Structure, Magnetism, and Thermal Stability of LaNiOF: A Ruddlesden-Popper Oxyfluoride Crystallizing in Space Group 4/.

We report on the new Ruddlesden-Popper (RP) oxyfluoride LaNiOF containing an unprecedented high amount of fluorine and Ni. This oxyfluoride was prepared by topochemical low-temperature fluorination of LaNiO, which was obtained by a soft chemistry synthesis, with poly(vinylidene difluoride) (PVDF) as fluorinating agent. LaNiOF is the first = 1 RP compound crystallizing in the tetragonal space group 4/ ( = 5.7297(6) Å and = 13.0106(2) Å). The crystal structure shows a unique tilting scheme of the NiOF 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 NiOF 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 NiOF 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 was found compared to LaNiO (3.4 eV vs 1.3 eV).