NMR Spin-lattice Relaxation Research Articles

Diffusion-induced 7Li NMR relaxation of layer-structured tin disulphide — Li diffusion along the buried interfaces in Li0.17SnS2

7Li NMR relaxation has been used to study lithium-ion diffusion in layer-structured SnS2. Keeping the Li intercalation degree in LixSnS2 below x=0.49, the Li ions preferentially occupy sites in the van der Waals gap between the SnS2 sheets. In contrast to conventional NMR spin-lattice relaxation (SLR) rate measurements in the laboratory frame of reference, which are sensitive to rather fast Li exchange processes, with the help of spin-locking SLR NMR slower Li motions were extracted from characteristic diffusion-induced rate peaks. The latter contain information on both Li+ activation energies Ea and Li ion jump rates τ−1 characterizing the elementary steps of Li+ hopping. Our results point to two different diffusion processes (Ea(I)=0.38eV; Ea(II)=0.28eV), a slower and a faster one, observable directly after chemical Li insertion. Interestingly, the diffusion behaviour irreversibly changes when the sample has been exposed to temperatures as high as 573K. Diffusion-induced NMR rates and corresponding line shapes are discussed with respect to an inhomogenous distribution of Li ions in SnS2, which seems to be present directly after Li intercalation.

Phase analysis and determination of local charge carrier concentration in eutectic Mg2Si–Si alloys

Multiphase materials attract attention due to possible combination of various properties attributed to each phase. The phase diagram of Mg–Si system shows that solidification of a melt containing about 45 and 55 at.% of Mg and Si should result in formation of Mg2Si and Si. Two alloys, Mg45Si55 and Mg46Si54 + 0.5 wt.% Cu have been synthesized and studied using XRD, SEM, and 29Si NMR at 300 K, and the Seebeck effect, electrical resistivity, and thermal conductivity in the temperature range of 300–750 K have been measured. 29Si NMR detects two distinct signals, at −177 and −80 ppm, in both materials, which are assigned to Mg2Si and Si phases, respectively. Both phases are slightly nonstoichiometric and doped with Mg. Two phases also are found by XRD and electron microscopy. 29Si NMR spin-lattice relaxation measurements in Mg2Si and Si phases show at least two components, short and long, which can be attributed to different local carrier concentrations, high and low, respectively, reflecting a local electronic inhomogeneity in each phase. The carrier concentrations range between 0.6 × 1019 and 9 × 1019 cm−3. The Seebeck coefficient in both alloys is mostly determined by the Si phase, while the thermal conductivity is limited by the Mg2Si phase with a lower value than that of the Si phase. By utilizing all characterization tools, we show how various experimental methods can be used as complementary methods to better understand the individual and combined properties of multiphase alloys.

NMR spin–lattice relaxation time T1 of thin films obtained by magnetic resonance force microscopy

We obtained the NMR spectrum and the spin–lattice relaxation time (T1) for thin film samples by magnetic resonance force microscopy (MRFM). The samples were CaF2 thin films which were 50nm and 150nm thick. T1 was measured at 18K using a cyclic adiabatic inversion method at a fixed frequency. A comparison of the bulk and two thin films showed that T1 becomes shorter as the film thickness decreases. To make the comparison as accurate as possible, all three samples were loaded onto different beams of a multi-cantilever array and measured in the same experimental environment.

Computational Identification and Experimental Realization of Lithium Vacancy Introduction into the Olivine LiMgPO4

Calculation of the energetics of aliovalent substitution into the olivine LiMgPO4 suggests that replacement of Mg2+ by In3+ is the most effective way to introduce lithium vacancies and thus generate Li ion conductivity. Experimental synthesis accesses materials with up to 17% Li vacancy content. An order-of-magnitude increase in the high-temperature hopping rates probed by 7Li NMR spin–lattice relaxation, and over 2 orders of magnitude increase in the room-temperature Li+ ion conductivity measured by impedance spectroscopy is observed upon the introduction of In3+ ions and Li vacancies. NMR spectroscopy and calculations reveal that the energy barrier to site-to-site hopping is 0.3–0.5 eV, comparable with best-in-class nonoxide systems such as argyrodite, but NMR-derived hopping rates, and impedance spectroscopy shows that longer range transport is less facile with activation energies in the range of 0.7–1 eV. Calculations suggest that this is because the Li vacancies are strongly bound to the In3+ dopants...

NO adsorption in amino-modified Cu3(btc)2-type MOFs studied by solid-state NMR

Adsorption of nitric oxide – NO – in the MOFs Cu3(btc)2 (HKUST-1) and Cu3(NH2btc)2 (UHM-30) was studied via 1H solid-state NMR. Changes to the electronic properties by adding paramagnetic molecules to antiferromagnetically coupled copper ions is evident in 1H NMR shift, hyperfine coupling, and spin-lattice relaxation times (T1). Furthermore, in the case of Cu3(NH2btc)2, the results indicate the formation of NONOates.

Phase Behavior, Diffusion, Structural Characteristics, and pH of Aqueous Hydrophobic Ionic Liquid Confined Media: Insights into Microviscosity and Microporsity in the [C4C4im][NTf2] + Water System

We present our studies on the physicochemical properties of water confined in Dibutylimidazolium bis(trifluoromethanesulfonylimide) ([C4C4im][NTf2]) reverse micelles through the NMR relaxation measurements that provide us an understanding of microviscosity and pH in the confined condition. We present experimental results on phase behavior, diffusion, structural characteristics and pH in aqueous ionic liquid-confined media. The ternary phase diagram was constructed by the cloud point measurements and the microheterogeneous regions were detected by the measurement of bulk viscosity and diffusion coefficients of K4[Fe(CN)6] inside the homogeneous microemulsion systems through the cyclic voltammetric (CV) measurements. The size of the microemulsion systems was characterized by the dynamic light scattering (DLS) method. The (1)H NMR spectra of homogeneous microemulsion systems were taken which indicates the presence of bound and free water molecules inside the microemulsion system. The NMR spin-lattice relaxation time (T1) of water molecules in its homogeneous microemulsion systems were measured and the reorientational correlation time (τc) of water molecules obtained from it indicates that the fluidity of homogeneous confined media decreases with the decrease in the composition of water. Microviscosity of the aqueous confined media was calculated from the measured T1 relaxation time values by applying the Debye-Stokes equation and correlated with the bulk viscosity of the samples. It was observed that both the microviscosity and bulk viscosity show inverse relationship. The fraction of bound and free water molecules were calculated from the measured T1 values. NMR spin-spin relaxation time (T2) of water molecules in its homogeneous microemulsion systems were measured with the varying pH of the aqueous core. A change in the T2 relaxation time of the water proton was observed proposing an exchange of proton between the H2O and -OH group of the TX-100 molecules. Finally, methyl orange (MO) was used as a UV-vis spectrophotometric molecular probe and the measured λmax values of the probe were used for the detection of micropolarity of the homogeneous aqueous confined media and was found to be increase with the increase in the size of the confined media.

Molecular dynamics simulation of spin-lattice NMR relaxation in poly-L-lysine dendrimers: manifestation of the semiflexibility effect.

NMR relaxation experiments are widely used to investigate the local orientation mobility in dendrimers. In particular, the NMR method allows one to measure the spin-lattice relaxation rate, 1/T1, which is connected with the orientational autocorrelation function (ACF) of NMR active groups. We calculate the temperature (Θ) and frequency (ω) dependences of the spin-lattice NMR relaxation rates for segments and NMR active CH2 groups in poly-L-lysine (PLL) dendrimers in water, on the basis of full-atomic molecular dynamics simulations. It is shown that the position of the maximum of 1/T1(ω) depends on the location of the segments inside the dendrimer. This dependence of the maximum is explained by the restricted flexibility of the dendrimer. Such behavior has been predicted recently by the analytical theory based on the semiflexible viscoelastic model. The simulated temperature dependences of 1/T1 for terminal and inner groups in PLL dendrimers of n = 2 and n = 4 generations dissolved in water are in good agreement with the NMR experimental data, which have been obtained for these systems previously by us. It is shown that in the case of PLL dendrimers, the traditional procedure of the interpretation of NMR experimental data - when smaller values of 1/T1 correspond to higher orientation mobility - is applicable to the whole accessible frequency interval only for the terminal groups. For the inner groups, this procedure is valid only at low frequencies.

Na(+) mobility in sodium strontium silicate fast ion conductors.

We present the first direct evidence of Na-ion mobility in sodium strontium silicate fast ion conductors, based on variable temperature (23)Na solid state NMR spectroscopy and spin-lattice relaxation measurements.

Exploring the Role of Phosphate Structural Distortions on the Sodium Jump Dynamics in NASICON Phases

ABSTRACTHigh temperature solid state sodium (23Na) magic angle spinning (MAS) NMR spin lattice relaxation times (T1) were evaluated for a series of NASICON (Na3Zr2PSi2O12) materials to directly determine Na jump rates. Simulations of the T1 temperature variations that incorporated distributions in Na jump activation energies, or distribution of jump rates, improved the agreement with experiment. The 23Na NMR T1 relaxation results revealed that distributions in the Na dynamics were present for all of the NASICON materials investigated here. The 23Na relaxation experiments also showed that small differences in material composition and/or changes in the processing conditions impacted the distributions in the Na dynamics. The extent of the distribution was related to the presence of a disordered or glassy phosphate phase present in these different sol-gel processed materials. The 23Na NMR T1 relaxation experiments are a powerful tool to directly probing Na jump dynamics and provide additional molecular level details that could impact transport phenomena.

Universal Knight shift anomaly in the periodic Anderson model

We report a Determinant Quantum Monte Carlo investigation which quantifies the behavior of the susceptibility and the entropy in the framework of the periodic Anderson model (PAM), focussing on the evolution with different degree of conduction electron (c) -local moment (f) hybridization. These results capture the behavior observed in several experiments, including the universal behavior of the NMR Knight shift anomaly below the crossover temperature, $T^{\ast}$. We find that $T^{\ast}$ is a measure of the onset of c-f correlations and grows with increasing hybridization. These results suggest that the NMR Knight shift and spin-lattice relaxation rate measurements in non-Fermi liquid materials are strongly influenced by temperature-dependent hybridization processes. Our results provide a microscopic basis for the phenomenological two-fluid model of Kondo lattice behavior, and its evolution with pressure and temperature.

β-detected NMR ofLi8+in Bi, Sb, and the topological insulatorBi0.9Sb0.1

We report the NMR Knight shift and spin-lattice relaxation of $^{8}\mathrm{Li}{}^{+}$ implanted $\ensuremath{\sim}100$ nm into single crystals of semimetallic Sb, Bi, and topologically insulating ${\mathrm{Bi}}_{0.9}{\mathrm{Sb}}_{0.1}$. We find small negative shifts (of order 100 ppm) in all three. In the insulator, the shift is nearly temperature independent, while in Bi and Sb it becomes more negative at low temperature without following the bulk susceptibility, suggesting two distinct temperature dependent contributions, possibly from the orbital and spin response. However, a simple model is unable to account for the observed shift. The spin-lattice relaxation differs in both scale and temperature dependence in all three. It is Korringa-like in Bi and remarkably is fastest in the insulating alloy and slowest in Sb with the highest bulk carrier density. These surprising results call for detailed calculations, but phenomenologically demonstrate that $\ensuremath{\beta}$-detected NMR of implanted $^{8}\mathrm{Li}{}^{+}$ is sensitive to the magnetic response of low-density carriers. The prospects for depth-resolved studies of conventional and topological surface states at lower implantation energies are good.

A variable temperature study of the transport properties of aqueous solutions of VOSO4 and NH4VO3 in 2 M H2SO4

Variable temperature ionic conductivity, viscosity and 1H NMR spin-lattice relaxation times, linewidths and chemical shifts measurements were determined for various concentrations of aqueous ammonium metavanadate (NH4VO3) and vanadyl sulfate (VOSO4) over the temperature range of 20–100 °C. Concentration ranges of 0.05–2 M and 0.05–0.2 M were investigated for VOSO4 and NH4VO3 respectively in 2 M H2SO4. Results show ion and proton transport processes that are mediated strongly by increasing solution viscosity, especially in the case of VOSO4. Activation energies calculated from ionic conductivity and solution viscosity show greater energy being needed for translational ion dynamics with increasing concentration for both solutions. Additionally, data suggest that with greater metal ion concentration ion association is more prevalent for the vanadyl ion. The differences between the transport capabilities of the two solution are discussed in terms of the 1H NMR, ionic conductivity and viscosity data.

Complex dynamics of 1.3.5-trimethylbenzene-2.4.6-D3 studied by proton spin–lattice NMR relaxation and second moment of NMR line

Molecular dynamics of a solid 1.3.5-trimethylbenzene-2.4.6-D3 in phase I is studied on the basis of the proton T1 (24.7MHz and 15MHz) relaxation time measurements and the proton second moment of NMR line, M2. The measurements of the T1 were performed for temperatures from 20 to 167K, while those of the second moment M2 from 23 to 220K. The phase I was accurately prepared. The obtained second moment, M2 values were correlated with those based on T1 relaxation time measurements. The proton spin pairs of the methyl groups perform a complex motion being a resultant of two components characterized by the correlation times τ3T and τ3H, referring to the tunneling and over the barrier jumps in a triple potential. Forτ3Hthe Arrhenius temperature dependence was assumed, while forτ3T – the Schrödinger one. The jumps over the barrier causes a minimum in T1 (24.7MHz) at temperature about 35K. The high temperatures slope of this minimum permits evaluation of the activation energy as EH=2.0kJ/mol. The relaxation time T1 is temperature independent in the lowest temperature regime. This indicates that tunnelling correlation time assumes a constant value of about 1.3·10-10s according to the Schrödinger equation (τ3T≈τ03TeBEH at lowest temperatures). The tunneling jumps of methyl protons reduce M2 from the rigid lattice value 22.6G2 to the value 5.7G2 at zero Kelvin temperature. The second reduction to the value 1.41G2 at 4.5–7K is due to C3 jumps over the barrier. According to the Schrödinger equation the tunnelling jumps ceases above Ttun temperature where the thermal energy is equal to the activation energy. The Ttun equals 43.8K (from T1 data fit, EH=2.0kJ/mol) or 35K (from M2 data fit, EH=1.47kJ/mol). The second moment assumes again the value 5.7G2 above Ttun temperature.The tunneling splitting, ωT, was estimated equal 2.47GHz as best fit parameter from the T1 fit. The symmetrical T1 minimum indicates the same value of ωT for the all methyl groups. This frequency is in good agreement with the value of ωT (ℏωT=10.2μeV, tunnel splitting energy) obtained from the neutron powder scattering method. This high tunneling splitting is responsible for the long and Larmor frequency independent of T1 relaxation time. The presented results are compared to those of Köksal et al.

Pressure-Collapsed Amorphous Mg(BH4)2: An Ultradense Complex Hydride Showing a Reversible Transition to the Porous Framework

Hydrogen-storage properties of complex hydrides depend of their form, such as a polymorphic form or an eutectic mixture. This Paper reports on an easy and reproducible way to synthesize a new stable form of magnesium borohydride by pressure-induced collapse of the porous γ-Mg(BH4)2. This amorphous complex hydride was investigated by temperature-programmed synchrotron X-ray diffraction (SXRD), transmission electron microscopy (TEM), thermogravimetric analysis, differential scanning calorimetry analysis, and Raman spectroscopy, and the dynamics of the BH4– reorientation was studied by spin–lattice relaxation NMR spectroscopy. No long-range order is observed in the lattice region by Raman spectroscopy, while the internal vibration modes of the BH4– groups are the same as in the crystalline state. A hump at 4.9 Å in the SXRD pattern suggests the presence of nearly linear Mg–BH4–Mg fragments constituting all the known crystalline polymorphs of Mg(BH4)2, which are essentially frameworks built of tetrahedral Mg nodes and linear BH4 linkers. TEM shows that the pressure-collapsed phase is amorphous down to the nanoscale, but surprisingly, SXRD reveals a transition at ∼90 °C from the dense amorphous state (density of 0.98 g/cm3) back to the porous γ phase having only 0.55 g/cm3 crystal density. The crystallization is slightly exothermic, with the enthalpy of −4.3 kJ/mol. The volumetric hydrogen density of the amorphous form is 145 g/L, one of the highest among hydrides. Remarkably, this form of Mg(BH4)2 has different reactivity compared to the crystalline forms. The parameters of the reorientational motion of BH4 groups in the amorphous Mg(BH4)2 found from NMR measurements differ significantly from those in the known crystalline forms. The behavior of the nuclear spin–lattice relaxation rates can be described in terms of a Gaussian distribution of the activation energies centered on 234 ± 9 meV with the dispersion of 100 ± 10 meV.

Probing the structural evolution during the geopolymerization process at an early age using proton NMR spin-lattice relaxation

Proton nuclear magnetic resonance (1H NMR) was used to characterize the geopolymerization process of a metakaolin-based geopolymer. In the spin-lattice relaxation time (T1) distribution curve obtained for the first 15-min time point, two relaxation peaks were observed, which were assigned to two different water phases in the initial paste. Based on the evolution curve of the average mean value of T1, the geopolymerization process in the early age can be divided into four successive stages: the induction period, acceleration period, deceleration period and stabilization period. This work reveals that 1H NMR is a noninvasive and non-destructive method for the analysis of structural evolution during the geopolymerization process.

Structure and tunneling splitting spectra of methyl groups of tetramethylpyrazine in complexes with chloranilic and bromanilic acids.

The crystal and molecular structure of the 2,3,5,6-tetramethylpyrazine (TMP) complex with 2,5-dibromo-3,6-dihydroxy-p-quinone (bromanilic acid, BRA) has been studied and the results are compared with TMP CLA (2,5-dichloro-3,6-dihydroxy-p-quinone (chloranilic acid, CLA) complex. The X-ray structure of TMP BRA complex indicates the formation of dimeric units, in which two BRA(-) anions are connected by two O-H···O (2.646(2) Å) hydrogen bonds, whereas the cations and anions are joined together by strong N(+)-H···O(-) (2.657(2) Å) hydrogen bonds. The results are analyzed in terms of both the methyl group surroundings and the C-H···O and N(+)-H···O(-) (or N···H-O) bridge formations. Both effects, the strength of the N(+)-H···O(-) hydrogen bonds and steric hindrance for the rotations, are responsible for the CH3 group dynamics. For the TMP CLA and TMP BRA complexes, the inelastic neutron backscattering spectra were also investigated. In the case of TMP CLA, four tunneling signals have been observed in the energy range ±30 μeV, which indicates four inequivalent methyl groups in the crystal structure at the lowest temperature. No tunneling splitting is observed in the case of the TMP BRA complex, most probably due to the overlapping with the elastic peak. The tunneling results are consistent with the (1)H NMR spin-lattice relaxation time investigations in a wide temperature range, which also point to the CH3 group tunneling effect in the case of TMP CLA.

Water confinement in faujasite cages: a deuteron NMR investigation in a wide temperature range. 2. Spectra and relaxation at high temperature.

Deuteron NMR spectra and spin-lattice relaxation were measured for D2O confined in NaX, NaY, and DY faujasites with various loadings at temperatures ranging from 200 to 310 K with the aim to study molecular mobility of confined water. Hysteresis of spin-lattice relaxation was observed for both DY and NaY(2.4) samples at 500% loading (280 water molecules per unit cell) in a heating-cooling cycle between 264.5 and 277.7 K. The hysteresis is most likely reflecting formation and decomposition of water clusters at different temperature. Spin-lattice relaxation rates obtained from the experiment are consistent with a picture of the fast magnetization exchange between two dynamically different deuteron populations. The observed relaxation behavior as a function of temperature and loading is most likely an effect of interplay between translational and rotational diffusion. Translational diffusion of water molecules is found to be related to the strength of the electrostatic interaction of water oxygen atoms to faujasite sodium cations, whereas water molecule reorientations seem to depend on the strength of hydrogen bonding to faujasite oxygen atoms and the strength of hydrogen bonds between water molecules, at outer and inner positions in water clusters, respectively.

Nucleus-phonon interactions of MCsSO4(M = Na, K, or Rb) single crystals studied using spin-lattice relaxation time

The structural properties and relaxation processes of <TEX>$MCsSO_4$</TEX> (M = Na, K, or Rb) crystals were investigated by measuring the NMR spectra and spin-lattice relaxation rates <TEX>$1/T_1$</TEX> of their <TEX>$^{23}Na$</TEX>, <TEX>$^{39}K$</TEX>, <TEX>$^{87}Rb$</TEX>, and <TEX>$^{133}Cs$</TEX> nuclei. According to the NMR spectra, the <TEX>$MCsSO_4$</TEX> crystals contain two crystallographically inequivalent sites each for the M and Cs ions. Further, the relaxation rates of all these nuclei do not change significantly over the investigated temperature range, indicating that no phase transitions occur in these crystals in this range. The variations in the <TEX>$1/T_1$</TEX> values of the <TEX>$^{23}Na$</TEX>, <TEX>$^{39}K$</TEX>, <TEX>$^{87}Rb$</TEX>, and <TEX>$^{133}Cs$</TEX> nuclei in these three crystals with increasing temperature are approximately proportional to <TEX>$T^2$</TEX>, indicating that Raman processes may be responsible for the relaxation. Therefore, for nuclear quadrupole relaxation of the <TEX>$^{23}Na$</TEX>, <TEX>$^{39}K$</TEX>, <TEX>$^{87}Rb$</TEX>, and <TEX>$^{133}Cs$</TEX> nuclei, Raman processes with n = 2 are more effective than direct processes.

Local composition and carrier concentration in Pb0.7Ge0.3Te and Pb0.5Ge0.5Te alloys from 125Te NMR and microscopy

Pb0.7Ge0.3Te and Pb0.5Ge0.5Te alloys, (i) quenched from 923K or (ii) quenched and annealed at 573K for 2h, have been studied by 125Te NMR, X-ray diffraction, electron and optical microscopy, as well as energy dispersive spectroscopy. Depending on the composition and thermal treatment history, 125Te NMR spectra exhibit different resonance frequencies and spin-lattice relaxation times, which can be assigned to different phases in the alloy. Quenched and annealed Pb0.7Ge0.3Te alloys can be considered as solid solutions but are shown by NMR to have components with various carrier concentrations. Quenched and annealed Pb0.5Ge0.5Te alloys contain GeTe- and PbTe-based phases with different compositions and charge carrier concentrations. Based on the analysis of non-exponential 125Te NMR spin-lattice relaxation, the fractions and carrier concentrations of the various phases have been estimated. Our data show that alloying of PbTe with Ge results in the formation of chemically and electronically inhomogeneous systems. 125Te NMR can be used as an efficient probe to detect the local composition in equilibrium as well as non-equilibrium states, and to determine the local carrier concentrations in complex multiphase tellurides.

Experimental and solid-state computational study of structural and dynamic properties in the equilibrium form of temazepam.

Structural properties and rotational dynamics of methyl groups in the most stable form of temazepam were investigated by means of (13)C CP MAS NMR, quasielastic neutron scattering (QENS), and (1)H NMR spin-lattice relaxation methods. The QENS and (1)H NMR studies reveal the inequivalency of methyl groups, delivering their activation parameters. The structural properties of the system were explored in frame of periodic density functional theory (DFT) computations, giving insight into the reorientational barriers and providing understanding of the solid-state NMR results. The theoretical computations are shedding light on the intermolecular interactions along their relation with particular asymmetric structural units.