Low‐Temperature Synthesis, Characterization, and Stability of Spinel‐Type Li₂NiF₄ and Solid‐Solutions Li₂Ni_1–xCo_xF₄

Abstract

A new synthesis route to Li2NiF4 based on a fluorolytic process using Ni(acac)2 or Ni(OAc)2·4H2O as precursor is presented. Variation of the synthesis conditions allows crystallite size control of the obtained powders. 6Li and 7Li MAS NMR experiments were carried out to study local environments of the lithium ions. Several attempts were made to synthesize Li2CoF4, which are, unfortunately, hitherto not successful. Nevertheless, our studies clearly reveal that solid solutions Li2NiF4–Li2CoF4 are stable up to ca. 30 % cobalt. High‐temperature X‐ray diffraction measurements also show no evidence for the existence of pure Li2CoF4. These findings are supported by quantum chemical calculations.

Ultraslow Li Exchange Processes in Diamagnetic Li2ZrO3 As Monitored by EXSY NMR

“…resulting in (i) short NMR spin–lattice relaxation (SLR) rates ranging from milliseconds to some seconds ,, and (ii) large, site-specific NMR paramagnetic shifts. ,, While the first circumstance drastically reduces the measurement time but limits the application of time-domain NMR techniques to study lithium-ion dynamics, the second fact makes it possible to resolve the distinct Li sites present in those materials, which is a prerequisite for the application of 2D MAS NMR. However, even under fast sample rotation, in some cases Fermi-contact interactions may become very large leading to extremely broad and unresolved NMR lines, which can make the interpretation of 6 Li NMR spectra of paramagnetic compounds extremely difficult . In particular, this becomes a challenging task in complex materials with a large number of magnetically inequivalent Li sites.…”

Ternary transition-metal fluoride precursors for the fluorolytic sol–gel route: new insights into speciation and decomposition

“…7 Absolute pyridine (5 mL) was added to the Li 2 NiF 4 precursor (500-600 mg). A Li 2 NiF 4 precursor was prepared from [Ni(acac) 2 ] in ethanol according to a standard procedure.…”

“…A Li 2 NiF 4 precursor was prepared from [Ni(acac) 2 ] in ethanol according to a standard procedure. 7 Absolute pyridine (5 mL) was added to the Li 2 NiF 4 precursor (500-600 mg). The mixture was stirred overnight at 50 °C.…”

“…7 The precursors contain highly amorphous fractions, making it impossible to get information about their composition by X-ray powder diffraction techniques. Using this protocol, either modification of Li 3 VF 6 can be prepared in excellent yield: heating of the sample to 300°C followed by slow cooling to room temperature results in the formation of monoclinic β-Li 3 VF 6 , whereas heating the precursor to 700°C and quenching to room temperature results in the formation of orthorhombic α-Li 3 VF 6 .…”

“…In contrast to this, the method can be successfully applied to the system Li-Ni-F: Li 2 NiF 4 is prepared in high yields with domain sizes of 55 nm. 7 The precursors contain highly amorphous fractions, making it impossible to get information about their composition by X-ray powder diffraction techniques. Only the existence of LiF and Li 2 SiF 6 in the precursor materials has been confirmed so far.…”