Topochemical Fluorination of La₂NiO_4+d: Unprecedented Ordering of Oxide and Fluoride Ions in La₂NiO₃F₂

Abstract

The Ruddlesden-Popper (KNiF) type phase LaNiOF was prepared via a polymer-based fluorination of LaNiO. The compound was found to crystallize in the orthorhombic space group Cccm ( a = 12.8350(4) Å, b = 5.7935(2) Å, c = 5.4864(2) Å). This structural distortion results from an ordered half occupation of the interstitial anion layers and has not been observed previously for KNiF-type oxyfluoride compounds. From a combination of neutron and X-ray powder diffraction and F magic-angle spinning NMR spectroscopy, it was found that the fluoride ions are only located on the apical anion sites, whereas the oxide ions are located on the interstitial sites. This ordering results in a weakening of the magnetic Ni-F-F-Ni superexchange interactions between the perovskite layers and a reduction of the antiferromagnetic ordering temperature to 49 K. Below 30 K, a small ferromagnetic component was found, which may be the result of a magnetic canting within the antiferromagnetic arrangement and will be the subject of a future low-temperature neutron diffraction study. Additionally, density functional theory-based calculations were performed to further investigate different anion ordering scenarios.

High cycle life all-solid-state fluoride ion battery with La2NiO4+d high voltage cathode

“…Owing to the fact that no superstructure reflections could be observed, all orthorhombic phases were refined using the highest symmetry orthorhombic space group of Fmmm, which can be derived from the aristotype I4/mmm symmetry for a √2 × √2 × 1 supercell. Such an orthorhombic distortion was also found previously in La 2 NiO 3 F 2 , which was prepared via a nonoxidative fluorination of La 2 NiO 4+d using polyvinylidene fluoride 41 and resulted from ordering of vacancies, unresolvable within the composite mixtures used here. The orthorhombic distortion of this phase increases on further charging to 30 mAh/g (see Fig.…”

Layered electrides as fluoride intercalation anodes

“…We note that while Ca2NCl and Ca2NBr are experimentally reported to exist in the 3 ̅ phase, 34,35 Ca2NF has only been made in the I41/amd phase, 36 which we find is 5 meV/atom higher in energy than the 3 ̅ phase. Therefore, we expect that low-temperature fluorination, which has emerged as a successful strategy to produce fluoride structures topochemically, 37,38 should yield 3 ̅ Ca2NF from 3 ̅ Ca2N.…”

Anion Doping of Ferromagnetic Thin Films of La0.74Sr0.26MnO3−δ via Topochemical Fluorination

“…In this regard, F − doping may trigger antiferromagnetic coupling or extinguish any exchange interaction between adjacent Mn ions. This observation is in principle agreement with the recently reported weakening of Fe-F-Fe superexchange interactions as compared to Fe-O-Fe for the series LaFe 3+ O 3 F 0 (T N ~750 K), (Sr/Ba/Pb) Fe 3+ O 2 F 1 (T N = 650–700 K) and AgFe 3+ O 1 F 2 (T N ~500 K) [40], as well as the weakening of Ni-F-F-Ni superexchange interactions in La 2 Ni 2+ O 3 F 2 (T N = 49 K) as compared to Ni-O-O-Ni in La 2 NiO 4 (T N = 330 K) [14]. …”

“…Low-temperature topotactic fluorination, implying the incorporation of monovalent F − anions, was found to be [14,15,16,17] a powerful tool to change material properties of transition metals with perovskite (or perovskite-related) crystal structure by turning them into oxyfluorides. As the synthesized oxyfluorides systems are mostly metastable, low temperature topotactic fluorination routes are often required for the synthesis [18].…”