratio on the Structure

Abstract

≤ 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 (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 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.

Ruddlesden–Popper Oxyfluorides La2Ni1–xCuxO3F2 (0 ≤ x ≤ 1): Impact of the Ni/Cu Ratio on the Thermal Stability and Magnetic Properties

“…By incorporation of Cu 2+ at Ni 2+ positions, AFM super exchange interactions are expected between Cu 2+ centers, and additional weak ferromagnetic interactions of half occupied Ni-d-orbitals and occupied Cu-d-orbitals are expected based on the GKA rules. An additional Jahn–Teller elongation of the Cu-containing octahedra was derived from our previous structural investigations . This elongation was additionally found to yield different octahedral tilting components depending on x.…”

“…The substitution of different transition metal cations within the oxyfluoride structure can have a strong impact on the local arrangement of oxygen and fluorine ions, influencing the structural distortions and stability, as well as the physical properties. In a previous article, we discussed how the replacement of Ni with Cu in La 2 Ni 1– x Cu x O 3 F 2 affects the crystal structure, and we found that the anionic ordering observed for La 2 NiO 3 F 2 persists throughout the substitution series. On the other hand, incorporation of the Jahn–Teller active Cu 2+ results in a symmetry lowering from orthorhombic ( Cccm ; x ≤ 0.1) to monoclinic ( C 2 /c ; 0.2 ≤ x ≤ 0.9) and finally to triclinic ( P ; x = 1.0) symmetry.…”

“…The oxyfluorides La 2 Ni 1– x Cu x O 3 F 2 (in steps of x = 0.1) were synthesized as reported before . Fluorination was achieved by mixing the precursor oxides with polyvinylidene fluoride ((PVDF/CH 2 CF 2 ) n ) (Alfa Aesar) in a molar ratio of 1:1.05 (oxide: CH 2 CF 2 ).…”