NMR and impedance studies of nanocrystalline and amorphous ion conductors: lithium niobate as a model system

Abstract

Lithium niobate has been chosen as a model system for spectroscopic studies of the influence of different structural forms and preparation routes of an ionic conductor on its ion transport properties. The Li diffusivity in nanocrystalline LiNbO 3 , prepared either mechanically by high-energy ball-milling or chemically by a sol-gel route, was studied by means of impedance and solid state 7 Li NMR spectroscopy. The Li diffusivity turned out to be strongly correlated with the different grain boundary microstructures of the two nanocrystalline samples and with the degree of disorder introduced during preparation, as seen especially by HRTEM and EXAFS. Although in both samples nanostructuring yields an enhancement of the Li diffusivity compared to that in coarse grained LiNbO 3 , the Li diffusivity in ball-milled LiNbO 3 is much higher than in chemically prepared nanocrystalline LiNbO 3 . The former LiNbO 3 sample has a large volume fraction of highly disordered interfacial regions which seem to be responsible for fast Li diffusion and to have a structure being very similar to that of the amorphous form. This is in contrast to the chemically prepared sample where those regions have a smaller volume fraction.

F anion dynamics in cation-mixed nanocrystalline LaF3: SrF2

“…During ball milling the number density of defect sites usually increases which causes the bulk ionic conductivity also to increase as compared to the more ordered, single crystalline state. This effect has, in particular, been shown for oxides such as LiNbO 3 [16] and LiTaO 3 [17]. Recently, a quite similar enhancement has been reported for LiAlO 2 [18] and Li 2 TiO 3 [19].…”

“…The effect of abraded material from vial sets and milling balls has only a negligible effect if other factors, such as cation mixing, governs ion dynamics [14]. As has been documented for other nanocrystalline systems, the main change in ionic conductivity is caused by structural disorder, lattice strain introduced and the mixed cation effect [15][16][17]. The mismatch in size of the cations sensitively changes the potential landscape the mobile F anions are exposed to.…”

Effects of mechanical processing and annealing on optical coherence properties ofEr3+:LiNbO3 powders

“…While it is clear that the extended highenergy milling required to produce nanocrystalline powders produces significant damage, it is unknown if the damage is intrinsically linked to the size reduction or whether lower energy grinding methods might better preserve the properties of the bulk crystal. For LiNbO 3 , some of the same measurements that reveal the large degradation in nanocrystals indicate that microcrystals have nearly the same properties as the bulk crystal [47]. In our previous SHB studies of Tm 3+ -doped yttrium aluminum garnet powders, we found that high-energy planetary ball milling dramatically affected 169 Tm 3+ nuclear hyperfine state lifetimes; however, we also found that the lifetimes were still significantly reduced in relatively large microcrystals produced by low-energy ball milling [49].…”

“…Since very large internal electric fields can be produced through the piezoelectric and pyroelectric effects in LiNbO 3 [10], which are also known to contribute to optical damage processes in the bulk crystals [42], we might expect even greater potential for crystal damage from high-energy milling of this material. High-energy ball milling has been used by a number of groups to produce LiNbO 3 nanopowders, mostly undoped, and the resulting materials have been extensively characterized [43][44][45][46][47][48]. A range of techniques including nuclear magnetic resonance spectroscopy, extended X-ray absorption fine structure spectroscopy, impedance spectroscopy, X-ray diffraction, transmission electron microscopy, differential scanning calorimetry, thermogravimetic analysis, infrared absorption spectroscopy, and Raman spectroscopy have all indicated a significant increase in amorphous behavior in the LiNbO 3 crystallites when they are reduced to sizes below 100 nm by high-energy ball milling [43][44][45][46][47].…”

Lithium Permeation through Thin Lithium–Silicon Films for Battery Applications Investigated by Neutron Reflectometry

“…Thed eposited structure is schematically depicted in Figure 1, and the resultingd epth distribution of scattering length densities as deduced from neutron reflectivity experiments is showni nF igure 2w hile further details can be found in the Experimental Section.I n this setup,a morphous LiNbO 3 serves solely as lithium tracer reservoir. Amorphous LiNbO 3 is as olid electrolyte,e xhibiting ah igh lithium-ionc onductivity and diffusivity [18,19] while still retaining ar igid Nb-O skeleton [20] which makes it ideally suited for these experiments.I np rinciple,a lso other types of materials can be used as tracer reservoirs.T he sample design is tailored to neutron reflectometry measurements,w hich is best explained throughF igure 3a.W hile the isotope contrast from the alternating depositiono f 6 LiNbO 3 and nat LiNbO 3 gives rise to ad istinctB ragg peak around Q z % 0.019 À1 in the reflectivity pattern, the chemical contrast between LiNbO 3 and Li x Si results in an additional Bragg peak at ahigher scattering vector Q z % 0.033 À1 .T he continuous line in Figure 3a represents af it based on calculations using the program Parratt32. With the variationofgiven input parameters this program uses the Parratta lgorithm [21] to simulate the resultingn eutronr eflectivity pattern.…”