Structure, Magnetism, and Thermal Stability of La₂NiO_2.5F₃: A Ruddlesden–Popper Oxyfluoride Crystallizing in Space Group P4₂/nnm

Abstract

We report on the new Ruddlesden–Popper (RP) oxyfluoride La2NiO2.5F3 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. La2NiO2.5F3 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).

Topochemical Fluorination of LaBaInO4 to LaBaInO3F2, Their Optical Characterization, and Photocatalytic Activities for Hydrogen Evolution

“…This can be explained by the larger ionic radii 42 of Ba 2+ than La 3+ and In 3+ , which increase the in-plane space requirements, resulting in a significant increase of the long crystallographic axis (+0.62 Å for LaBaInO 4 → LaBaInO 3 F 2 as compared to +0.17 Å for La 2 NiO 4 → La 2 NiO 3 F 2 45 ). Similar behavior was also observed recently by Jacobs et al 47 for the La 2 NiO 4 → La 2 NiO 3 F 2 → La2NiO 2.5 F 3 series, where the Ni−F ap distance decreases with increasing interstitial anion occupation.…”

“…LaBaInO 4 appears dark brown to the eye, whereas LaBaInO 3 F 2 powders possess a dull hue white represented in the Supporting Information (see SI, insets of Figure S9). In a recent study, Jacobs et al 47 reported on the oxyfluoride system prepared by topochemical low-temperature fluorination of La 2 NiO 4 with PVDF to La 2 NiO 3 F 2 and La 2 NiO 2.5 F 3 , which show a strong bandgap increase compared to La 2 NiO 4 (3.2, 3.4 vs 1.3 eV, respectively).…”

Cuprate Oxyfluorides La2Cu0.8Ni0.2O3F2 and La2CuO3F2 with “Channel-like” Anion Ordering

“…These results demonstrate that the B-oxidation state is not significantly changed by the fluorination, confirming the non-oxidative nature of the fluorination process with PVDF as fluorine source. The same was previously found for La 2 NiO 3 F 2 and La 2 NiO 2.5 F 3 . IR-spectra of the reaction products were recorded to check for any residual PVDF or other carbon-containing compounds.…”

“…The same was previously found for La 2 NiO 3 F 2 22 and La 2 NiO 2.5 F 3 . 26 IR-spectra of the reaction products were recorded to check for any residual PVDF or other carbon-containing compounds. The corresponding spectra are shown in Figure S1, and the absence of impurities indicates a complete reaction.…”

Ruddlesden–Popper Oxyfluorides La2Ni1–xCuxO3F2 (0 ≤ x ≤ 1): Impact of the Ni/Cu Ratio on the Structure

“…The use of more reactive precursor oxides obtained by the citrate-based route at lower calcination temperatures has been successful in the synthesis of La 2 CuO 3 F 2 as well as La 2 NiO 2.5 F 3 . Therefore, this approach was used for the precursor oxides in this work.…”