Motional Narrowing of Nuclear Magnetic-Resonance Lines in Manganese-Doped Lithium Fluoride (original) (raw)
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Antiferromagnetic spin structure and lithium ion diffusion in Li2MnO3 probed by <m...
Physical Review B, 2013
In order to elucidate the antiferromagnetic (AF) spin structure below T N ∼ 35 K and to clarify the diffusive behavior of Li + ions in the layered compound Li 2 MnO 3 , we have performed a muon-spin rotation and relaxation (μ + SR) experiment using a powder sample in the temperature range between 2 and 500 K. Below T N , the zero-field (ZF-) μ + SR spectrum showed a clear oscillation that consists of two muon-spin precession signals with different frequencies. Combining with the dipole field calculations, it was found that the most probable spin structure for Li 2 MnO 3 is the C x -type AF order in which Mn moments align parallel or antiparallel to the a axis in the [Li 1/3 Mn 2/3 ]O 2 layer, and a ferromagnetic chain along the a axis aligns antiferromagnetically along both the b and c axes. The ordered Mn moment was estimated as 2.62μ B at 2 K. In the paramagnetic state, ZF-and longitudinal-field μ + SR spectra exhibited a dynamic nuclear field relaxation. From the temperature dependence of the field distribution width, the Li + ions were found to diffuse mainly along the c axis through the Li ion in the [Li 1/3 Mn 2/3 ]O 2 layer. Also, based on the field fluctuation rate, a self-diffusion coefficient of Li + ions (D Li ) at 300 K was estimated as 4.7(4) × 10 −11 cm 2 /s with the thermal activation energy E a = 0.156(3) eV.
Magnetic properties of the trirutile compound lithium manganese vanadium fluoride (LiMnVF6)
Inorganic Chemistry, 1985
The magnetic susceptibility from 4.2 to 300 K and spontaneous magnetization up to T, of the trirutile compound LiMnVF, were measured. A plot of x-' vs. T displays two linear regions. One possible explanation for this phenomenon employs a binuclear model in which the intradimer interaction (J,) is 1 order of magnitude or more greater than the interdimer interaction (Jz). This model presumes Mn2+ and V3+ ions are ordered within the trirutile lattice such that all nearest-and second-nearest-neighboring magnetic ions of MnZt are V", and vice versa. Intracoupling between nearest-neighboring MnZt-V3+ pairs occurs first, forming magnetically ordered dimers. These dimers then behave as paramagnetic ions, giving rise to a quasi-paramagnetic region. At lower temperatures, intercoupling of the dimers leads to three-dimensional, long-range ordering with a predictable spontaneous magnetic moment. The experimental magnetic constants for LiMnVF6 are C M = 5.32 cm3 K mol-', CM' = 1.92 cm3 K mol-I, 8 = -25 K, 8' = +18.5 K, T, = 23 K, and uo = 2.6 wcg. Values of the exchange constants that were fitted to the susceptibility data are J l / k = -26 f 3 K and J2/k = -0.83 i 0.05 K.
19F nuclear spin relaxation and spin diffusion effects in the single-ion magnet LiYF4:Ho3+
The European Physical Journal B, 2008
Temperature and magnetic field dependences of the 19 F nuclear spin-lattice relaxation in a single crystal of LiYF 4 doped with holmium are described by an approach based on a detailed consideration of the magnetic dipole-dipole interactions between nuclei and impurity paramagnetic ions and nuclear spin diffusion processes. The observed nonexponential long time recovery of the nuclear magnetization after saturation at intermediate temperatures is in agreement with predictions of the spin-diffusion theory in a case of the diffusion limited relaxation. At avoided level crossings in the spectrum of electron-nuclear states of Ho 3+ ions, rates of nuclear spin-lattice relaxation increase due to quasi-resonant energy exchange between nuclei and paramagnetic ions in contrast to the predominant role played by electronic cross-relaxation processes in the low-frequency ac-susceptibility. PACS. 76.60.-k Nuclear magnetic resonance and relaxation -75.40.Gb Dynamic properties -76.30.Kg Magnetic resonances and relaxations in condensed matter Rare-earth ions and impurities __________________________________________ a
6Li and 7Li MAS NMR Studies of Lithium Manganate Cathode Materials
Journal of the American Chemical Society, 1998
6 Li MAS NMR has been used to study the lithium local environments and manganese electronic structures of a number of lithium manganese oxides with manganese oxidation states varying from (III) to (IV). Most samples were chosen with compositions within the LiMn 2 O 4-Li 2 Mn 4 O 9-Li 4 Mn 5 O 12 phase diagram, but Li 2 Mn 2 O 4 with Mn(III) was also synthesized for comparison. Despite the presence of unpaired electrons, high-resolution spectra could still be acquired, allowing a number of different local environments to be detected. Assignments of the resonances to different lithium local environments were made by comparing the observed shifts and local structures in a number of lithium manganates. Two 6 Li NMR resonances were observed for the spinel phase Li 4 Mn 5 O 12 at 1980 and 847 ppm, which were assigned to Li + in the octahedral and tetrahedral sites of the spinel structure, respectively. A shift was observed for Li + in the tetrahedral site of Li 2 Mn 4 O 9 , which also contains Mn(IV), at 687 ppm. The shifts are ascribed primarily to a Fermi-contact shift mechanism, and possible mechanisms to account for the directions and sizes of the shift are discussed. Shifts to lower frequency are observed as the manganese oxidation state is reduced (i.e., for manganates containing occupied e g orbitals). The 6 Li MAS NMR spectra of the spinels with a Li:Mn ratio of 1:2 are extremely sensitive to the synthesis conditions. When relatively high synthesis temperatures (850°C) are used, a single resonance at 520 ppm, from the normal spinel phase, dominates. In contrast, several resonances are observed for samples synthesized at lower temperatures (550-650°C), as a result of defects in the normal spinel structure. These resonances collapse into the main spinel resonance at high temperatures (250°C) and are assigned to electronic defects associated with higher oxidation state manganese ions (Mn 4+). No evidence for a Jahn-Teller distortion is observed in both the NMR and by diffraction for samples that contain considerable disorder. In contrast, samples that were prepared at temperatures of 650°C or higher show a cubic-to-tetragonal phase change below room temperature. This is accompanied by the appearance of at least three additional 6 Li resonances, indicating some ordering of the Mn 3+ and Mn 4+ cations below the phase transition, in the time scale of the NMR experiment.
J. Phys. …, 2010
Two-dimensional (2D) exchange nuclear magnetic resonance (NMR) spectroscopy carried out under magic angle spinning (MAS) conditions is ideally suited to study site-specific Li diffusion parameters of cathode materials required for the target-oriented development of so-called high-energy density 4 V-lithium-ion batteries. In the present study, we took advantage of Li NMR hyperfine shifts to record temperature-variable 1D and mixing-time dependent 2D exchange MAS 6 Li NMR spectra on R-Li 3 VF 6 serving as both a potential cathode material as well as an application-oriented model substance with three magnetically inequivalent Li sites. By comparing the NMR results with structural details of the material we were able to obtain detailed insights into the migration pathways and Li exchange rates which are of the order of some hundreds of Li jumps per second at approximately 340 K. Site-specific Li jump rates τ-1 reveal the electrochemically active sites and provide information how to modify the material in order to increase its relatively low Li diffusivity found at room temperature.
Magnetism and structural phase transitions in LiTmF 4 powders
Jetp Letters, 1997
The field (0–5.5 T) and temperature (2–300 K) dependences of the magnetization of LiTmF4 powders with particle sizes of 1 μm and 56–400 μm are investigated experimentally and theoretically. It is concluded that a transition layer exists between the thulium ions in the bulk and the ions at the surface. Two magnetic-field-induced structural phase transitions are observed at low temperatures, and the temperature dependence of the critical magnetic fields is established.
The Journal of Physical Chemistry C, 2010
Two-dimensional (2D) exchange nuclear magnetic resonance (NMR) spectroscopy carried out under magic angle spinning (MAS) conditions is ideally suited to study site-specific Li diffusion parameters of cathode materials required for the target-oriented development of so-called high-energy density 4 V-lithium-ion batteries. In the present study, we took advantage of Li NMR hyperfine shifts to record temperature-variable 1D and mixing-time dependent 2D exchange MAS 6 Li NMR spectra on R-Li 3 VF 6 serving as both a potential cathode material as well as an application-oriented model substance with three magnetically inequivalent Li sites. By comparing the NMR results with structural details of the material we were able to obtain detailed insights into the migration pathways and Li exchange rates which are of the order of some hundreds of Li jumps per second at approximately 340 K. Site-specific Li jump rates τ-1 reveal the electrochemically active sites and provide information how to modify the material in order to increase its relatively low Li diffusivity found at room temperature.
Nuclear spin relaxation of 8 Li in a thin film of La 0.67 Ca 0.33 MnO 3
We report b-NMR measurements of the nuclear spin relaxation rate ð1=T 1 Þ in a thin film of La 0.67 Ca 0.33 MnO 3 (LCMO) using a low-energy beam of spin-polarized 8 Li. In a small magnetic field of 150 G, there is a broad peak in 1=T 1 near the Curie temperature ðT c ¼ 259 KÞ and a dramatic decrease in 1=T 1 at lower temperatures. This is attributed to a critical slowing down of the spin fluctuations near T c and freezing of the magnetic excitations at low temperatures, respectively. In addition, there is a small amplitude, slow relaxing component at high temperatures, which we attribute to 8 Li in the SrTiO 3 substrate. There is an indication that the spin relaxation rate in the substrate is also peaked at T c due to close proximity to the magnetic film. These results establish that low-energy b-NMR can be used as a probe of magnetic fluctuations in magnetic thin films over a wide range of temperatures.
NMR Study on Li+ Ionic Motion in LixV2O5 (0.4 ≦x≦1.4)
Journal of the Physical Society of Japan, 2008
The temperature dependences of the 7 Li NMR line width Á and spin-lattice relaxation rate 1=T 1 have been measured in lithium vanadium bronze Li x V 2 O 5 with 0:4 5 x 5 1:4 over the temperature range of 77-550 K. The narrowing of Á has been observed for each sample, which is interpreted in terms of the motional narrowing due to the Li þ ionic diffusion. In Li 0:8 V 2 O 5 with a single-phase, an activation energy E m for the hopping of Li þ ions is estimated to be 0:16 AE 0:05 eV, whereas in Li 0:4 V 2 O 5 the value of 0:10 AE 0:05 eV is deduced for the-phase. The ionic conductivity evaluated from the diffusion coefficient D in Li 0:8 V 2 O 5 at 400 K is 6:3 Â 10 À7 À1 cm À1 , which is consistent with the reported value of dc ' 10 À7 À1 cm À1. Below 300 K, 1=T 1 is considered to be due to magnetic interaction of 7 Li nuclear spins with V 4þ electronic spins. Above 350 K, 1=T 1 is dominated by Li þ ionic diffusion, from which E m is estimated to be 0:10 AE 0:05 eV.
Influence of Morphology of LaF3 Nano-crystals on Fluorine Dynamics Studied by NMR Diffusometry
Applied Magnetic Resonance, 2018
Ionic dynamics in nano-structured 2D superionic conductors LaF 3 obtained by a synthesis at the gas-solution interface has been analyzed using 19 F NMR Static Field Gradient (SFG) diffusometry in a temperature range up to 800 K. The fluorine diffusion in 2D materials is significantly higher as compared to that in bulk LaF 3 and is strongly dependent on the nano-crystalline sheet thickness. Its decrease from 18 nm to 6 nm leads to an increase of mobility by almost two orders of magnitude, resulting in an overall mobility enhancement of more than three orders of magnitude compared to mono-crystalline LaF 3. Moreover, the activation energy is reduced from 1.2 eV for mono-crystals to 0.23 eV for 6 nm thin nano-crystalline powder samples. 1 Introductions Lanthanum fluoride LaF 3 is a well-known high-temperature superionic conductor [1-3] with tysonite structure having practical applications in various fields, for example, as solid-state sensor [4], luminescent material [5, 6], and biocompatible material [7]. The tysonite belongs to the very few structures where the superionic properties are found to be very pronounced. The fluorine ionic conductivity in tysonite is governed by a disorder in the anionic sublattice, and a usual way to enhance it is the partial heterovalent substitution of La 3+ by M 2+ metal (M=Sr, Ca, Ba etc.), which preserves the tysonite structure. In this way, the best result has been