NMR study of pure and doped β-LiAl

The NMR spin-lattice relaxation time, T_I, has been measured as a function of temperature for both ⁷Li and ²⁷Al in pure and doped β-LiAl alloys. Compositions with ⁷Li concentration in the range 48.3–54.5% and doping in the form Li₅₀Al_50?xM_x, where M = Ag or In, were studied. The relaxation rates T₁^?1 for the ²⁷Li and the ²⁷Al resonances were found to be peaked functions of temperature with the maxima for ⁷Li appearing at composition dependent temperatures. The ²⁷Al maxima always appeared at a lower temperature, independent of composition, and the ²⁷Al maximum relaxation rate was a strong function of composition in contrast with ⁷Li where the maximum rate was only weakly dependent on composition. The principle relaxation mechanisms are identified as dipole-dipole coupling in the ⁷Li and coupling of the ²⁷Al quadrupole moment to electric field gradients. The temperature dependence of these rates is attributed to the thermally activated diffusion of vacancies of a non-thermal origin in the Li sub-lattice. These vacancies are also responsible for the fluctuating electric field gradients. The results have been analyzed to give the Li diffusion coefficients with associated activation energies and estimates of the vacancy concentration as functions of alloy composition.     

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.     

A nuclear magnetic resonance study of the structural properties and molecular motions of Li2KH(SO4)2 and LiKSO4 single crystals

The structural properties and relaxation mechanisms of Li₂KH(SO₄)₂ crystals were determined using the temperature dependences of NMR spectra and the spin-lattice relaxation times (T₁) of their ¹H, ⁷Li, and ³⁹K nuclei. The results obtained were compared with the previously reported physical properties of LiKSO₄ crystals. The substitution of the potassium ions with protons in the LiKSO₄ crystals were variations in the phase transition temperatures, and the non-appearance of ferroelastic properties. The ⁷Li T₁ for the Li₂KH(SO₄)₂ crystals was much shorter than the ⁷Li T₁ for the LiKSO₄ crystals, and these findings indicate that the presence of the protons in Li₂KH(SO₄)₂ causes the Li ions to move with greater freedom.     

199Hg and63,65Cu NMR studies of mercury based high-T c superconductors

Hg-oxide ceramic high temperature superconductors were studied by¹⁹⁹Hg and^63,65Cu NMR spectroscopy. Room temperature spectra, spin-spin and spin-lattice relaxation times of samples with different superconducting transition temperatures are presented. A spin-lattice relaxation time ofT ₁=35 msec and a spin-spin relaxation time ofT ₂=1.6 msec were found for the¹⁹⁹Hg NMR. All samples exhibit similar characteristic powder spectra caused by an axially symmetric¹⁹⁹Hg spin interaction. The isotropic value and the anisotropy of the tensor relative to solid HgCl₂ as a standard substance is estimated. Furthermore, results of^63,65Cu NMR measurements at a temperature of 4.2 K which exhibit a typical powder line shape (forI=3/2) are presented.     

H NMR study of the phase transitions of trissarcosine calcium chloride single crystals at low temperature

The ¹H NMR line-width and spin-lattice relaxation time T₁ of TSCC single crystals were studied. Variations in the temperature dependence of the spin-lattice relaxation time were observed near 65 and 130 K, indicating drastic alterations of the spin dynamics at the phase transition temperatures. The changes in the temperature dependence of T₁ near 65 and 130 K correspond to phase transitions of the crystal. The anomalous decrease in T₁ around 130 K is due to the critical slowing down of the soft mode. The abrupt change in relaxation time at 65 K is associated with a structural phase transition. The proton spin-lattice relaxation time of this crystal also has a minimum value in the vicinity of 185 K, which is governed by the reorientation of the CH₃ groups of the sarcosine molecules. From this result, we conclude that the two phase transitions at 65 and 130 K can be discerned from abrupt variations in the ¹H NMR relaxation behavior, and that ¹H nuclei play important roles in the phase transitions of the TSCC single crystal.     

Li3VO4-Li4TO4(T=Ge,Si)固溶体离子导体7Li的NMR研究

在150—573K温度范围内,研究了固溶体Li₃VO₄-Li₄TO₄(T=Ge,Si)系统不同成分的⁷Li的NMR谱。发现γ_II相固溶体室温⁷Li的NMR线宽和自旋晶格弛豫时间T₁的值都比Li₄GeO₄,Li₄SiO₄和Li₃VO₄小约一个数量级。这表明在γ_II相固溶体离子导体中,Li⁺离子运动有可能比固溶前有数量级增长。同时还发现⁷Li的电四极分裂伴线数随成分和温度而异,以及伴线强度百分比依赖于温度。这反映γ_II相的不同成分中,间隙Li⁺离子占有的不等价位置个数不同,而Li⁺离子在每个不等价位置上的占有率又随温度而变化。 关键词：     

NMR studies of U0.2Y0.8Pd3 and YPd3 at low temperatures

We report results of ⁸⁹Y-NMR measurements on U_0.2Y_0.8Pd₃ as well as YPd₃, performed at frequencies between 9 and 16MHz and at temperatures between 0.25 and 2 K. In this temperature range the average Knight shift is +0.5%, temperature-independent and the same for both materials. Also the NMR linewidth for both compounds was found to be temperature and field independent but one order of magnitude larger for U_0.2Y_0.8Pd₃ than for YPd₃. For U_0.2Y_0.8Pd₃ our results indicate a distribution of internal static fields at the Y sites and a small temperature-dependent enhancement of the spin-lattice relaxation rate T ₁ ^?1 with respect to YPd₃. The NMR spectra are consistent with the presence of very small frozen U moments, but the temperature dependence of the spin-lattice relaxation rate indicates a more complicated situation. In particular (T₁T)^?1 shows an anomalous temperature dependence.     

7Li NMR analysis on perovskite structured Li0.15La0.28TaO3

The nanocrystalline perovskite material Li_0.15La_0.28TaO₃ has been synthesized by alkoxide-free Pechini type sol gel method. ⁷Li NMR measurements were carried out using a Bruker Avance 300 spectrometer at 116 MHz over the temperature range 150 to 400 K. Longitudinal spin-lattice relaxation times (T₁) measured by saturation recovery and longitudinal relaxation times in the rotating frame (T_1ρ) measured using the pulse sequence (π/2–spin lock τ acquisition) with lock radio-frequency field υ = 62.5 kHz and the T₂ relaxation time measured by Hahn echo are presented. The static Hahn-echo spectra show two different lithium sites in this perovskite oxide. Further, the relaxation measurements T₁ and T_1ρ show two different types of lithium cations with fast and slow dynamics.     

NMR relaxation rate in metallic carbon nanotubes

NMR relaxation rate, T₁⁻¹, of the metallic carbon nanotube is discussed based on Tomonaga–Luttinger-liquid theory. It is found that the Coulomb interaction leads to increase of (T₁T)⁻¹ by a power law with decreasing temperature, T. The dependence on temperature of (T₁T)⁻¹ in the multi-wall nanotube (MWNT) is shown to be strongly suppressed by existence of the metallic shells in the MWNTs.     

Phase transition and relaxation processes by 87Rb and 39K nuclear magnetic measurements of RbKSO4 single crystals

⁸⁷Rb and ³⁹K nuclear magnetic resonance (NMR) spectra of RbKSO₄ single crystals were measured at room temperature. ⁸⁷Rb central line has the angular dependences of second-order quadrupolar shifts. From these results, the quadrupole coupling constant and the asymmetry parameter were determined at room temperature. In addition, the spin–lattice relaxation rate, 1/T₁, and the spin–spin relaxation rate, 1/T₂, were measured as a function of temperature. The values of 1/T₁ for the ⁸⁷Rb and ³⁹K nuclei were found to increase with increasing temperature, and 1/T₁ was determined to be proportional to Tⁿ. Therefore, for the ⁸⁷Rb and ³⁹K nuclei, Raman processes with n=2 are more significantly in nuclear quadrupole relaxation than direct processes.     

NMR spectral densities and two dimensional diffusion in TiS2(NH3)1.0 and TaS2(NH3)x

NMR measurements of proton spin-lattice relaxation times T₁ and T_1? in the layered intercalation compounds TiS₂(NH₃)_1.0 and TaS₂(NH₃)_x (x = 0.8, 0.9, 1.0) are reported as functions of frequency and temperature (100 K – 300 K). These observations probe the spectral density of magnetic fluctuations due to motions of the intercalated molecules at frequencies accessible to the T₁ (4–90 MHz) and T_1? (1–100 kHz) measurements. Since the average molecular hopping time (τ) can be changed by varying temperature, different regions of the spectral density can be examined. For T > 200 K, both T^?1₁ and T^?1_1? vary logarithmically with frequency, reflecting the two dimensional character of the molecular diffusion. The temperature dependence of T₁ suggests that a more accurate picture of the short time dynamics is required. No dependence of relaxation rate on vacancy concentration is found.     

Temperature dependence of the spin-lattice relaxation time for quadrupole nuclei under conditions of NMR line saturation

The contributions of different mechanisms of nuclear spin-lattice relaxation are experimentally separated for ⁶⁹Ga and ⁷¹Ga nuclei in GaAs crystals (nominally pure and doped with copper and chromium), ²³Na nuclei in a nominally pure NaCl crystal, and ²⁷Al nuclei in nominally pure and lightly chromium-doped Al₂O₃ crystals in the temperature range 80–300 K. The contribution of impurities to spin-lattice relaxation is separated under the condition of additional stationary saturation of the nuclear magnetic resonance (NMR) line in magnetic and electric resonance fields. It is demonstrated that, upon suppression of the impurity mechanism of spin-lattice relaxation, the temperature dependence of the spin-lattice relaxation time T₁ for GaAs and NaCl crystals is described within the model of two-phonon Raman processes in the Debye approximation, whereas the temperature dependence of T₁ for corundum crystals deviates from the theoretical curve for relaxation due to the spin-phonon interaction.     

NMR study of one-dimensional ionic conductor with hollandite-type structure. II. Frequency dependence of spin-lattice relaxation time of 27Al

Frequency dependence of spin-lattice relaxation time T₁ of ²⁷Al in one-dimensional K⁺ ion conductor, K-Al-priderite, was measured at 45 K in the frequency range from 10.1 MHz to 55 MHz. It is found that T₁ is proportional to ω^1.49±0.05 and agrres well with the $\text{ω}{}^{\mathrm{<mtext>3</mtext><mtext>4</mtext>}}$ dependence derived by the continuum diffusion model. The intrinsic activation energy is determined to be 0.058 eV by doubling the slope E_NMR=0.029 eV of the d(ln T₁)/dT curve in the low temperature region. The frequency dependence of T₁ in the high temperature region measured in the frequency range from 11.5 MHz to 20.8 MHz shows a tendency that the frequency dependence becomes smaller than the $\text{ω}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ dependence as temperature is raised above 450 K.     

1H NMR study of the α phase of Mg2NiHx system

Hydrogen behavior in the α phase of Mg₂NiH_x system was studied by ¹H NMR. ¹H NMR spectra and spin-lattice relaxation times, T₁ and T_1ρ, of Mg₂NiH_0.22 were measured in the temperature range between 100 and 480 K. The drastic change in the linewidth is observed between 170 and 340 K, and ¹H rigid lattice is observed below 170 K, from which it is deduced that the hydrogen atoms are randomly distributed in α-Mg₂NiH_x. The relaxation mechanism for t₁ is the paramagnetic one, while the T_1ρ value is determined partially by hydrogen diffusion. The hydrogen diffusion rate has been determined from the linewidth and the T_1ρ value. The paramagnetic relaxations observed in T₁ and T_1ρ have been discussed relating to the hydrogen diffusion.     

27Al pulsed nuclear magnetic resonance study of CeAl2

The magnetic properties and the electronic structures of a rare-earth aluminum intermetallic compound CeAl₂ are investigated by magnetic susceptibility measurements and ²⁷Al pulsed nuclear magnetic resonance (NMR) techniques. The magnetic susceptibility is strongly temperature-dependent, following a Curie–Weiss law down to ∼12 K, and shows an antiferromagnetic transition at 4 K. The ²⁷Al NMR spectra show a typical powder pattern for a nuclear spin I of 5/2 with the second-order nuclear quadrupole interaction at high temperature and an additional large dipolar broadening between the 4f electron spins of cerium and the ²⁷Al nuclear spins at low temperature. The ²⁷Al NMR Knight shift follows the same temperature dependence as the magnetic susceptibility, suggesting that the ²⁷Al NMR Knight shift originates from the transferred hyperfine field of the Ce 4f electron spins with the hyperfine coupling constant of A = +5.7 kOe/μ_B. The spin-lattice relaxation rate 1/T₁ is roughly proportional to temperature, as with most non-magnetic metals at high temperature, and then strongly temperature-dependent, increasing rapidly with a peak near the antiferromagnetic transition temperature and decreasing at lower temperature. The temperature dependence of the Korringa ratio K, however, suggests that the antiferromagnetic spin fluctuation signature, which is an enhancement in the Korringa ratio, is washed out owing to the geometrical cancellation of Ce 4f fluctuations at the Al sites.     

Low-temperature spin-lattice relaxation of8Li in the glass Li2O·2SiO2

Nuclear spin-lattice relaxation in Li₂0·2Si0₂ glass below 200 K has been studied using the asymmetric Β-decay radiation of polarized⁸Li (T_1/2=0.Bs) nuclei produced by capture of polarized neutrons. Transients of the⁸Li polarization follow an exp(?√E/T₁) law. The dependence of the spin-lattice relaxation rate ⊥_¯1 ¹ on temperature T and magnetic field B can roughly be described by T_¯1 ¹～T/B. The interpretation is based on the assumption that for⁸Li, contrary to⁷Li in the same glass, spin-diffusion is absent and that each probe nucleus is coupled by quadrupolar interaction to an individual distribution of nearby centres typical of glasses. The fluctuation of these centres causing relaxation may be induced by either a multi-phonon or a thermally activated motional process.     

Nuclear spin-lattice relaxation due to non-linear excitations in magnetic chains

A simple classical model for nuclear spin-lattice relaxation due to solitons in XY one-dimensional magnetic chains in an external magnetic field is discussed. The results show that one should expect opposite behavior for the H/T dependence of T^-1₁ for the ferromagnetic and the antiferromagnetic case.     

H and K spin-lattice relaxation, ionic motion and Raman processes in the proton conducting KHSeO4 single crystal

The spin-lattice relaxation rates of ¹H and ³⁹K nuclei in KHSeO₄ crystals were studied in the temperature range 160-400 K. The spin-lattice relaxation recovery of ¹H nucleus in this crystal can be represented with a single exponential function, and the relaxation T₁⁻¹ curve of ¹H can be represented with the Bloembergen-Purcell-Pound (BPP) function. The relaxation process of ³⁹K with dominant quadrupole relaxation can be described by a linear combination of two exponential functions. T₁⁻¹ for the ³⁹K nucleus was found to have a very strong temperature dependence, T₁⁻¹=βT⁷. Rapid variations in relaxation rates are associated with critical fluctuations in the electronic spin system. The T⁷ temperature dependence of the Raman relaxation rate is shown here to be due to phonon-magnon coupling.     

Charge density wave transition in Na2Ti2Sb2O probed by 23Na NMR

Herein we investigated the electronic properties of layered transition-metal oxides Na2Ti2Sb2O by23Na nuclear magnetic resonance(NMR)measurement.The resistivity,susceptibility and specific heat measurements show a phase transition at approximately 114 K(TA).No splitting or broadening in the central line of23Na NMR spectra is observed below and above the transition temperature indicating no internal field being detected.The spin-lattice relaxation rate divided by T(1/T1T)shows a sharp drop at about 110 K which suggests a gap opening behavior.Below the phase transition temperature zone,1/T1T shows Fermi liquid behavior but with much smaller value indicating the loss of large part of electronic density of states(DOS)because of the gap.No signature of the enhancement of spin fluctuations or magnetic order is found with the decreasing temperature.These results suggest a commensurate charge-density-wave(CDW)phase transition occurring.     

6,7Li NMR study of ion mobility on the molecular scale in lithated imidazole complexes

Two model compounds, lithium imidazolium (LiIm) and lithium 2-undecylimidazolium (und-LiIm), were synthesized. These materials are chosen as models of potential lithium ion conductors for use as electrolytes in lithium batteries. Solid-state NMR was used to provide information on the microscopic interactions including ionic mobility and ring reorientations which govern the efficiency of conductivity. Lithium imidazolium was mixed with lithium methylsulfonate, generating a doped complex in which a doubly lithiated imidazole ring was inferred based on the ⁷Li NMR chemical shifts. Our research includes ^6,7Li variable temperature MAS NMR experiments at intermediate spinning speeds, relaxation studies to determine spin-lattice relaxation times (T₁) of lithium ion hopping, and 2D exchange spectroscopy to determine possible chemical exchange processes. The possibility of 2-site ring reorientation for the doubly lithiated imidazole ring was supported by exchange spectroscopy. Comparisons of spin-lattice relaxation times and corresponding activation energies of the lithium imidazolium and the doped complex point to a higher degree of mobility in the latter.Lithium 2-undecylimidazolium was prepared and exhibited a lower melting point than the parent lithium imidazolium, as expected. This small molecule was chosen as representative of a side-chain functionalized polyethylene-based material. ⁷Li MAS spectra show mainly the presence of the doubly lithiated imidazole ring in pure und-LiIm, and in the LiCH₃SO₃–und-LiIm mixture. The data clearly indicate local mobility of the lithium ions in the materials.