Vacancy distribution and ionic motion in LaF3 studied by 19F NMR

Synthesis of LaF3 nanosheets with high fluorine mobility investigated by NMR relaxometry and diffusometry

Ionically conducting lanthanum fluoride (LaF3), displaying a nanoscopic lamellar structure, has been synthesized at the surface of an aqueous solution of LaCl3 and HF. The structure and the chemical composition of the conductor have been analyzed by SEM, electron probe microanalysis, X-ray powder diffraction, FTIR, and (19)F magic angle spinning nuclear magnetic resonance (NMR) spectroscopy. The fluorine dynamics have been studied by NMR diffusometry and relaxometry in a temperature range from room temperature up to 875 K. The fluorine self-diffusion coefficient of the nanostructured LaF3 is about two orders of magnitude larger than that of bulk LaF3. This novel material is highly promising for many typical applications of fluorine ionic systems.

Fluoride solid electrolytes: investigation of the tysonite-type solid solutions La1−xBaxF3−x (x < 0.15)

La_1−xBa_xF_3−x solid solutions for x < 0.15 were prepared by solid state synthesis in a platinum tube under an nitrogen atmosphere with subsequent quenching for 0.07 ≤ x < 0.15.

Simultaneous effects of relaxation and polarization transfer in LaF3-type crystals as sources of dynamic information

Fluorine nuclear magnetic resonance (NMR) spin-lattice relaxation dispersion has been measured for pure LaF(3) and La(1-x)Sr(x)F(3-x) for admixture concentrations x ranging from 0.01% up to 16%. The relaxation dispersion experiments have been carried out in a wide frequency range (20 kHz-40 MHz) at temperatures between 300 and 1400 K. The data have been analyzed using the recently published [J. Magn. Res. 179 (2006) 250] relaxation model for multispin systems of mutually interacting quadrupolar and dipolar nuclei. Rate constants of the fluorine ionic jumps within and among distinct fluorine sublattices have been extracted. Characteristic effects of the polarization transfer between fluorine and lanthanum spins have been observed and attributed to slow dynamics within one of the fluorine sublattices.

Multinuclear Solid-State NMR Spectroscopy of Doped Lanthanum Fluoride Nanoparticles

Multinuclear solid-state NMR spectroscopy and powder X-ray diffraction (XRD) experiments are applied to comprehensively characterize a series of pure and lanthanide-doped LaF3 nanoparticles (NPs) that are capped with di-n-octadectyldithiophosphate ligands (Ln3+ = diamagnetic Y3+ and Sc3+ and paramagnetic Yb3+ ions), as well as correlated bulk microcrystalline materials (LaF3, YF3, and ScF3). Solid-state 139La and 19F NMR spectroscopy of bulk LaF3 and the LaF3 NPs reveal that the inorganic core of the NP retains the LaF3 structure at the molecular level; however, inhomogeneous broadening of the NMR powder patterns arises from distributions of 139La and 19F NMR interactions, confirming a gradual change in the La and F site environments from the NP core to the surface. 139La and 19F NMR experiments also indicate that low levels (5 and 10 mol %) of Ln3+ doping do not significantly change the LaF3 structure in the NP core. Similar doping levels of paramagnetic Yb3+ ions severely broaden 19F resonances, but only marginally effect 139La powder patterns, suggesting that the dopant ions are uniformly distributed throughout the NP core and occupy vacant La sites. Measurements of 139La T1 and T2 relaxation constants are seen to vary between the bulk material and NPs and between samples with diamagnetic and paramagnetic dopants. 45Sc NMR experiments confirm that the dopants are integrated into the La sites of the LaF3 core. Solid-state 1H and 31P magic-angle spinning (MAS) NMR spectra aid in probing the nature of the capping ligands and their interactions at the NP surface. 31P cross-polarization (CP)/MAS NMR experiments identify not only the dithiophosphate head groups but also thiophosphate and phosphate species which may form during NP synthesis. Finally, 19F-31P CP/MAS and 1H MAS experiments confirm that ligands are coordinated to the NP surface.

Field-dependent nuclear relaxation of spins 1/2 induced by dipole-dipole couplings to quadrupole spins: LaF3 crystals as an example

A general theory of spin-lattice nuclear relaxation of spins I=1/2 caused by dipole-dipole couplings to quadrupole spins S1, characterized by a non-zero averaged (static) quadrupole coupling, is presented. In multispin systems containing quadrupolar and dipolar nuclei, transitions of spins 1/2 leading to their relaxation are associated through dipole-dipole couplings with certain transitions of quadrupole spins. The averaged quadrupole coupling attributes to the energy level structure of the quadrupole spin and influences in this manner relaxation processes of the spin 1/2. Typically, quadrupole spins exhibit also a complex multiexponential relaxation sensed by the dipolar spin as an additional modulation of the mutual dipole-dipole coupling. The proposed model includes both effects and is valid for an arbitrary magnetic field and an arbitrary quadrupole spin quantum number. The theory is applied to interpret fluorine relaxation profiles in LaF3 ionic crystals. The obtained results are compared with predictions of the 'classical' Solomon relaxation theory.