Multiple quadrupolar spin echoes in the La-filled skutterudite LaOs4As12

We report experimental results of ¹³⁹La pulse NMR studies in LaOs₄As₁₂. Measurements have been performed on a powder sample obtained from high quality single crystals. For the first time the pattern of quadrupole echoes for ¹³⁹La nuclei (I=7/2) was obtained. All the allowed quadrupolar echoes expected for spin I=7/2 were observed at times t=(4/3)τ, (3/2)τ, (5/3)τ, 2τ, (5/2)τ, 3τ, 4τ. The presence of quadrupolar echoes is the fingerprint of the deviation from perfect cubic symmetry of the structure and can be used as a simple and fast test of the sample quality.     

Electric field gradients at cubic lattice sites

Electric field gradients are observable by NMR at the nuclei of Gd³⁺ and Eu²⁺ at nominally cubic lattice sites in ferromagnetically ordered intermetallic compounds. The origin of these EFGs and experimental details influencing the resolution of the anisotropic quadrupolar splitting in zero-field NMR are discussed.     

A heteronuclear intermolecular single-quantum coherence scheme for high-resolution 2D J-resolved 1H NMR spectra in inhomogeneous magnetic fields

Based on heteronuclear intermolecular single-quantum coherences between proton (¹H) and quadrupolar nuclei (i.e. deuterium ²H), a three-dimensional nuclear magnetic resonance (NMR) pulse sequence is proposed for recovering high-resolution two-dimensional J-resolved NMR spectra from samples mixed with a deuterated solvent in the presence of large magnetic field inhomogeneities. Benefitting from excitation of spins via two different radio frequency (RF) transmit channels, this sequence is suitable for applications in randomly large inhomogeneous fields and the solvent suppression generally required in homonuclear intermolecular multiple-quantum coherence approaches is no longer necessary. Systematic theoretical analyses are given based on the distant dipolar field treatment. Experiment on a sample of corn oil in deuterated acetone and ethyl 3-bromopropionate and acetone dissolved in DMSO-d₆ in a deshimmed field with severe inhomogeneous broadening is performed to show the feasibility and applicability of this sequence.     

19F-decoupling of half-integer spin quadrupolar nuclei in solid-state NMR: application of frequency-swept decoupling methods

In solid-state NMR studies of minerals and ion conductors, quadrupolar nuclei like ⁷Li, ²³Na or ¹³³Cs are frequently situated in close proximity to fluorine, so that application of ¹⁹F decoupling is beneficial for spectral resolution. Here, we compare the decoupling efficiency of various multi-pulse decoupling sequences by acquiring ¹⁹F-decoupled ²³Na-NMR spectra of cryolite (Na₃AlF₆). Whereas the MAS spectrum is only marginally affected by application of ¹⁹F decoupling, the 3Q-filtered ²³Na signal is very sensitive to it, as the de-phasing caused by the dipolar interaction between sodium and fluorine is three-fold magnified. Experimentally, we find that at moderate MAS speeds, the decoupling efficiencies of the frequency-swept decoupling schemes SW_f-TPPM and SW_f-SPINAL are significantly better than the conventional TPPM and SPINAL sequences. The frequency-swept sequences are therefore the methods of choice for efficient decoupling of quadrupolar nuclei with half-integer spin from fluorine.     

粉末样品自旋3/2四极核中心跃迁的高分辨核磁共振谱

本文利用脉冲对自旋3/2四极核中心跃迁激发的选择性,在粉末样品中用一个90°脉冲将电场梯度张量在空间某一取向的核自旋磁化矢量转到旋转坐标系中XY平面进行检测,从而得到与单晶样品类似的高分辨谱线,在理论上用密度算符公式作了分析,实验上用NaNO₂和Na₂SO₄粉末样品进行了验证。 关键词：     

Double rotation27Al NMR in molecular sieves

Although high-resolution NMR spectra can be obtained in solids, the use of²⁷Al NMR to investigate the structure of aluminosilicate and aluminophosphate molecular sieves has been severely limited because anisotropic second-order quadrupolar interactions, responsible for spectral broadening, cannot be eliminated by conventional magic angle spinning (MAS) or multiple pulse techniques. Here we give the principles of the double rotation (DOR) NMR technique which can remove not only the first-order broadenings but also the second-order broadenings in the NMR spectra of quadrupolar nuclei in solids. High-resolution²⁷Al NMR using DOR is capable of resolving discrete framework aluminum sites in aluminophosphate molecular sieves, permitting quantitative investigation of site-specific adsorbate-host interactions, and of discriminating different aluminum species in zeolites.     

Sensitivity enhancement in quadrupolar nuclei double resonance detection

NMR imaging of quadrupolar nuclei in liquid system has received less attention than the imaging ofS=1/2 nuclei due to difficulties associated with the quadrupolar interaction. The¹⁰B spatial distribution in Na₂ ¹⁰B₁₂H₁₁SH in phantom has been obtained using an indirect J-coupling Double Resonance method. This procedure allows one to localize the quadrupolar nuclei by observing the protons J-coupled with them. The sensitivity gain in the indirect spatial localization of the¹⁰B nuclei with respect to to the direct one, is here computed and verified on the Na₂ ¹⁰B₁₂H₁₁SH images.     

Iterative deconvolution of quadrupole split NMR spectra

We propose a simple method to deconvolute NMR spectra of quadrupolar nuclei in order to separate the distribution of local magnetic hyperfine field from the quadrupole splitting. It is based on an iterative procedure which allows to express the intensity of a single NMR line directly as a linear combination of the intensities of the total experimental spectrum at a few related frequencies. This procedure is argued to be an interesting complement to Fourier transformation since it can lead to a significant noise reduction in some frequency ranges. This is demonstrated in the case of the ¹¹B-NMR spectrum in SrCu₂(BO₃)₂ at a field of 31.7 T, where a magnetization plateau at 1/6 of the saturation has been observed.     

High resolution heteronuclear correlation NMR spectroscopy between quadrupolar nuclei and protons in the solid state

A high resolution two-dimensional solid state NMR experiment is presented that correlates half-integer quadrupolar spins with protons. In this experiment the quadrupolar nuclei evolve during t1 under a split-t1, FAM-enhanced MQMAS pulse scheme. After each t1 period ending at the MQMAS echo position, single quantum magnetization is transferred, via a cross polarization process in the mixing time, from the quadrupolar nuclei to the protons. High-resolution proton signals are then detected in the t2 time domain during wPMLG5* homonuclear decoupling. The experiment has been demonstrated on a powder sample of sodium citrate and 23Na-1H 2D correlation spectra have been obtained. From the HETCOR spectra and the regular MQMAS spectrum, the three crystallographically inequivalent Na+ sites in the asymmetric unit were assigned. This MQMAS-wPMLG HETCOR pulse sequence can be used for spectral editing of half-integer quadrupolar nuclei coupled to protons.     

Chemisorption and surfaces studied by nuclear magnetic resonance spectroscopy

This paper presents an introduction to the study of surfaces and chemically adsorbed species with nuclear magnetic resonance (NMR) spectroscopy. The analysis is based on nuclear magnetic interactions in the solid state: dipole-dipole couplings, chemical shift anisotropy, Knight shifts, and quadrupolar splitting. The physical origins and characteristics of each interaction, as well as relative intensities for different nuclei, are discussed. In particular, emphasis is placed on the relation of these interactions to quantities of interest to studies in adsorption and catalysis: motional properties of the adsorbate, the distribution of adsorption sites, the chemical state of atoms adsorbed at the surface, electrostatic field gradients, and the metallic character of surface atoms. Techniques to observe these interactions are described; subdivided by the type of nucleus: strongly coupled nuclei (e.g. ¹H, ¹⁹F), weakly coupled nuclei (e.g. ¹³C, ¹⁵N, ²⁹Si, ¹⁹⁵Pt), and quadrupolar nuclei (e.g. ²H, ¹⁴N, ²⁷Al). The techniques described to isolate and identify the overlapping effects in the spectra include multiple-pulse spin echoing and decoupling, double-resonance irradiation, multiple-quantum excitation, and mechanical sample spinning. A review of the recent application of these techniques to studies of adsorption and surfaces illustrates the potentials and limitations. Finally, a procedure for formulating a NMR study of surface samples is proposed, with respect to sample composition and character, and the type of information desired.     

59Co nuclear magnetic resonance study in GdCo2Si2 compound

The spin echo NMR spectra of⁵⁹Co for GdCo₂Si₂ are reported. A weak negative hyperfine field on Co nuclei was found in magnetically ordered state of the compound. The splitting of resonance lines in external magnetic field indicates on antiferromagnetic structure of Gd moments lying in thea-a plane with energetic minima in [100] and [110] directions as in GdFe₂Si₂ and GdNi₂Si₂.     

NMR and magnetic susceptibility of the intermetallic pseudo-binary compounds Ce1−xRxAl3 with R=Gd,Tb and Er

The NMR and magnetic susceptibility of the intermetallic pseudo-binary compounds Ce_1?xGd_xAl₃ (x=0·01; 0·03; 0·05; 0·07), Ce_0·95Tb_0·05Al₃ and Ce_0·975Er_0·025Al₃ were investigated. The susceptibility is given by the sum of a temperature independent term and a Curie-Weiss one. The last one results from the contribution of the localized free-ion magnetic moments of all the rare-earth ions which are also responsible for the strong temperature dependent Knight shifts of the NMR lines of ²⁷Al nuclei via the exchange polarization of the conduction electrons. The ²⁷Al NMR spectra exhibit besides the line due to Al sites surrounded in the first coordination sphere as in CeAl₃ a second peak with a lower Knight shift due to Al nuclei positioned in the vicinity of a Gd, Tb or Er ion.     

A Review of NMR Studies in RCo3 Systems Presenting d-Moment Instability

For the past few years, intermetallic systems with the formula RCo₃, where R is a rare earth, have attracted the attention of the magnetism community. In these systems, the magnetism associated to the Co ions presents a magnetic instability which gives rise to metamagnetic phase transitions observed as a change on the Co magnetic moment for some critical values of the molecular field acting on this sublattice. A number of publications about these systems is concerned with basic magnetic measurements and ⁵⁹Co NMR in compounds such as Y_1–x R _x Co₃, where R = Gd, Nd, Er. The Co ions are distributed among three crystal sites with different symmetries, one of these sites being further split into two magnetic sites, a fact in itself that considerably complicates the NMR spectra. Besides, as we have observed, the linewidths and even the number of lines appearing on the NMR spectra are strongly dependent on some NMR features, such as the radiofrequency power applied to the sample. Adding to this fact, these magnetic peculiarities depend on the compounds stoichiometry; therefore it is no surprise to find conflicting results in the literature. In the present paper a review of the NMR published data will be made, including some new results of our own.     

Separation of quadrupolar and magnetic contributions to spin-lattice relaxation in the case of a single isotope

We present a NMR pulse double-irradiation method which allows one to separate magnetic from quadrupolar contributions in the spin–lattice relaxation. The pulse sequence fully saturates one transition while another is observed. In the presence of a Δm = 2 quadrupolar contribution, the intensity of the observed line is altered compared to a standard spin-echo experiment. We calculated analytically this intensity change for spins I = 1, , , thus providing a quantitative analysis of the experimental results. Since the pulse sequence we used takes care of the absorbed radiofrequency power, no problems due to heating arise. The method is especially suited when only one NMR sensitive isotope is available. Different cross-checks were performed to prove the reliability of the results obtained. The applicability of this method is demonstrated by a study of the plane oxygen ¹⁷O (I = ) in the high-temperature superconductor YBa₂Cu₄O₈: the ¹⁷O spin–lattice relaxation rate consists of magnetic as well as quadrupolar contributions.     

Zur Bestimmung chemischer Verschiebungen der NMR-Frequenzen bei Quadrupolkernen aus den MAS-NMR-Spektren

Determination of Chemical Shifts of NMR-Frequencies of Quadrupolar Nuclei from the MAS-NMR Spectra The general expressions for the NMR central transition of rotating samples with quadrupolar nuclei of half-integer spins, derived by BEHRENS [1, 2] for arbitrary angles of inclination of the spinning axis considering second-order quadrupolar effects, are presented for the practically interesting case of magic angle spinning (MAS) in a form analogous to the expressions for the resting sample. The theory is tested and used for the exact determination of the chemical shift values from the MAS-²⁷Al-NMR spectra of two representative aluminates.     

Mössbauer Studies and Magnetic Properties of Y3−x Ce x Fe5O12

It is shown that in-situ ^166mHo (I = 7) in a spherical single crystal of HoF₃ can be used as sensitive internal thermometer to thermally detect NMR (NMR-TDNO) from the 100% abundant stable ¹⁶⁵Ho (I = 7/2) nuclei. In addition, new ^166mHo NMRON results are reported. Both the ^166mHo NMRON and ¹⁶⁵Ho NMR-TDNO spectra show three distinct quadrupolar split sub-resonances, in zero applied field. The data is used to make estimates of the Ho magnetic moments and quadrupole parameters for the ^166mHo and ^166mHo sites.     

Single- and multiple-quantum cross-polarization in NMR of quadrupolar nuclei in static samples

Cross-polarization from a spin I=1/2 nucleus (e.g., ¹H) to a spin S = 3/2 nucleus (e.g., ²³Na) or a spin S = 5/2 nucleus (e.g., ²⁷A1 or ⁿO) in static powder samples is investigated. The results of conventional (single-quantum), three-quantum, and five-quantum cross-polarization experiments are presented and discussed. Based on a generalization of an existing theory of cross-polarization to quadrupolar nuclei, computer simulations are used to model the intensity and lineshape variations observed in cross-polarized NMR spectra as a function of the radio-frequency field strengths of the two simultaneous spin-locking pulses. These intensity and lineshape variations can also be understood in terms of the spin S = 3/2 or 5/2 nutation rates determined from experimental quadrupolar nutation spectra. The results of this study are intended as a preliminary step towards understanding single- and multiple-quantum cross-polarization to quadrupolar nuclei under MAS conditions and the application of these techniques to the MQMAS NMR experiment.     

NMR studies of single crystals of the topological insulator Bi<Subscript>2</Subscript>Te<Subscript>3</Subscript> at low temperatures

A powder sample and single-crystal plates of the topological insulator Bi₂Te₃ have been investigated using the ¹²⁵Te NMR method at room temperature and at low temperatures in the range from 12.5 to 16.5 K. The NMR spectra of the single-crystal plates have been studied in the orientation where the crystallographic axis c is directed parallel or perpendicular to the magnetic field. The spectra have been obtained by means of recording spin-echo signals and plotting their envelopes. It has been shown that the NMR spectra for the bismuth telluride powder and plates with the orientation c ⊥ B consist of two lines, which are presumably attributed to tellurium nuclei in two crystallographic positions in the bulk of the sample. The position and shape of the lines are determined by the chemical shift and the Knight shift. For the orientation of the plates c || B, the spectrum contains an additional component in the high-frequency region, which cannot appear due to the angular dependence of the line shifts caused by tellurium nuclei in the bulk of the topological insulator. At a low temperature, the additional line dominates in the spectrum.     

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.     

Probing through bond connectivities with MQMAS NMR

Polarization transfer from quadrupolar (27Al) to spin-1/2 (31P) nuclei via J-coupling is employed to measure two-dimensional 27Al-31P heteronuclear correlation spectra with isotropic resolution. The proposed experiment, MQ-J-HETCOR, uses multiple quantum magic angle spinning (MQMAS) NMR for elimination of the second-order quadrupolar broadening and INEPT, INEPTR, INEPT+ and DEPT sequences for the polarization transfer. The experimental conditions leading to best sensitivity and resolution are detailed using AlPO4-14 as a test sample.