Central Transition Line Research Articles

71Ga NMR characterization of an n-doped free-standing gallium nitride wafer

The crystallinity of an n-doped free-standing GaN wafer sliced from a GaN crystal grown on a GaN template with a mask pattern was characterized by 71Ga NMR. The quadrupolar frequency determined from the quadrupolar-split 71Ga spectra agreed with that of free-standing GaN single crystals reported in the literature. On the other hand, the broad satellite transition lines compared to the central transition line manifested structural deviation. The satellite linewidths were notably anisotropic with respect to the angle between the magnetic field and the crystalline axes; this phenomenon was attributed to the strain distribution inside the crystal. Assuming uniform deformation of the crystal as the main origin of the satellite linewidths, the longitudinal strain distribution was estimated to be on the order of 10−5. The strain distribution was also found to be slightly larger at the center of the wafer than at the edge.

Fast Li-Ion Dynamics in Stoichiometric Li2S–Ga2Se3–GeSe2 Glasses

The temperature dependence of Li-ion transport is explored in novel stoichiometric chalcogenide glasses in the ternary system Li2S–Ga2Se3–GeSe2 using electrochemical impedance spectroscopy (EIS) and 7Li nuclear magnetic resonance (NMR) spectroscopy. The dc conductivity in these glasses monotonically increases with Li2S content with the highest room-temperature conductivity being ∼10–4 S/cm in the glass of composition: 50% Li2S–10% Ga2Se3–40% GeSe2 with an activation energy of 0.37 eV. Analysis of the 7Li NMR central transition line shape reveals that Li ions are randomly distributed in the structure of these glasses and their rapid hopping transport is responsible for the electrical conductivity. The excellent agreement between the experimentally measured dc conductivity and that calculated from the Li-ion hopping frequency obtained from EIS and NMR implies that the ionic transference number in all glasses is close to unity and these glasses behave as strong electrolytes.

Lithium Polymer Electrolytes Based on Sulfonated Poly(ether ether ketone) for Lithium Polymer Batteries

AbstractWe studied a lithium‐ion conducting polymer based on sulfonated poly(ether ether ketone) (SPEEK) doped with lithium bis(trifluoromethane)sulfonimide (LiTFSI). Self‐standing membranes were prepared by the solvent‐casting technique with a LiTFSI loading of 0 to 30 wt.‐%. The thermogravimetric analysis curves showed that the SO3H groups decompose earlier in the SPEEK–LiTFSI membranes than in pure SPEEK, owing to interactions between the Li+ ions and the SO3H groups. X‐ray diffraction and differential scanning calorimetry studies showed that the addition of LiTFSI decreased the crystallinity and the glass‐transition temperature of the polymer, which revealed the plasticizing effect of the lithium salt on the polymer matrix. The 7Li NMR spectroscopy results showed a single central transition line at around δ = –1.2 ppm, which indicated the presence of free mobile lithium ions. Dynamic mechanical analysis of the membrane showed it to be mechanically stable up to 100 °C, a prerequisite for flexible lithium polymer batteries. The highest room‐temperature conductivity in the order of 10–5 S cm–1 was observed for the 20 wt.‐% LiTFSI‐doped SPEEK membrane, which increased to 5 × 10–4 S cm–1 at 100 °C.

Defects in half-Heusler type antimonides ScTSb (T = Ni, Pd, Pt)

The ScTSb (T = Ni, Pd, Pt) antimonides adopt the cubic MgAgAs type structure ( F ¯ 4 3 m , a = 605.21(6), wR2 = 0.0241, 94 F 2 values and 7 variables for ScNi 0.871(6)Sb; a = 630.00(6), wR2 = 0.0254, 147 F 2 values and 7 variables for ScPd 0.966(3)Sb; a = 631.1(1), wR2 = 0.0220, 89 F 2 values and 5 variables for ScPtSb). The refinement of the occupancy parameters revealed vacancies on the transition metal sites for the Ni and Pd compounds. This results in additional signals in the 45Sc MAS NMR spectra, having intensities in accordance with the refined vacancy concentrations. Furthermore, the quenched samples are showing a third 45Sc resonance that vanishes upon annealing, suggesting a partly disordered transition metal substructure. The higher degree of ordering due to annealing is also manifested by a reduction of the central transition line widths in the 121Sb MAS NMR spectra. 121Sb Mössbauer spectra are showing a single absorption line for all samples.

In-plane anisotropy of the magnetic fluctuations in [formula omitted

We found two components of the in-plane Co 59 NMR Knight shift K and different nuclear spin–lattice relaxation rates 1 / T 1 in bilayer hydrated superconductor Na x CoO 2 · y H 2 O , an oriented powder sample by a magnetic field in Fluorinert FC70. Distinguishable two peaks in the central transition line of the Co 59 NMR spectrum at 10 K can be explained by an anisotropic Knight shift in a triangular plane. The hyperfine coupling constants seem to change around 150 K, in which the bulk susceptibility χ shows a broad minimum, suggesting a change of the electronic state in the CoO 2 plane.

Phase Behavior and Dynamics of the ABA Triblock Copolymer Poly(ethylene glycol) Distearate Doped with Alkali Metal Salts

The ABA triblock copolymer poly(ethylene glycol) distearate (PEGD), average Mn ca. 930, complexed with lithium and sodium perchlorates has been studied by 1H, 7Li, 13C, and 23Na solid-state nuclear magnetic resonance (NMR), SAXS, DSC, and polarized-light optical microscopy. Unlike other solid polymer electrolytes, highly Li+-doped PEGD samples exhibit sharp 7Li NMR quadrupolar powder patterns even at temperatures well above the melting point, indicating that this triblock copolymer is microphase separated and the dynamics in the PEG phase are anisotropic. Measurements of the 7Li central transition line width in highly doped samples show three distinct line narrowings, due to the poly(ethylene glycol) glass transition (∼−20 °C), the stearate melting point of the polymer (∼35 °C), and an order−disorder transition (∼72 °C). 23Na NMR measurements yield similar results. SAXS, DSC, and optical microscopy with polarized light confirm the presence of a microphase-separated state up to ∼72 °C. 13C and 1H NMR show that the segmental mobility in the ordered state is reduced compared to the isotropic melt. The results confirm the previously proposed order−disorder model to explain the dependence of the ionic conductivity on the lithium concentration for Li+-doped PEGD samples.

11B NMR investigations in xV 2O 5–B 2O 3 and xV 2O 5–B 2O 3–PbO glasses

11B ( I=3/2) MAS NMR in the binary glass system xV 2O 5–B 2O 3 ( x=0.053, 0.43) and the ternary glass system xV 2O 5–B 2O 3–PbO (0.1⩽ x⩽1.5) has been investigated at room temperature. In the xV 2O 5–B 2O 3 glasses, one NMR line due to BO 3 unit was observed. Meanwhile in the xV 2O 5–B 2O 3–PbO, two NMR lines which arise from BO 3 and BO 4 units were detected, where the appearance of BO 4 units is produced by the presence of PbO. From the computer-simulation of the 11B NMR central transition line ( m=−1/2↔1/2), the quadrupole parameters ( e 2 qQ/ h and η) for BO 3 units in xV 2O 5–B 2O 3, and those for BO 3 and BO 4 units in xV 2O 5–B 2O 3–PbO were obtained as a function of x. As the V 2O 5 content increases in xV 2O 5–B 2O 3–PbO, the e 2 qQ/ h and η values of the BO 3 − associated resonance are found to slightly decrease and increase, respectively. Meanwhile, the e 2 qQ/ h and η values of BO 4 − associated resonance in xV 2O 5–B 2O 3–PbO are found to slightly increase and decrease, respectively. By comparing the intensities of the total transitions ( m=−3/2↔−1/2, m=−1/2↔1/2, and 1/2↔3/2) for the 11B NMR line of BO 3 and BO 4 units contained in xV 2O 5–B 2O 3–PbO with those of respective standard samples of 0.053V 2O 5–B 2O 3 and NaBH 4, the quantitative fractions of BO 3 and BO 4 in xV 2O 5–B 2O 3–PbO were obtained as a function of x.

Multinuclear NMR Study of HFeCo3(CO)9[P(OCH3)3]3 in the Solid State and in Solution

31P and 59Co NMR have been applied to study the structure and dynamics of the tetrahedral mixed-metal cluster HFeCo3(CO)9[P(OCH3)3]3 both in the solid state and in solution. The 31P chemical shift (CS) anisotropy, the direct (D) and indirect (J) dipolar 31P−59Co interactions, and the relative orientation of the CS, D, and J tensors have been determined by iterative fitting of the 31P MAS NMR spectra at two magnetic field strengths (4.7 and 7.1 T). The quadrupole coupling constant as well as the isotropic part and anisotropy of the CS tensor at the 59Co nucleus has been evaluated by a moment analysis of the solid-state central transition line shape. The 31P and 59Co NMR data, which are both influenced by second-order quadrupolar shifts, clearly show a departure from the C3v molecular symmetry due to a solid-state packing effect. Using the static interaction parameters obtained by solid-state NMR, it was possible to evaluate the overall rate of molecular motion in solution from 59Co relaxation measurements....

23Na NMR Spectroscopy of Solids: Interpretation of Quadrupole Interaction Parameters and Chemical Shifts

Quadrupole coupling constants (QCC), asymmetry parameters of the electric field gradients (η), and isotropic chemical shifts (δ cs ) of sodium-23 were determined by computer simulation of the central transition line shapes of the 23 Na MAS NMR spectra of a series of crystalline sodium compounds in which sodium has exclusively oxygen atoms as nearest neighbors. The relations between the crystal structure and the above parameters were discussed qualitatively for typical examples. The bond-valence method is applied to derive empirical models for the calculation of a «shift parameter» (A) and of oxygen atomic charges (model I) from which QCC and η were calculated by a simple point charge model

Determination of the quadrupolar coupling constant in powdered samples with a two in-phase rf pulse sequence in solid-state NMR

We propose a new method for the determination of the quadrupolar coupling constant e 2 qQ/h of a spin I = 3 2 in powdered samples. It is suitable for characterizing a nucleus with a featureless central transition line shape where it is difficult to extract the value of e 2 qQ/h from the spectrum. The pulse sequence is a two in-phase rf pulse with a short delay between them. The method consists of measuring the central transition line intensity for increasing lengths of the second pulse and comparing it to calculated data. We illustrate the method with the sodium nuclei 23Na in powdered NaNO 3.

Field dependence of the nuclear magnetic resonance static powder line shape: Studies of the mutual orientation of interaction tensors from the field dependence of the critical frequencies

A procedure for determining the interaction tensor orientations and the interaction parameters for mutually oriented electric field gradient (efg) or dipolar and shielding tensors has been developed based upon the magnetic field dependence of the critical frequencies from the polycrystalline nuclear magnetic response (NMR) spectrum. The central transition line shape of half-integer quadrupolar nuclei in the presence of mutually oriented shift anisotropy and second order quadrupolar interaction is discussed for the first time. Analytical expressions for the field dependent critical frequencies have been determined for special orientations when the shift principal Z axis lies on the XZ, YZ, or XY plane of the efg (or dipolar) tensor for the first order as well as the second order interactions. The plots resulting from the analytical expressions provide a convenient graphical approach in determining approximate tensorial orientations and interaction parameters through pattern recognition. For general orientations, a numerical procedure has been developed to determine these parameters by iteratively minimizing the squares of the differences of the calculated and the experimental critical frequencies. Higher order perturbation terms can be incorporated in the present treatment. The method is demonstrated by variable field static proton spectra of trichloro–acetic acid at three different fields (1.3, 2.3, 5.2 T). The near-orthogonal orientation between the dipolar and shielding tensors and the interaction parameters obtained from this approach are consistent with those obtained previously from single crystal studies.