7Li-NMR Study of Diffusion-Induced Spin-Lattice Relaxation in Glassy and Crystalline LiAlSi2O6

Diffusion and Ionic Conduction in Nanocrystalline Ceramics

Diffusion and ionic conduction in nanocrystalline ceramics, both monophase and composite, was studied by NMR relaxation and NMR lineshape as well as impedance spectroscopy. Measurements were mainly done on Li ion conductors prepared by high-energy ball milling. It was possible to discriminate between mobile ions in the interfacial regions and immobile ions in the grains. In general the diffusivity and conductivity are enhanced in the nanocrystalline monophase system as compared to the microcrystalline one, e. g. by about four orders of magnitude in the case of CaF2. An exception is, e. g., Li2O where the nano- and microcrystalline forms have similar conductivities. However, when the nanocrystalline insulator B2O3 is added to nanocrystalline Li2O the conductivity of the composite increases whereas it decreases in the corresponding microcrystalline system.

Li Ion Diffusion in Nanocrystalline and Nanoglassy LiAlSi2O6 and LiBO2 - Structure-Dynamics Relations in Two Glass Forming Compounds

In the present study the Li diffusivity in nanostructured samples of two glass forming model systems, spodumene (LiAlSi2O6) and lithium metaborate (LiBO2), was examined using 7Li nuclear magnetic resonance (NMR) spin-lattice relaxometry and dc conductivity measurements. The nanostructured samples were prepared by high-energy ball milling of the respective crystalline starting material on the one hand and the corresponding glass on the other hand. The diffusivity of the glass exceeds that of the crystalline sample for both systems. However, when the crystalline samples are mechanically treated by ball milling the diffusivity is enhanced. Nevertheless, the diffusivity of these nanocrystalline samples remains lower than that of the corresponding glass. Surprisingly, when the glassy samples are treated in the same way the diffusivity decreases. After sufficiently long milling times the diffusivity of these nanoglassy samples approaches that of the nanocrystalline samples. This convergence effect seems to be due to structural relaxation processes as is suggested by supplementary infrared spectroscopy and 27Al, 12B magic angle spinning NMR measurements.

Spin relaxation and ultra-slow Li motion in an aluminosilicate glass ceramic

The ion dynamics in a lithium aluminosilicate glass ceramic was studied using stimulated-echo 7Li-NMR. For temperatures 300 K<T<450 K the hopping correlation times, tau(C), were determined from the decay of the quadrupolar spin-alignment amplitude. The decay times were thermally activated with an energy barrier of 0.61 eV, in agreement with conductivity measurements. For T<300 K the temperature dependence of the decay times, now corresponding to T1Q, was much weaker. The T1Q times followed the same trend as the independently measured spin-lattice relaxation times, T1, but were a factor of about three shorter than those. The theoretical ratio of T1/T1Q is shown to be 25/8 in the slow-motion regime if quadrupolar spin relaxation prevails. This result explains the present observation and similar ones made for several Li ion conductors. The relaxation of the octupolar spin-alignment order is discussed.

From ultraslow to fast lithium diffusion in the 2D ion conductor Li0.7TiS2 probed directly by stimulated-echo NMR and nuclear magnetic relaxation

7Li stimulated-echo NMR and classical relaxation NMR techniques are jointly used for the first time for a comprehensive investigation of Li diffusion in layer-structured Li0.7TiS2. One single 2D Li diffusion process was probed over a dynamic range of almost 10 orders of magnitude. So far, this is the largest dynamic range being measured by 7Li NMR spectroscopy directly, i.e., without the help of a specific theoretical model. The jump rates obey a strict Arrhenius law, determined by an activation energy of 0.41(1) eV and a preexponential factor of 6.3(1)x10(12) s-1, and range between 1x10(-1) s-1 and 7.8x10(8) s-1 (148-510 K). Ultraslow Li jumps in the kHz to sub-Hz range were measured directly by recording 7Li spin-alignment correlation functions. The temperature and, in particular, the frequency dependence of the relaxation rates fully agree with results expected for 2D diffusion.

Tracer diffusion measurements in solid lithium: a test case for the comparison between NMR in static and pulsed magnetic field gradients after upgrading a standard solid state NMR spectrometer

This paper reports on the upgrading of a standard solid state NMR spectrometer, which has been used in combination with a field variable 7 T cryomagnet, to a low-cost combined SFG and PFG NMR spectrometer. Both methods are applied to solid lithium as a simple test case. The results show that under the given conditions SFG NMR and PFG NMR can provide tracer diffusion coefficients for 7 Li diffusion down to about 10(-14) and 10(-13) m2/s, respectively. SFG and PFG NMR are complementary methods. The paper demonstrates advantages and disadvantages of each method with a concrete example and why it is desirable to be able to apply both methods to the same sample.

The importance of paramagnetic impurities to the nuclear magnetic resonance relaxation of ion-conducting glasses

Paramagnetic impurities have been shown to affect the 7Li nuclear magnetic resonance relaxation rates in cation-conducting glasses, and wrong data may then be extracted from the experiments. Frequency- and temperature-dependent T(-1)1 studies of lithium borate and thioborate glasses revealed that iron impurities cause frequency-independent relaxation and "shoulders" of the low-temperature slopes in high fields, whereas manganese produces enhanced relaxation peaks. The results of the T(-1)1 (and some T(-1)1p and T(-1)2) measurements are discussed.