Large Knight Shift Research Articles

Light-Induced Metallic and Paramagnetic Defects in Halide Perovskites from Magnetic Resonance.

Halide perovskites are promising next-generation solar cell materials, but their commercialization is hampered by their propensity to degrade under operating conditions, particularly under heat, humidity, and light. Identifying degradation products and linking them to the degradation mechanism at the atomic scale is necessary to design more stable perovskite materials. Here we use magnetic resonance methods to identify and characterize the formation of both metallic lead clusters and Pb3+ defects upon light-induced degradation of methylammonium lead halide perovskite using nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) measurements. Paramagnetic relaxation enhancement (PRE) of the 1H NMR resonances demonstrates the presence of localized paramagnetic Pb3+ defects, a large Knight shift of the 207Pb NMR proves the presence of lead metal, and their relative proportions are determined by the differing temperature dependence in variable-temperature EPR. This work reconciles previous conflicting literature results, enabling the use of EPR spectroscopy to monitor photodegradation of perovskite devices.

Theoretical Studies on Off-Axis Phase Diagrams and Knight Shifts in UTe2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document}: Tetra-Critical Point, d-Vector Rotation, and Multiple Phases

Inspired by recent remarkable sets of experiments on UTe2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document}: discoveries of the fourth horizontal internal transition line running toward a tetra-critical point (TCP) at H = 15 T, the off-axis high-field phases, and abnormally large Knight shift (KS) drop below Tc\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$T_\ extrm{c}$$\\end{document} for H‖a\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$H \\parallel a$$\\end{document}-magnetic easy axis, we advance further our theoretical work on the field (H)-temperature (T) phase diagram for H‖b\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$H \\parallel b$$\\end{document}-magnetic hard axis which contains a positive sloped Hc2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$H_\ extrm{c2}$$\\end{document} departing from TCP. A nonunitary spin-triplet pairing with three components explains these experimental facts simultaneously and consistently by assuming that the underlying normal electron system with a narrow bandwidth characteristic to the Kondo temperature ∼\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim$$\\end{document}30 K unsurprisingly breaks the particle-hole symmetry. This causes a special invariant term in Ginzburg–Landau (GL) free energy functional which couples directly with the 5f magnetic system, giving rise to the Tc\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$T_\ extrm{c}$$\\end{document} splitting and ultimately to the positive sloped Hc2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$H_\ extrm{c2}$$\\end{document} and the horizontal internal transition line connected to TCP. The large KS drop can be understood in terms of this GL invariance whose coefficient is negative and leads to a diamagnetic response where the Cooper pair spin is antiparallel to the applied field direction. The present scenario also accounts for the observed d-vector rotation phenomena and off-axis phase diagrams with extremely high Hc2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$H_\ extrm{c2}$$\\end{document}≳\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\gtrsim$$\\end{document}70 T found at angles in between the b- and c-axes and between the bc-plane and a-axis, making UTe2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_2$$\\end{document} a fertile playground for a possible topological superconductor.

Sub-millisecond 125Te NMR spin-lattice relaxation times and large Knight shifts in complex tellurides: Validation of a quadratic relation across the spectrum

125Te NMR spectra and spin-lattice relaxation times, T1, have been measured for several GeTe-based materials with Te excess. The spectra show inhomogeneous broadening by several thousand ppm and a systematic variation in T1 relaxation time with resonance frequency. The quadratic dependence of the spin-lattice relaxation rate, 1/T1, on the Knight shift in the Korringa relation is found to be valid over a wide range of Knight shifts. This result confirms that T1 relaxation in GeTe-based materials is mostly dominated by hyperfine interaction between nuclei and free charge carriers. In GeTe with 2.5% excess of Te, about 15% of the material exhibits a Knight shift of ≥4500ppm and a T1 of only 0.3ms, indicating a high hole concentration that could correspond to close to 50% vacancies on the Ge sublattice in this component. Our findings provide a basis for determining the charge carrier concentration and its distribution in complex thermoelectric and phase-change tellurides, which should lead to a better understanding of electronic and thermal transport properties as well as chemical bonding in these materials.

29Si,47Ti,49Ti and195Pt solid state MAS NMR spectroscopic investigations of ternary silicides TPtSi, germanides TPtGe (T = Ti, Zr, Hf) and stannide TiPtSn

Eight ternary tetrelides TPtX (T = Ti, Zr, Hf; X = Si, Ge, Sn) were synthesized from the elements by arc-melting and subsequent annealing. TiPtSi, ZrPtSi, ZrPtGe, HfPtSi and HfPtGe crystallize with the orthorhombic TiNiSi type structure, in the space group Pnma. The structures of HfPtSi (a = 654.44(9), b = 387.97(6), c = 750.0(1) pm, wR2 = 0.0592, 411 F(2) values, 20 variables) and HfPtGe (a = 660.36(7), b = 395.18(4), c = 763.05(8) pm, wR2 = 0.0495, 430 F(2) values, 20 variables) were refined from single crystal X-ray diffractometer data. TiPtSn adopts the cubic MgAgAs type. TiPtGe is dimorphic with a TiNiSi type high-temperature modification which transforms to cubic LT-TiPtGe (MgAgAs type). All phases were investigated by high resolution (29)Si, (47)Ti, (49)Ti and (195)Pt solid state MAS NMR spectroscopy. In the cubic compounds, the (47/49)Ti NMR signals are easily detected owing to the absence of quadrupolar broadening effects. The (195)Pt resonances of the orthorhombic compounds are characterized by strongly negative isotropic Knight shifts and large Knight shift anisotropies, whereas positive isotropic Knight shifts and no anisotropies are observed for the cubic compounds. These results indicate that the phase transition in TiPtGe is associated with dramatic changes in the electronic properties. Within each group of isotypic compounds the isotropic (29)Si, (47/49)Ti and (195)Pt Knight shifts show systematic dependences on the transition metal or tetrel atomic number, suggesting that the numerical values are influenced by the electronegativities of the metallic (or metalloid) neighbours.

Magnesium and cadmium containing Heusler phases REPd2Mg, REPd2Cd, REAg2Mg, REAu2Mg and REAu2Cd

Twenty-eight new Heusler phases REPd2Mg, REPd2Cd, REAg2Mg, REAu2Mg and REAu2Cd with different rare earth elements were synthesized from the elements in sealed niobium ampoules in a water-cooled sample chamber of an induction furnace. The samples were characterized by powder X-ray diffraction. The cell volumes show the expected lanthanide contraction. The structures of YPd2Cd, GdPd2Cd, GdAu2Cd, Y1.12Ag2Mg0.88 and GdAg2Mg were refined based on single crystal diffractometer data. The magnetic properties were determined for fifteen phase pure samples. LuAu2Mg is a weak Pauli paramagnet with a susceptibility of 1.0(2) × 10−5 emu mol−1 at room temperature. The remaining samples show stable trivalent rare earth ions and most of them order magnetically at low temperatures. The ferromagnet GdAg2Mg shows the highest ordering temperature of TC = 98.3 K. 113Cd and 89Y MAS NMR spectra of YAu2Cd and YPd2Cd confirm the presence of unique crystallographic sites. The resonances are characterized by large Knight shifts, whose magnitude can be correlated with electronegativity trends.

The Silicides YT2Si2 (T =Co, Ni, Cu, Ru, Rh, Pd): A Systematic Study by 89Y Solid-state NMR Spectroscopy

The ThCr2Si2-type silicides YT2Si2 (T =Co, Ni, Cu, Ru, Rh, Pd) were synthesized from the elements by arc-melting. They were characterized by powder X-ray diffraction, and the structures were refined on the basis of single-crystal X-ray diffractometer data. The course of the lattice parameters shows a distinct anomaly for YRu2Si2 which has by far the smallest c/a ratio along with elongated Y- Si distances. Systematic 89Y solid-state NMR spectra show large Knight shifts arising from unpaired conduction electron spin density near the Fermi edge. The Knight shift decreases with increasing valence electron count (VEC), reflecting the sensitivity of this parameter to electronic properties. The particularly strong structural distortion observed in YRu2Si2 manifests itself in a sizeable magnetic shielding anisotropy. Electronic structure calculations for YRu2Si2 and YRh2Si2 reveal similar projected density of states (PDOS) shapes with an energy upshift of the Fermi level in YRh2Si2 due to the extra electron brought in by Rh. As a consequence, the PDOS at the Fermi energy is twice as large in the Ru compound as in the Rh compound. While both compounds show the major bonding interaction within the T2Si2 layers, YRh2Si2 exhibits significantly stronger Y-Si bonding

Doping effects in low dimensional antiferromagnets

The study of impurities in low-dimensional antiferromagnets has been a very active field in magnetism ever since the discovery of high-temperature superconductivity. One of the most dramatic effects is the appearance of large Knight shifts in a long range around non-magnetic impurities in an antiferromagnetic background. The dependence of the Knight shifts on distance and temperature visualizes the correlations in the system. In this work, we consider the Knight shifts around a single vacancy in the one- and two-dimensional Heisenberg model.

EPR and ferromagnetism in diluted magnetic semiconductor quantum wells.

Motivated by recent measurements of electron paramagnetic resonance spectra in modulation-doped CdMnTe quantum wells [Phys. Rev. Lett. 91, 077201 (2003)]], we develop a theory of collective spin excitations in quasi-two-dimensional diluted magnetic semiconductors. Our theory explains the anomalously large Knight shift found in these experiments as a consequence of collective coupling between Mn-ion local moments and itinerant-electron spins. We use this theory to discuss the physics of ferromagnetism in (II,Mn)VI quantum wells and to speculate on the temperature at which it is likely to be observed in n-type modulation-doped systems.

Collective character of spin excitations in a system of Mn2+ spins coupled to a two-dimensional electron gas.

We have studied the low energy spin excitations in n-type CdMnTe based dilute magnetic semiconductor quantum wells. For magnetic fields for which the energies for the excitation of free carriers and Mn spins are almost identical, an anomalously large Knight shift is observed. Our findings suggest the existence of a magnetic-field-induced ferromagnetic order in these structures, which is in agreement with recent theoretical predictions [Phys. Rev. Lett. 91, 077202 (2003)]].

Synthesis and characterization of nano-LiMn2O4 powder by tartaric acid gel process

Polycrystalline nano-LiMn 2 O 4 spinel powder was synthesized by the tartaric acid gel process and developed without any detectable minor phase at 300 °C. The powders synthesized by the tartaric acid gel process had a relatively smaller particle size, larger specific surface area and narrower particle size distribution than those prepared by the conventional solid state reaction. The average valence of manganese decreased with increasing synthesis temperature and resulted in an increase of the lattice parameter with calcination temperatures. As temperatures were increased to above 500 °C, LiMn 2 O 4 underwent phase transition from a cubic to a tetragonal phase by removing oxygen ion in the lattice. From the results of MAS NMR, LiMn 2 O 4 spinels formed at 300–800 °C had a large Knight shift of ∼520 and 560 ppm in reference to LiC1.

NMR and X-ray Spectroscopy of Sodium−Silicon Clathrates

The 23Na and 29Si NMR spectra of the sodium−silicon clathrate materials Na8Si46 and NaxSi136 (1 < x < 24) and the parent Zintl phase NaSi have been studied in detail with a view to ironing out a number of ambiguities in the published literature and to determining properties of these potential thermoelectric materials. Sharp spectra are obtained only when the clathrate cages are close to fully occupied by Na, so that crystallographic symmetry is achieved. Signals from Na in the small and large cages of both structures have been unequivocally assigned. The pseudospherical cages give isotropic 23Na lines, whereas the other cages produce first-order quadrupolar line shapes. Electric field gradients derived from these spectra, and ab initio calculations are in remarkable agreement. The large Knight shifts of both types of nuclei have rather unusual temperature dependences, which reflect the changing distributions of unpaired electron density in the conduction bands. As the Na content of NaxSi136 is reduced there is drastic broadening of the 23Na and 29Si spectra due to a random distribution of vacant cages and hence of environments, and the Si spectra shift to lower frequencies, indicating a reduction in conduction s-electron density on the Si. Intensity data suggest a preferential loss of Na from the large cages. XPS results show a protective coating of silica on the surface of the clathrates. NMR, XPS, and XANES results all indicate transfer of electrons from Na to the Si framework, as predicted by calculations. XANES shows a lowering of the absorption edge, and hence of the conduction band, of both clathrates relative to crystalline Si.

NMR Knight shifts and the electronic properties ofRb8Na16Si136clathrate

A silicon framework clathrate type-II compound was synthesized with rubidium and sodium atoms in cages. A single crystal of this material was characterized by both conventional and synchrotron x-ray diffraction; the structure belongs to the cubic space group $Fd\ensuremath{-}3m,$ with a cell edge of 14.738(1) \AA{}. The alkali metals are ordered in the structure, with the small cages containing sodium, and the large cages containing rubidium. Variable temperature magic-angle-spinning NMR of all three nuclei show large Knight shifts with a strong temperature dependence, unlike conventional metals. The low conductivity (200 S/cm) and high paramagnetic susceptibility $(5\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}\mathrm{e}\mathrm{m}\mathrm{u}/\mathrm{g})$ indicate that as the temperature is lowered, the electrons become more localized on the alkali atoms, resulting in properties consistent with a correlated narrow band metal system.

133Cs and 23Na NMR Studies of Cs8NaxGe136 Clathrates

ABSTRACTWe report 133Cs and 23Na spectra of Cs8NaxGe136 clathrates. Fully loaded Cs8Na16Ge136 shows only ionic signals from both Cs and Na nuclei. In contrast to the Cs8Na16Si136 clathrate, germanium Cs8Na16Ge136 clathrate shows no large Knight or paramagnetic shifts. However, when sodium is removed from Cs8Na16Ge136, the resulting Cs8Ge136 clathrate shows a very large shift in the 133Cs NMR spectrum.

A 23Na NMR study of NaxSi136 and Na8SI46 silicon clathrates

Static and MAS 23Na NMR techniques have been applied to the characterisation of several compositions of the non-stoichiometric clathrate NaxSi136 in order to evaluate the electronic state of the alkali species. This study has been extended to the stoichiometric clathrate Na8Si46. In the low x compounds, a hardly observable broad NMR signal suggests that sodium is only partially ionised, with a single electron remaining localised on the nucleus. For the larger x values as well as for Na8Si46, the materials become metallic conductors and two narrow NMR lines with a very large Knight shift (in the 1500–2000 ppm range) are observed for each phase, corresponding to sodium atoms in the two types of cages. Unlike other 23Na NMR data from the literature, these results are compatible with the preferential occupancy of the larger Si28 cages in NaxSi136, determined in a separate paper by Rietveld refinement of X-ray diffraction data. Étude de clathrates de silicium NaxSi136 et Na8Si46 par RMN du 23Na. La RMN du 23Na a été appliquée en MAS et en statique sur diverses compositions de la phase clathrate non stœchiométrique NaxSi136 dans le but d'évaluer l'état électronique des espèces alcalines. Cette étude a été élargie au clathrate stcechiométrique Na8Si46. Dans les composés à faible x, un signal RMN large et difficilement observable suggère que le sodium n'est que partiellement ionisé, avec un électron célibataire localisé sur le noyau. Pour des valeurs de x plus importantes ainsi que pour Na8Si46, ces matériaux deviennent des conducteurs métalliques et l'on observe, pour chaque phase, deux pics de RMN très fins avec un important Knight shift (dans un domaine compris entre 1500 et 2000 ppm), correspondant aux atomes de sodium dans les deux types de cages. Contrairement aux conclusions d'une autre étude par RMN du sodium publiée récemment, nos résultats ne sont pas incompatibles, avec une occupation préférentielle des grandes cages Si28 dans NaxSi136, mise en évidence dans un autre article par affinement Rietveld de diagrammes de diffraction X sur poudre.

Determination of the spin density distribution in the organic conductor DMTM(TCNQ)2 with 13C magic angle spinning NMR

13C magic angle spinning (MAS) NMR experiments on the organic conductor N,N-dimethylthiomorpholinium bis-tetracyanoquino-dimethane [DMTM(TCNQ)2]isotopically enriched at the position of the C-2 carbons of TCNQ are reported. Two isotropic resonances are resolved above the structural phase transition at 272 K, both exhibiting large positive isotropic and anisotropic Knight shifts. Sideband analysis of the MAS spectra has yielded the complete Knight shift tensors above T c, in the insulating state. From the Knight shifts the hyperfine couplings and spin densities at the C-2 sites were calculated and were found to be in good agreement with molecular calculations. Below T c, the sideband pattern displays two sets of 13C-2 lines indicating a spin density distribution between two sites of 0·59 :0·41.

NMR in the silicon clathrate compounds Na xBa ySi 46 and Na xSi 136

29Si, 23Na, 137Ba, and 135Ba NMR experiments were carried out in the superconducting silicon clathrate compound Na xBa ySi 46 to study the electronic states above the superconducting transition temperature. At all the atomic sites, considerable Knight shift and the Korringa relation in NMR relaxation were found, which indicates metallic electronic structure with moderate electronic correlation. The conduction-electron spin/charge density was found to be strongly site-dependent. The Na atoms are partially ionized as well as the Ba atoms, which shows noticeable difference from metal-doped fullerides. The results of comparative NMR experiments with non-superconducting clathrate compound Na xSi 136 suggested that the existence of the Si site with remarkably large Knight shift in Na xBa ySi 46 has some relevance to the superconductivity.

Hyperfine interaction of13O and23Mg implanted in Pt

The spin relaxation timeT 1 for short-lived beta emitters13O and23Mg implanted in Pt have been measured for the first time;T 1T13O) = 2.90 ±0.65 Ks andT 1 T(23Mg) = 1665 ±140 Ks. The Knight shift for13O in Pt was measured at 300 K to beK(13O) = +(4.23 ±0.14) × 10−3. In the case of13O, the Knight shift is unusually large and the relaxation time is unusually fast compared with other interstitial impurities in Pt. A KKR band-structure calculation reproduces the present large Knight shift fairly well.

Physical and Electrochemical Characterization of Quaternary Li‐Mn‐V‐O Spinel as Positive Materials for Rechargeable Lithium Batteries

Quaternary Li‐Mn‐V‐O spinels formed from heating mixtures of , and around 350° have been characterized by electrochemical measurement, x‐ray photoelectron spectroscopy, x‐ray diffractometry, , and magnetic susceptibility measurements. These results were compared with those for ternary Li‐Mn‐O spinels prepared at 350 and 850°C. The compositions of the quaternary and ternary spinels formed at 350°C were shown to be (x = 0.5 ∼ 1, y = 0 ∼ 0.4). The cubic a0‐lattice parameter and specific density of the quaternary spinels increased with increasing vanadium content in the spinel structure. The measurements revealed that the quaternary spinels did not show any Knight shift, while the ternary Li‐Mn‐O spinels formed at 350 and 850°C had a large Knight shift of 490 ∼ 535 ppm in reference to LiCl. Furthermore, the measurement of the magnetic susceptibility indicated that the ternary and quaternary spinels formed at 350°C are paramagnetic and the ternary prepared at 850°C is antiferromagnetic.

Ultra-high-resolution multiple-pulse 1H NMR of organic conductors

it is shown that the use of multiple-pulse decoupling and magic-angle spinning can give extremely highly resolved 1H NMR spectra of some organic conductors. Although the scaled linewidths approach 2.5 ppm, the dispersion of the resonances, due to large Knight shifts, is an order of magnitude larger than the usual 'H chemical-shift dispersion. This point is illustrated with spectra from two TCNQ salts in which up to eight chemically equivalent 1H's, distinguished only by the amount of resident spin density on the bound carbon, are resolved.

Studies of adsorbate structure and dynamics on catalytic surfaces with NMR spectroscopy: CO on metals

NMR studies of adsorbed CO illustrate the methodology of NMR spectroscopy and demonstrate results attainable in a comprehensive application to a heterogeneous catalytic system. This review is divided into two sections: adsorbate structure and dynamics. CO adsorbs on dispersed metals in at least three forms: linear- and bridge-bonded CO on metal particles and multicarbonyls on isolated metal atoms. Broadline 13C NMR spectra of CO on Ru and Rh reveal distinct powder patterns for each type, which provides quantitative site distributions of the site geometries. High resolution spectra obtained by magic-angle spinning provide isotropic shifts which support the separation of the overlapping broadline components. The metallic character of the dispersed metal particles is revealed through susceptibility broadening and large Knight shifts. Further details of the adsorbed states are available through analysis of the various nuclear dipolar couplings; examples include measurements of the CO bond length, the local density of CO on the metal particles, and the intramolecular distance in dicarbonyls. The rates of surface diffusion and site exchange have been characterized by the application of three different methods — relaxation studies, lineshape narrowing, and spin-population labeling — which together allow measurements of rate constants spanning 8 orders of magnitude.