Solid-state diffusion and NMR

Diffusion in solids, which requires the presence of crystal defects or disorder, has both microscopic and macroscopic aspects. Nuclear magnetic resonance techniques provide access to microscopic diffusion parameters like atomic jump rates and activation energies as well as to the tracer diffusion coefficient for macroscopic transport. Microscopic NMR methods include spin-lattice relaxation spectroscopy of stable and beta-radioactive nuclei, spin-spin relaxation or linewidth and spin alignment decay measurements, whereas macroscopic NMR methods are represented by the techniques of static and pulsed field gradient NMR. We recall some basic principles of the mentioned techniques and review case studies for their application to various solids like glassy and crystalline aluminosilicates, nanocrystalline composites, an intercalation compound and a simple bcc metal. Taken together, jump rates in solids are covered over about 10 decades by the microscopic, and diffusion coefficients over 3 decades by the macroscopic NMR methods

A “Solvent-Free” Crystal Structure of [FeN(SiMe3)23] – Synthesis, Structure and Properties

For the synthesis of the ferric bis(trimethylsilyl)amido complex [Fe{N(SiMe3)2}3] literature gives differing synthetic protocols based on crystallization from solution. In this report we present a ‘solvent‐free’ structural phase of [Fe{N(SiMe$_{3}$)$_{2}$}$_{3}$] which was isolated by sublimation of the product obtained from the reaction of 2 eq FeCl$_{3}$ with 3 eq LiN(SiMe$_{3}$)$_{2}$ in benzene. It could be characterized by single crystal as well as powder XRD and elemental analysis. However, $_{57}$Fe Mößbauer spectroscopy suggests a contamination of the main product with an Fe(II) species. Also, a part of the solid reaction byproducts from the reactions in solution were identified by powder XRD and $_{7}$Li MAS NMR which indicate distinct redox side reactions between oxidizing FeCl$_{3}$ and reducing LiN(SiMe$_{3}$)$_{2}$, a fact which rationalizes the lower than expected yields and the observation of an Fe(II) impurity compound. AC magnetic measurements of [Fe{N(SiMe$_{3}$)$_{2}$}$_{3}$] ave been performed in an extended frequency range up to 10$^{4}$ s$^{-1}$ allowing for a more precise evaluation of the magnetic relaxation parameters when compared to previously published measurements

AC and DC Conductivity in Nano- and Microcrystalline Li2O : B2O3 Composites: Experimental Results and Theoretical Models

We report on impedance measurements of nano- and microcrystalline composites of the Li ion conductor Li2O and the ionic insulator B2O3 as well as their interpretation in the frame of percolation models. In the experimental part, besides the dc conductivity and its dependence on composition and temperature (i.e. its activation energy) also the ac conductivity and its dependence on composition, temperature and frequency (i.e. the conductivity exponent) are presented. Striking differences between the nanocrystalline and the corresponding microcrystalline composites were found. Deviations of the ac from the dc results can be explained by the fact that the experiments probe ion dynamics on different time and thus length scales. In the theoretical part, a continuum percolation model, a brick-layer type bond percolation approach and a Voronoi construction are alternatively used to model the dc behaviour. Based merely on the largely different volume fractions of the interfaces between ionic conductor and insulator grains in the nano- and microcrystalline composites, good overall agreement with the experimental dc results is obtained. The high critical insulator content above which the experimental conductivity vanishes in the nanocrystalline composites suggests the presence of an additional Li diffusion passageway of nanometer length in the interface between nanocrystalline insulator grains. © 2005, Oldenbourg Wissenschaftsverlag Gmb

Local electronic structure in a LiAl O2 single crystal studied with Li7 NMR spectroscopy and comparison with quantum chemical calculations

The local electronic structure of a γ−LiAlO2 single crystal was investigated with 7Li nuclear magnetic resonance measurements. We observed different sets of spectra which originate from the four crystallographically equivalent but magnetically inequivalent Li sites per unit cell. We find a coupling constant e2qQ/h=115.1±0.6kHz and an asymmetry parameter η=0.69±0.01. The directions of the principal axes of the electric field gradient tensor at the sites of the Li nuclei have also been determined. We compared these experimental results with quantum chemical calculations at density-functional level and found good agreement. © 2006 The American Physical Society

Nanocrystalline versus microcrystalline Lo2O:B2O 3 composites: Anomalous ionic conductivities and percolation theory

We study ionic transport in nano- and microcrystalline (1−x)Li2O:xB2O3 composites using standard impedance spectroscopy. In the nanocrystalline samples (average grain size of about 20 nm), the ionic conductivity σdc increases with increasing content x of B2O3 up to a maximum at x≈0.5. Above x≈0.92, σdc vanishes. By contrast, in the microcrystalline samples (grain size about 10μm), σdc decreases monotonically with x and vanishes above x≈0.55. We can explain this strikingly different behavior by a percolation model that assumes an enhanced conductivity at the interfaces between insulating and conducting phases in both materials and explicitly takes into account the different grain sizes. © 2000 The American Physical Society

Enhanced conductivity at the interface of Li2O:B2O3 nanocomposites: Atomistic models

A theoretical investigation at density-functional level of Li ion conduction at the interfaces in Li2O:B2O3 nanocomposites is presented. The structural disorder at the Li2O(111):B2O3(001) interface leads to reduced defect formation energies for Li vacancies and Frenkel defects compared to Li2O surfaces. The average activation energy for Li+ diffusion in the interface region is in the range of the values for Li2O. It is therefore concluded that the enhanced Li conductivity of Li2O:B2O3 nanocomposites is mainly due to the increased defect concentration. © 2007 The American Physical Society

Influence of the Cation on the Reaction Mechanism of Sodium Uptake and Release in Bivalent Transition Metal Thiophosphate Anodes: A Case Study of Fe2_2P2_2S6_6

The layered active material Fe$_2$P$_2$S$_6$ was examined as anode material in sodium-ion batteries (SIBs) and compared to previously investigated Ni$_2$P$_2$S$_6$. A reversible specific capacity of 540 mAh g$^{−1}$ was achieved after 250 cycles, depicting similar electrochemical performance as observed for Ni$_2$P$_2$S$_6$. The rate capability and long-term behavior of these two materials are also very similar. Another objective was to elucidate the reaction mechanism during discharging and charging by applying several techniques such as X-ray diffraction, pair distribution function analysis as well as X-ray absorption and solid state NMR spectroscopy. The results clearly demonstrate that the majority of Fe$^{2+}$ is reduced to elemental Fe during the uptake of 5 Na/f.u., while an amorphous intermediate is generated, which was identified as Na$_4$P$_2$S$_6$ by solid state NMR spectroscopy. Completely discharging against a Na metal counter electrode leads to the formation of nanocrystalline Na$_2$S and indications of the formation of polymeric phosphorus were found. In sum, the Na uptake reaction process observed for Fe$_2$P$_2$S$_6$ coincides with the previously unraveled reaction pathway of Ni$_2$P$_2$S$_6$. We therefore conclude that a universal reaction takes places for bivalent transition metal thiophosphate (M$_2$P$_2$S$_6$) electrodes in SIBs