NMR Spin-lattice Relaxation Measurements Research Articles

Fast Na+ ion dynamics in the Nb5+ bearing NaSICON Na3+x−zNbzZr2−zSi2+xP1−xO12 as probed by 23Na NMR and conductivity spectroscopy

NaSICON-type (Na superionic conductor, Na1+yZr2SiyP3−yO12, 0 < y < 3) represent a class of fast Na+ ion conductors with bulk ion conductivities in the mS range near room temperature. Starting from Na3Zr2(SiO4)2(PO4), i.e., from y = 2, we investigated the change in ion transport parameter when x in Na3+xZr2Si2+xP1−xO12 is increased from 0 to 0.4 and Nb5+ is substituted for Zr4+ according to Na3+x−zNbzZr2−zSi2+xP1−xO12 (z = 0.04). The later might be beneficial also to change the transport properties in or near the grain boundary regions of our polycrystalline samples. To detect such changes, we used low-temperature conductivity and electric modulus spectroscopy to differentiate between the total and the bulk (intragrain) electrical responses. However, the main purpose of the present study is to corroborate the extraordinarily high bulk ionic conductivity of approximately 7.3 mS cm−1 (x = 0.4 and z = 0.04, room temperature) by diffusion-controlled 23Na NMR spin-lattice relaxation measurements. Indeed, such experiments directly sense the 23Na spin-fluctuations at the nuclear sites as a result of the fast intragrain motional processes. Both methods, nuclear and non-nuclear, point to a mean activation energy of 0.3 eV for Na+ exchange among the multiple sites in the NaSICON framework. In addition, NMR reveals local barriers characterized by activation energies as low as 0.1 eV. Altogether, from a dynamic point of view, Na3.36Nb0.04Zr1.96Si2.4P0.6O12 turned out to be indeed a very promising candidate for the breakneck development of Na-based all-solid-state battery applications.

Investigation of Dynamic Behavior of Confined Ionic Liquid [BMIM]+[TCM]- in Silica Material SBA-15 Using NMR.

The molecular dynamics of 1-butyl-3-methyl imidazolium tricyanomethanide ionic liquid [BMIM]+[TCM]- confined in SBA-15 mesoporous silica were examined using 1H NMR spin-lattice (T1) relaxation and diffusion measurements. An extensive temperature range (100 K-400 K) was considered in order to study both the liquid and glassy states. The hydrogen dynamics in the two states and the self-diffusion coefficients of the cation [BMIM]+ above the glass transition temperature were extracted from the experimental data. The results were then compared to the corresponding bulk substance. The effects of confinement on the dynamic properties of the ionic liquid clearly manifest themselves in both temperature regimes. In the high-temperature liquid state, the mobility of the confined cations reduces significantly compared to the bulk; interestingly, confinement drives the ionic liquid to the glassy state at a higher temperature Tg than the bulk ionic liquid, whereas an unusual T1 temperature dependence is observed in the high-temperature regime, assigned to the interaction of the ionic liquid with the silica-OH species.

On the Dependence of Ionic Transport on Crystal Orientation in Nasicon-Type Solid Electrolytes

The dependence of ionic transport on crystal orientations in NaSICON-type solid electroytes is studied on flux-grown M3Sc2(PO4)3 (M = Na, Ag) single crystals with well-defined facets. Herein, we provide the first impedance spectroscopy study to characterize ion conduction along different crystallographic orientations in this important class of materials for electrochemical energy storage systems. Moreover, we used single crystal X-ray diffraction, differential scanning calorimetry, 23Na NMR spin-lattice relaxation measurements, and ab initio molecular dynamics simulations to study the interplay of structure and ion transport taking place at different length scales. We conclude that the phase behavior in NaSICON-type materials is strongly linked to ion diffusion. At room temperature, ionic conductivity is slightly anisotropic along the crystallographic orientations [001] and [100]. The slightly different activation energies are related to diffusion bottlenecks solely changing along [001]. This change is caused by anisotropic thermal lattice expansion. With increasing temperature, ion transport increasingly becomes isotropic finally resulting in an order-disorder phase transition from C2/c to R−3c. This phase transition is associated with a clear change in activation energy solely along [001]; it can be traced back to the increasing jump distance along this crystal orientation with temperature. Astonishingly, changing the ionic charge carrier, i.e., when going from Na+ to Ag+, shifts the phase transition temperature by 140 K toward lower temperature. The Arrhenius behaviour remains, however, similar. This finding is related to the higher mobility of Ag+ in the NaSICON framework leading to isotropic ion diffusion at much lower temperatures. Overall, flux-grown M3Sc2(PO4)3 allowed us to show that ionic transport parameters and phase stability sensitively depend on crystal chemistry.

On the dependence of ionic transport on crystal orientation in NaSICON-type solid electrolytes

The dependence of ionic transport on crystal orientations in NaSICON-type solid electrolytes is studied on flux-grown M3Sc2(PO4)3 (M = Na, Ag) single crystals with well-defined facets. Herein, we provide the first impedance spectroscopy study to characterize ion conduction along different crystallographic orientations in this important class of materials for electrochemical energy storage systems. Moreover, we used single crystal x-ray diffraction, differential scanning calorimetry, 23Na NMR spin-lattice relaxation measurements, and ab initio molecular dynamics simulations to study the interplay of structure and ion transport taking place at different length scales. We conclude that the phase behavior in NaSICON-type materials is strongly linked to ion diffusion. At room temperature, ionic conductivity is slightly anisotropic along the crystallographic orientations [001] and [100]. The slightly different activation energies are related to diffusion bottlenecks solely changing along [001]. This change is caused by anisotropic thermal lattice expansion. With increasing temperature, ion transport increasingly becomes isotropic finally resulting in an order-disorder phase transition from C2/c to R−3c. This phase transition is associated with a clear change in activation energy solely along [001]; it can be traced back to the increasing jump distance along this crystal orientation with temperature. Astonishingly, changing the ionic charge carrier, i.e. when going from Na+ to Ag+, shifts the phase transition temperature by 140 K towards lower temperature. The Arrhenius behavior remains, however, similar. This finding is related to the higher mobility of Ag+ in the NaSICON framework leading to isotropic ion diffusion at much lower temperatures. Overall, flux-grown M3Sc2(PO4)3 allowed us to show that ionic transport parameters and phase stability sensitively depend on crystal chemistry.

Evaluating the Ion Transport of 1-Ethyl-3-Methylimidazolium Acetate Solutions Containing Carbohydrate Solutes

This study evaluated the ion transport properties of 1-ethyl-3-methylimidazolium acetate (EMIAc)-based solutions containing additional acetonitrile (AN), H2O, and up to 3.0 wt-% carbohydrate solutes. Solution ion conductivity was directly measured via frequency response, and ion self-diffusivity coefficients determined via pulsed-field gradient spin-echo (PGSE) NMR. These data combined to determine solution ionicity. Multiple linear regression analysis shows ionicity is primarily impacted by the added neutral solvent, and weakly impacted by temperature. Addition of equimolar AN or H2O increases both cation and anion self-diffusivity (D+ and D−, respectively), and each of these solvent systems decreases the D+/D− quotient. Addition of carbohydrate solutes decreases cation and anion self-diffusivity, but solute additions slightly increase the D+/D− quotient. Our observation that carbohydrate solutes reduce Ac self-diffusivity by a greater fraction than EMI supports the general assertion that acetate anions are more heavily involved in solvating cellulose than EMI cations. Finally, EMIAc solutions containing microcrystalline cellulose (MCC) were evaluated by NMR spin-lattice relaxation (T1) measurements. These T1 data reveal the microviscosity of EMIAc:MCC solutions is virtually unchanged with addition of up to 3.0 wt-% solute, in agreement with the ion transport properties measured at the same scale.

Exploring catalyst passivation with NMR relaxation.

NMR relaxation has recently emerged as a novel and non-invasive tool for probing the surface dynamics of adsorbate molecules within liquid-saturated mesoporous catalysts. The elucidation of such dynamics is of particular relevance to the study and development of solvated green catalytic processes, such as the production of chemicals and fuels from bio-resources. In this paper we develop and implement a protocol using high field 1H NMR spin-lattice relaxation as a probe of the reorientational dynamics of liquids imbibed within mesoporous oxide materials. The observed relaxation of liquids within mesoporous materials is highly sensitive to the adsorbed surface layer, giving insight into tumbling behaviour of spin-bearing chemical environments at the pore surface. As a prototypical example of relevance to liquid-phase catalytic systems, we examine the mobility of liquid methanol within a range of common catalyst supports. In particular, through the calculation and comparison of a suitable interaction parameter, we assess and quantify changes to these surface dynamics upon replacing surface hydroxyl groups with hydrophobic alkyl chains. Our results indicate that the molecular tumbling of adsorbed methanol is enhanced upon surface passivation due to the suppression of surface-adsorbate hydrogen bonding interactions, and tends towards that of the unrestricted bulk liquid. A complex analysis in which we account for the influence of changing pore structure and surface chemistry upon passivation is discussed. The results presented highlight the use of NMR spin-lattice relaxation measurements as a non-invasive probe of molecular dynamics at surfaces of interest to liquid-phase heterogeneous catalysis.

In Situ NMR Measurements of Vapor Deposited Ice

In situ NMR spin–lattice relaxation measurements were performed on several vapor deposited ices. The measurements, which span more than 6 orders of magnitude in relaxation times, show a complex spin–lattice relaxation pattern that is strongly dependent on the growth conditions of the sample. The relaxation patterns change from multitime scale relaxation for samples grown at temperatures below the amorphous–crystalline transition temperature to single exponential recovery for samples grown above the transition temperature. The slow-relaxation contribution seen in cold-grown samples exhibits a temperature dependence, and becomes even slower after the sample is annealed at 200 K. The fast-relaxation contribution seen in these samples, does not seem to change or disappear even when heating to temperatures where the sample is evaporated. The possibility that the fast relaxation component is linked to the microporous structures in amorphous ice samples is further examined using an environmental electron scanning microscope. The images reveal complex mesoscale microporous structures which maintain their morphology up to their desorption temperatures. These findings, support the possibility that water molecules at pore surfaces might be responsible for the fast-relaxation contribution. Furthermore, the results of this study indicate that the pore-collapse dynamics observed in the past in amorphous ices using other experimental techniques, might be effectively inhibited in samples which are grown by relatively fast vapor deposition.

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.

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.

Mechanism of Photoinduced Bending and Twisting in Crystalline Microneedles and Microribbons Composed of 9-Methylanthracene

The solid-state photodimerization of 9-methylanthracene is used as a model system to investigate how crystal morphology and reaction dynamics affect photomechanical deformations of single microcrystals. By varying the crystallization conditions, two different crystal shapes, microneedles and microribbons, are grown on a clean water surface. The microribbons twist under irradiation, while the microneedles bend. In both shapes, the maximum deformation occurs at roughly the midpoint of the reaction, while further dimerization causes the crystals return to their original shapes. Powder X-ray diffraction patterns establish that the needles and ribbons have the same crystal orientation and that the photoreaction proceeds in a crystal-to-crystal manner. NMR spin-lattice relaxation measurements are consistent with the rapid formation of large (>100 nm) dimer crystal domains. Simultaneous measurement of the needle bending and monomer fluorescence signal allows us to correlate the bending with the reaction progress. The behavior is qualitatively reproduced by a model in which the motion is driven by strain between spatially distinct reactant and product domains, also called heterometry. We consider several different mechanisms that could give rise to these spatially distinct domains. The ability to control the photoinduced crystal deformation by manipulating crystal shape and solid-state reaction kinetics suggests that photoreactive molecular crystals may be useful for generating well-defined motions on small length scales.

Proton Conducting Compound of Benzimidazole with Sebacic Acid: Structure, Molecular Dynamics, and Proton Conductivity

Benzimidazole salt of sebacic acid, a new proton conductor from the family of benzimidazole compounds of dicarboxylic acids, was crystallized to search for the factors which determine the hydrogen bond motif and the structure of the crystals. The molecular structure of benzimidazole-sebacic acid salt was solved by using X-ray diffractions and confirmed by 1H and 13C MAS NMR experiments combined with DFT calculations. The salt of sebacic acid, with 10 carbon atoms in the chain, was found to exhibit an undulated layer-type structure with banana-shaped acid molecules linked by O–H···O bonds into rectangular-type wavy chains and flat base molecules attached to the carboxylic groups by N–H···O bonds. The undulated layers are not linked with hydrogen bonds. Comparison of the architecture of benzimidazole salts with weak dicarboxylic acids of shorter carbon chains, studied by us earlier, points at the role of the acid chain length in the formation of structural and hydrogen bond motifs. The dynamics of protons in the ordered crystalline phase and disordered surface layers was characterized by NMR spin–lattice relaxation measurements, whereas complex impedance studies yielded information on the activation energy of proton diffusion in the both phases.

Studying Li Dynamics in a Gas-Phase Synthesized Amorphous Oxide by NMR and Impedance Spectroscopy

Abstract Li diffusion parameters of a structurally disordered Li-Al-Si-oxide prepared by gas-phase synthesis were complementarily investigated by both time-domain NMR techniques and impedance spectroscopy. The first include 7Li NMR spin-lattice relaxation (SLR) measurements in the laboratory as well as in the rotating frame of reference. An analysis of variable-temperature NMR line widths point to an activation energy Ea of approximately 0.6 eV. The value is confirmed by rotating-frame SLR NMR data recorded at approximately 11 kHz. Above room temperature the low-temperature flank of a diffusion-induced rate peak shows up which can be approximated by an Arrhenius law yielding Ea=0.56(1) eV. This is in very good agreement with the result obtained from 7Li spin-alignment echo (SAE) NMR being sensitive to even slower Li dynamics. For comparison, dc-conductivity measurements, probing long-range motions, yield Ea=0.8 eV. Interestingly, low-temperature SAE NMR decay rates point to localized Li motions being characterized with a very small activation energy of only 0.09 eV.

NMR investigation of vortex dynamics in the Ba(Fe0.93Rh0.07)2As2superconductor

75As NMR spin-lattice relaxation (1/T1) and spin-echo decay (1/T2) rate measurements were performed in a single crystal of Ba(Fe0.93Rh0.07)2As2 superconductor. Below the superconducting transition temperature Tc, when the magnetic field H is applied along the c axes, a peak in both relaxation rates is observed. Remarkably that peak is suppressed for H || ab. Those maxima in 1/T1 and 1/T2 have been ascribed to the flux lines lattice motions and the corresponding correlation times and pinning energy barriers have been derived on the basis of an heuristic model. Further information on the flux lines motion was derived from the narrowing of 75As NMR linewidth below Tc and found to be consistent with that obtained from 1/T2 measurements. All the experimental results are described in the framework of thermally activated vortices motions.

Proton dynamics in one-dimensional hydrogen-bonding system in molecular co-crystals TMP-D2ca and DMP-H2ca

The proton dynamics in one-dimensional hydrogen-bonding system in molecular co-crystals of tetramethylpyrazine (TMP) with chloranilic acid (H2ca), as well as 2,6-dimethylpyrazine (DMP) with H2ca is studied by 35Cl NQR and 2H NMR spin-lattice relaxation measurements. No transfer motion of proton between the acid and base molecules is observed in DMP-H2ca, while the motion of the acid proton is excited in TMP-H2ca and the activation energy for the motion increases from 35 kJ mol − 1 to 50 kJ mol − 1 by the deuteration.

Nuclear spin-lattice relaxation in carbon nanostructures

Abstract Interpretation of nuclear spin-lattice relaxation data in the carbon nanostructures is usually based on the analysis of fluctuations of dipole–dipole interactions of nuclear spins and anisotropic electron–nuclear interactions responsible for chemical shielding, which are caused by molecular dynamics. However, many nanocarbon systems such as fullerene and nanotube derivatives, nanodiamonds and carbon onions reveal noticeable amount of paramagnetic defects with unpaired electrons originating from dangling bonds. The interaction between nuclear and electron spins strongly influences the nuclear spin-lattice relaxation, but usually is not taken into account, thus the relaxation data are not correctly interpreted. Here we report on the temperature dependent NMR spectra and spin-lattice relaxation measurements of intercalated fullerenes C 60 ( MF 6 ) 2 ( M =As and Sb), where nuclear relaxation is caused by both molecular rotation and interaction between nuclei and unpaired electron spins. We present a detailed theoretical analysis of the spin-lattice relaxation data taking into account both these contributions. Good agreement between the experimental data and calculations is obtained. The developed approach would be useful in interpreting the NMR relaxation data in different nanostructures and their intercalation compounds.

The fast lithium-ion conducting oxides Li 3xLa 2/3 − xTiO 3 from fundamentals to application

The development of fast lithium conductors is attracting much attention because of their potential applications in electrochemical devices such as high-energy lithium ion batteries, electrochromic systems, supercapacitors and electrochemical sensors. Among the lithium electrolyte ceramics, the lithium lanthanum titanates, Li 3 x La 2/3 − x TiO 3, a disordered oxide with perovskite structure, have attracted much attention because of their high conductivity, of the order of 10 − 3 S cm − 1 at room temperature, for x = 0.11. The crystallographic structure of the solid solution, prepared by conventional solid-state chemistry, will be described. The dynamics of the lithium ions in Li 3 x La 2/3 − x TiO 3 has been investigated by conductivity spectroscopy in the wide frequency range from 1 Hz to 1 GHz and by 7Li NMR spin-lattice relaxation measurements ( T 1 and T 1ρ) in the temperature range from 150 K to 600 K. Furthermore, the preparation of nanomaterials (20–30 nm) by a modified Pechini-type in-situ polymerizable method will be presented. Finally, we will show the ability of the ceramic to be used as pH sensor.

17O NMR spin–lattice relaxation in [formula omitted]: Field-dependent cross-relaxation process driven by 237Np spins

We have performed 17O NMR spin–lattice relaxation measurements with conventional NMR techniques on 17O in NpO 2 . The results have been found to exhibit a field-dependent cross-relaxation effect from 237Np in addition to the field-independent exchange-fluctuation background rate which dominates at high fields. Analysis of the cross-relaxation process reveals a temperature-independent relaxation rate as well as a greatly enhanced unlike spin–spin coupling between the 237Np spins and their 17O nearest neighbors. We discuss the possibility of observing the cross-relaxation effects in other actinide dioxides.

NMR investigation of the dynamics of banana shaped molecules in the isotropic phase: a comparison with calamitic mesogens behaviour

The first translational self-diffusion NMR measurements in the isotropic phase of banana-shaped liquid crystals are reported. In this paper, two banana-shaped mesogens, having a similar molecular structure and showing a nematic phase, have been investigated by means of translational self-diffusion NMR, (2)H NMR spin-spin and (1)H NMR spin-lattice relaxation measurements in the isotropic phase. While (1)H diffusion and (2)H relaxation times reveal a peculiar slow dynamic behaviour of banana-shaped mesogens compared with calamitic mesogens, the (1)H relaxation times seem to be affected by fast dynamics only. The origin of these dynamic features is discussed in terms of overall and internal molecular motions, in the frame of recent speculations concerning the formation of molecular clusters or aggregates in the isotropic phase of banana-shaped liquid crystals.

On the low-energy excitations in superconductingYNi2B2CfromB11NMR relaxation around the critical field

$^{11}\mathrm{B}$ NMR spectra, spin-echo dephasing and spin-lattice relaxation measurements in a single crystal of superconducting ${\mathrm{YNi}}_{2}{\mathrm{B}}_{2}\mathrm{C}$ at two temperatures $T⪡{T}_{c}\phantom{\rule{0.3em}{0ex}}(H=0)$, as a function of the external magnetic field $H$ are reported, particularly for $H$ close to the critical field ${H}_{c2}$. For $H⪢{H}_{c1}$ the spin lattice relaxation rate increases linearly with field, consistently with a density of states at the Fermi level going as ${H}^{1∕2}$. It is pointed out how a relaxation process weighted average of the contributions from the intravortex and the intervortex regions also accounts for the field and the temperature dependences of the relaxation rate. For $H\ensuremath{\approx}{H}_{c2}$ a small extra contribution, possibly related to novel excitations in the vortex cores or to the motion of the flux lines at the melting of the flux lattice, is detected. When the field-induced metallic state is attained $(H&gt;{H}_{c2})$ the relaxation rate becomes field independent.

89Ynuclear magnetic resonance study of Ca-dopedY1−xCaxBa2Cu3Oyfrom the underdoped to the overdoped superconducting regime

${}^{89}\mathrm{Y}$ NMR linewidth, Knight shift, spin-echo dephasing, and spin-lattice relaxation measurements have been carried out in ${\mathrm{Y}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{Ba}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{y}.$ Underdoped and overdoped samples have been obtained by means of ${\mathrm{Y}}^{3+}$ for ${\mathrm{Ca}}^{2+}$ substitutions in the parent chain-empty antiferromagnetic (AF) ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.1}$ and in the chain-full ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{7},$ respectively. Unexpected effects, as the divergence of relaxation rate with the concurrent broadening of the NMR line in the underdoped superconducting phase and the inadequacy of the Korringa relation between the relaxation rate ${1/T}_{1}$ and Knight shift, even in the overdoped regime, suggest that a revision of the commonly accepted view of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6+y}$ compounds is required. In particular the linear temperature dependence of ${T}_{1}^{\ensuremath{-}1}$ and the temperature behavior of Knight shift cannot be accounted for over all the temperature range. In the underdoped superconducting phase the divergence of ${1/T}_{1}$ on cooling is associated with the slowing down of excitations possibly related to sliding motions of orbital currents, or with the concurrent freezing of AF correlated spins. Echo-dephasing measurements evidence an extreme slowing down of longitudinal spin fluctuations which appear to be driven by a different dynamic, related either to flux line motions or to ${}^{63,65}\mathrm{Cu}$ spin-lattice relaxation.