Magnetic correlations and their dependence on excess oxygen in La2NiO4+ delta

Structure, Magnetism, and Thermal Stability of La2NiO2.5F3: A Ruddlesden-Popper Oxyfluoride Crystallizing in Space Group P42/nnm

We report on the new Ruddlesden-Popper (RP) oxyfluoride La₂NiO_2.5F₃ containing an unprecedented high amount of fluorine and Ni²⁺. This oxyfluoride was prepared by topochemical low-temperature fluorination of La₂NiO₄, which was obtained by a soft chemistry synthesis, with poly(vinylidene difluoride) (PVDF) as fluorinating agent. La₂NiO_2.5F₃ is the first n = 1 RP compound crystallizing in the tetragonal space group P4₂/nnm (a = 5.7297(6) Å and c = 13.0106(2) Å). The crystal structure shows a unique tilting scheme of the NiO₄F₂ octahedra that has so far been only theoretically predicted. Combined neutron and X-ray powder diffraction experiments together with bond-valence-sum and DFT+U calculations reveal an unusual anion ordering with fluoride being located on the apical anion sites of the NiO₄F₂ octahedra. Excess fluorine ions were found to populate two of the four interstitial anion sites in an ordered fashion. A third interstitial anion position is occupied by oxygen ions while the fourth site remains unoccupied. This hitherto unobserved ordering scenario in RP oxyfluorides promotes a strong layerwise alternating tilting of the NiO₄F₂ octahedra. Magnetic measurements show strong antiferromagnetic interactions with a high Néel temperature of about 225 K and a pronounced ZFC/FC splitting most likely as the result of a small ferromagnetic moment arising from spin canting. The electronic structure was characterized by DFT and UV-vis spectroscopy, and a strong increase of E_g was found compared to La₂NiO₄ (3.4 eV vs 1.3 eV).