7Li NMR study of the mobility of lithium ions in one-dimensional channels of Nb3Se4

The temperature dependence of the spin-lattice relaxation time T ₁ and the ⁷Li NMR spectra of the Li_0.7Nb₃Se₄ intercalation compound with one-dimensional channel structure have been studied. It is found that the temperature dependence of T ₁ exhibits two relaxation minima, and the quadrupole splitting in the Li NMR spectra shows an anomalous temperature behavior. The inference is drawn that the observed effects are associated with the high-rate diffusive motion of lithium ions along one-dimensional channels and the interchannel transitions.     

Anomalous decline of molecular ion mobility in cooled helium gas

We present a first successful theoretical account of the ion mobilities of and in helium gas at 4.3 K. Measured mobilities of various molecular ions at low effective temperatures reportedly tend to values smaller than their polarization limits, with the exception of N(2)(+). The present theoretical results obtained by the classical trajectory calculations agree with the experimental ones very well, and make it definitive that the anomalous decline of molecular ion mobility is caused by a Feshbach-like resonance due to the anisotropic interaction potential between a molecular ion and a helium atom. The mechanism thus revealed is supported by quantitative quantum mechanical calculations. The process appears very similar to that of laser cooling.     

EPR and NMR study of molecular components mobility and organization in goat milk under ultrasound treatment

The effect of ultrasound treatment on molecular mobility and organization of the main components in raw goat milk was studied by EPR and NMR spectroscopies. NMR relaxation studies showed an increase in the spin–lattice T₁ and spin–spin T₂ relaxation times in goat milk products (cream, anhydrous fat) and change in the diffusion of proton-containing molecules during ultrasound treatment. The diffusion became more uniform and could be rather accurately approximated by one effective diffusion coefficient D_eff, which indicates homogenization of goat milk components, dispersion of globular and supermicellar formations under sonication. EPR studies have shown that molecular mobility and organization of hydrophobic regions in goat milk are similar to those observed in micellar formations of surfactants with a hydrocarbon chain length C12–C16. Ultrasound treatment did not affect submicellar and protein globule organization. Free radicals arising under ultrasound impact of milk reacted quickly with components of goat milk (triglycerides, proteins, fatty acids) and were not observed by spin trapping method.     

NMR study of 6Li in LiNbO3

An experimental NMR study of the ⁶Li isotope in single crystals of lithium niobate has been performed, along with a computer simulation of ⁶Li NMR spectra for a crystal of congruent composition, containing defects in the cation sublattice. It is found that the mean value of the principal component of the electric field gradient tensor at the ⁶Li nuclei is 1.48 times larger than at the ⁷Li nuclei. It is surmised that there is a substantial difference in the character of the mobility of the ⁶Li and ⁷Li nuclei in the LiO₆ octahedra at room temperature. Fiz. Tverd. Tela (St. Petersburg) 40, 122–125 (January 1998)     

NMR study of Li diffusion in β-LiAl

Diffusion of a Li atom in an anode material β-LiAl was studied over the composition range of 48.0–50.2 at % Li by the ⁷Li magnetic relaxation times. Dependence of T₁ upon temperature and the resonance frequency was successfully explained by a vacancy diffusion on the Li sublattice involving a distribution of the activation energy for migration, E_a. The central value of E_a was almost independent of the composition (? 13.8 kJ mol^-1), while the width of distribution increased with increase of the number of substituted Li on the Al sublattice. The obtained diffusion constants, extrapolated to 415°C, were larger by a factor of 1.8–3.4 than those obtained by the electrochemical method.     

锂原子同位素位移的计算(英文)

本文应用Grasp92程序包计算了~(5,7)LiI的2~2P_(3/2)到2~2S_(1/2)和2~2P_(1/2)到2~2S_(1/2)跃迁的同位素位移.2~2P_(3/2)到2~2S_(1/2)跃迁的同位素位移为10307 MHz,2~2P_(1/2)到2~2S_(1/2)跃迁的同位素位移为10207 MHz.这些理论结果和实验是相符合的.     

Li ion mobility in TeO2---LiO0.5---LiX (X = F, Cl) glasses determined by Li NMR and impedance spectroscopy

NMR and impedance spectroscopies have been used to study the Li⁺ ion mobility in the TeO₂---LiO_0.5---LiX (X = F, Cl) glasses. Activation energies determined from the temperature dependence of dc conductivity data decrease as the lithium or halogen content increases. Activation energies deduced from the analysis of the ac conductivity data in the complex modulus formalism and corresponding to the true one-particle energy barrier of ionic motion according to the coupling model, have been compared to those deduced from NMR data (T₁⁻¹ relaxation time): a good agreement is obtained. Finally, the influence of the glass composition and ion distribution on Li⁺ ion mobility has been discussed.     

The Challenge of Paramagnetism in Two-Dimensional <Superscript>6,7</Superscript>Li Exchange NMR

^6,7Li fast magic-angle spinning solid-state nuclear magnetic resonance (NMR) spectroscopy is used to study LiMn₂O₄ and Li₃V₂(PO₄)₃. The presence of paramagnetic transition metal centers in these materials has a profound effect on the resulting NMR spectra. Lithium ion mobility has been studied by two-dimensional (2-D) exchange spectroscopy (EXSY) in Li₃V₂(PO₄)₃ but an absence of lithium ion exchange was observed for LiMn₂O₄. Several differences between the two materials are explored to explain these results. LiMn₂O₄ experiences a greater donation of electron spin density to the Li nucleus via the Fermi-contact interaction when compared with Li₃V₂(PO₄)₃. This contributes to a greater hyperfine chemical shift and a larger dependence of chemical shift on temperature. The delocalized electrons in LiMn₂O₄ cause temperature-independent T ₁ relaxation rates and shorter relative T ₂ values. The relative rates of ionic conductivity and spin–lattice or spin–spin relaxation in LiMn₂O₄ and Li₃V₂(PO₄)₃ are contrasted to illustrate the constraints on the use of 2-D EXSY to characterize ion dynamics in paramagnetic materials. Authors' address: Gillian R. Goward, Department of Chemistry and Brockhouse Institute for Materials Research, McMaster University, 1280 Main St. West, Hamilton, ON L8S 4M1, Canada     

7Li NMR study on Li+ ionic diffusion and phase transition in LixCoO2

The temperature dependence of the spin-lattice relaxation time, T₁ and the line width of the ⁷Li nucleus were measured in delithiated Li_xCoO₂ (x = 0.6, 0.8, 1.0). Two different relaxation behaviors were observed in the temperature dependence of T₁^− 1 in a x = 0.8 sample. These would have arisen from inequivalent Li sites in two coexisting phases; an original hexagonal (HEX-I) and a modified hexagonal (HEX-II) phase in the x = 0.8 sample. We analyzed using a phenomenological non Debye-type relaxation model. Motional narrowing in the line width was observed in each sample, the result revealing that Li⁺ ions begin to move at low temperature in samples with less Li content. It was found that the activation energy associating with Li⁺ ion hopping in the HEX-II phase is smaller than that in the HEX-I phase. These results show that the HEX-II phase produced in the Li deintercalation process would be suitable for Li⁺ ionic diffusion in multi-phase Li_xCoO₂, and it is expected that this would enable fast ionic diffusion. Li⁺ ionic diffusion related to phase transition is discussed from ⁷Li NMR results.     

Diffusion of Li ion on graphene: A DFT study

Density functional theory investigations show that the Li⁺ ion is stabilized at the center of hexagonal carbon ring with the distance of 1.84 Å from graphene surface. The potential barrier of Li⁺ ion diffusion on the graphene surface, about 0.32 eV, is much lower than that of Li⁺ ion penetrating the carbon ring which is 10.68 eV. When a vacancy of graphene exists, potential barrier about 10.25 eV for Li⁺ ion penetrating the defect is still high, and the ability of the vacancy to sizing the Li⁺ ion is also observed. Electronic densities of states show that the formation of a localized bond between Li atom and edge carbon of vacancy is the main reason for high potential barrier when Li⁺ ion penetrate a vacancy. While Coulomb repulsion is the control factor for high potential barrier in case of Li⁺ ion penetrating a carbon ring.     

β NMR on8Li in the superiqnic conductor Li3N

Polarized⁸Li nuclei were produced in a Li₃N single crystal by irradiation with polarized neutrons, Β-ray detected NMR signals and spin-lattice relaxation of⁸Li were observed between B and 300 K. In Li₃N there are two non-equivalent Li sites. The corresponding two quadrupole split NMR spectra could be resolved. From the measured relaxation rates activation enthalpies for two diffusion processes were deduced.     

铜膜中锂离子扩散的第一性原理研究

基于第一性原理，采用“adiabatic trajectory”方法计算了锂离子在铜膜中的扩散势垒。通过比较可能的扩散机制，理论上证实了室温下锂离子可以最近邻空位扩散机制穿过铜膜。同时，解释了随着铜膜中空位数量的增加，锂离子的扩散势垒减小，从而更易在铜膜中扩散的物理现象。     

NMR study of molecular dynamics in selected hydrophilic polymers

The temperature dependencies of the 1H spin-lattice relaxation times T1 and of the proton NMR second moment M2 in the temperature range from about 90 to 420 K were measured for methyl cellulose, hydroxypropyl cellulose and hydroxypropyl methyl cellulose. The proton spin-lattice relaxation measurements reveal two minima due to the C3 reorientation of the methyl groups of the methoxy, methylenemethoxy or propylene oxide groups and the restricted motion of the segment of the polymer chain. The activation energy barriers for these motions were calculated.     

NMR relaxation study of molecular motions in liquid chlorobenzotrifluorides

The temperature dependences of the ¹H and ¹⁹F nuclear spin-lattice relaxation times T₁ in liquid o-, m-, and p-chlorobenzotrifluorides have been measured. The analysis of the temperature dependences of the ¹H spin-lattice relaxation times leads to the conclusion that the overall molecular reorientational motion in o-, m-, and p-benzotrifluorides is nearly the same. Data for ¹H and ¹⁹F spin-lattice relaxation times of o-chlorobenzotrifluoride jointly lead to the determination of the individual contributions to relaxation rate in the entire temperature range studied. A knowledge of these contributions for o-chlorobenzotrifluoride, together with the assumption of equal correlation times for overall molecular reorientation in o- and p-chlorobenzotrifluorides, leads to the determination of the spin-internal-rotation interaction contribution to relaxation for p-chlorobenzotrifluoride in the same range of temperature.     

Determination of the physical network of entanglements and the molecular mobility in linear flexible polymers by the NMR method

In the present paper, a theory of free induction decay in linear flexible chain polymers is developed. Theoretical and experimental dependences of the spin-spin relaxation time T ₂ on the average molecular weight M _w and temperature are compared. It is shown that, with increasing M _w , the topological structure of linear polymers changes, and at M _w > 10⁵, a physical network entanglements (quasi-network) is formed. A correlation function of molecular motions is obtained, which indicates whether the quasi-network is formed in polymer melts.