Low-temperature ESR Spectra Research Articles

Magnetic Resonance Study of the Spin-1/2 Quantum Magnet BaAg2Cu[VO4]2

Abstract BaAg2Cu[VO4]2 contains Cu(II) S=1/2 ions on a distorted two-dimensional triangular lattice interconnected via non-magnetic [VO4] entities. DFT band structure calculations, quantum Monte-Carlo simulations, and high-field magnetization measurements show that the magnetism of this compound is determined by a superposition of ferromagnetic (FM) and antiferromagnetic (AFM) uniform spin-1/2 chains with nearest neighbor exchange couplings of J FM=−19 K and J AFM=9.5 K (A. Tsirlin, A. Möller, B. Lorenz, Y. Skourski, H. Rosner, Phys. Rev. B 85 (2012) 014401). Here we report the study of BaAg2Cu[VO4]2 by high-field/frequency electron spin resonance (HF-ESR) and nuclear magnetic resonance (NMR) spectroscopies, which probe the local magnetic properties. In the HF-ESR measurements, we observe an anisotropic ESR spectrum typical for the Cu(II) ions and determine the g-tensor, g ||=2.38 and g ⊥=2.06. Moreover, we see a substantial change in the spectral shape of the ESR lines at low temperatures indicating the presence of short range magnetic correlations. The analysis of the low-temperature ESR spectra shows that its peculiar structure is due to the development of the anisotropic internal fields corresponding to FM and AFM correlations in the respective Cu spin chains. In the NMR spectra the signals from 51V nuclei in the two types of chains were identified. The analysis of the temperature evolution of these signals strongly supports the ESR findings on the occurrence of two types of Cu chains. Altogether, the HF-ESR and NMR results confirm theoretical predictions of the superposition of FM and AFM Cu(II) spin-1/2 chains in the studied material.

Phosphorus Doping in Si Nanocrystals/SiO2 multilayers and Light Emission with Wavelength compatible for Optical Telecommunication.

Doping in semiconductors is a fundamental issue for developing high performance devices. However, the doping behavior in Si nanocrystals (Si NCs) has not been fully understood so far. In the present work, P-doped Si NCs/SiO2 multilayers are fabricated. As revealed by XPS and ESR measurements, P dopants will preferentially passivate the surface states of Si NCs. Meanwhile, low temperature ESR spectra indicate that some P dopants are incorporated into Si NCs substitutionally and the incorporated P impurities increase with the P doping concentration or annealing temperature increasing. Furthermore, a kind of defect states will be generated with high doping concentration or annealing temperature due to the damage of Si crystalline lattice. More interestingly, the incorporated P dopants can generate deep levels in the ultra-small sized (~2 nm) Si NCs, which will cause a new subband light emission with the wavelength compatible with the requirement of the optical telecommunication. The studies of P-doped Si NCs/SiO2 multilayers suggest that P doping plays an important role in the electronic structures and optoelectronic characteristics of Si NCs.

Radiation effects in natural quartz crystals

Optically clear, as visually observed, natural quartz crystals of Brazilian and Arkansas origins, which exhibit Al–OH−-centers in their as-received conditions, have been measured for their low temperature ESR spectra to see the presence of Al-hole centers. The ESR spectra revealed the presence of Al-hole centers in their as-received conditions. These centers showed an increase in their strength upon irradiation with a 60C0 source. Such observations were exhibited by Al–OH−-centers as noticed earlier. The crystals were thus noticed to exhibit the presence of Al–OH−-centers and also Al-hole centers in their as-received conditions. The observations match with the results on cultured quartz where irradiation breaks away the Al-alkali centers into a mixture of Al–OH−- and Al-hole centers. It is thus concluded that the natural crystals, which exhibit the presence of these centers in their as-received condition, have been irradiated in nature with a low dose.

Influence of metal ions on the structures of Keggin polyoxometalate-based solids: Hydrothermal syntheses, crystal structures and magnetic properties

Three new Keggin polyoxometalate (POM)-based compounds linked to 3d metal complexes have been synthesized under hydrothermal conditions: [Cu(phen) 2] 2{[Cu(phen)] 2 [SiMo 12O 40(VO) 2]} ( 1), {[Zn(phen) 2] 2[GeMo 12O 40(VO) 2]}{[Zn(phen) 2(H 2O)] 2 [GeMo 12O 40(VO) 2]}·3H 2O ( 2) and {[Co(phen) 2] 2[PMo 12O 40(VO) 2]}{[Co(phen) 2(OH)] 2 [PMo 12O 40(VO) 2]}·2.5H 2O ( 3) (phen=1,10-phenanthroline). These three compounds present, as building blocks, the bicapped Keggin anions [ XMo 12O 40(VO) 2] ( X=Si, Ge and P). Compound 1 consists of a bicapped Keggin anion [SiMo 12O 40(VO) 2] 2− linked to two [Cu(phen)] + complexes with two [Cu(phen) 2] + countercations. Compound 2 contains two bicapped Keggin anions [GeMo 12O 40(VO) 2] 4−, one linked to two [Zn(phen) 2(H 2O)] 2+ cations and the other one linked to two [Zn(phen) 2] 2+ cations. Compound 3 is a two-dimensional POM-based square network formed by bicapped Keggin anions [PMo 12O 40(VO) 2] 4− connected by [Co(phen) 2] 2+ cations. Discrete bicapped Keggin anions [PMo 12O 40(VO) 2] linked to two [Co(phen) 2(OH)] + cations are located between the layers. The magnetic properties show the presence of antiferromagnetic interactions among the reduced Mo(V) atoms (in the three compounds) plus a paramagnetic contribution from the V(IV) atoms (in 1 and 2). Compound 3 shows, in addition, an antiferromagnetic interaction between the Co(II) and the V(IV) ions directly linked through an oxygen bridge. The low-temperature ESR spectra of compound 3 confirm the presence of the reduced Mo(V) ions and the antiferromagnetic coupling between the Co(II) and the V(IV) ions.

Simultaneous novel synthesis of conducting and non-conducting halogenated polymers by electroinitiation of (2,4,6-trichloro- or 2,6-dichlorophenolato)Ni(II) complexes

NiL 2(Ph) 2· xH 2O [L=3,5-dimethylpyrazole or N-methyl imidazole; Ph=DCP or TCP; x=0, 1 or 3] complexes were synthesised and characterised by analytical and spectroscopic methods using elemental analysis and FTIR. The electrochemical behavior of the complexes was studied by cyclic voltammetry in tetrabutylammoniumtetrafluoroborate– N, N-dimethylformamide electrolyte–solvent couple. Cyclic voltammogram of the complexes displayed two-step oxidation processes under the nitrogen gas atmosphere. The polymerization of the complexes was accomplished in the same solvent–electrolyte couple by the constant potential electrolysis of NiL 2(Ph) 2· xH 2O, synthesizing the poly(di- or monochlorophenylene oxide)s via free radical mechanism. The simultaneous polymerization of non-conducting polymer and conducting polymer (the conductivity of 0.7 S cm −2) were achieved by electroinitiated polymerization of Ni(DMPz) 2(TCP) 2. The structural analysis of the polymers were performed using FTIR, 1H NMR and 13C NMR spectroscopic techniques and DSC for the thermal analysis. The kinetics of the polymerization was followed by in situ UV–vis spectrophotometer during the electrolysis. The low temperature ESR spectrum of the electrolysis solution also confirmed the formation of phenol radical ( g=2.0028). One electron oxidation process of NiL 2(DCP) 2· xH 2O produces a new Ni(II) complex, Ni(L–L)(DCP) 2(S) by the rapid decomposition of Ni IIIL 2(DCP) 2 into a ligand radical producing a singlet with the g value of 2.0015. Second electron oxidation process generates oligemers, which could not be isolated from the electrolyte solution.

Synthesis, crystal structure and properties of a new complex constructed from coordinated Dy(III) unit and the polyanion [SiMo12O40]4−: [Dy(NMP)4(H2O)3]H[SiMo12O40]·2NMP

The synthesis, X-ray crystal structure, IR, UV and ESR spectra, and thermal properties of the new complex formulated [Dy(NMP)4(H2O)3]H[SiMo12O40]·2NMP (NMP = N-methyl-2-pyrrolidone), are reported. The crystal structure indicates that the coordinated ion combines with [SiMo12O40]4− by an electrostatic force. There are several kinds of hydrogen bonding in the complex and the crystal structure shows that the water molecules in the coordination sphere play a dual role: they coordinate to the rare earth ions on one hand and form strong hydrogen bonds on the other. The complex is strongly photosensitive under irradiation with sunlight, resulting in charge transfer by oxidation of the N-methyl-2-pyrrolidone and reduction of the polyoxometalate. The low-temperature ESR spectra indicate that thermal electron delocalization occurs among the Mo atoms in the complex.

A novel 1D polyoxometallate-based coordination polymer [{Pr(DMSO) 6(H 2O)}(PMo 12O 40)·CH 3CN] n: properties, crystal structure and characterization

A novel compound, [{Pr(DMSO) 6(H 2O)}(PMo 12O 40)·CH 3CN] n , has been synthesized and characterized by IR and single crystal X-ray structural analysis. It forms an one-dimensional zigzag chain built from alternating polyanions and cationic units through Mo–O t–Pr–O t–Mo links in the crystal. The results of the single crystal X-ray diffraction analyses and IR are agreement and both show the metal cation units are coordinately bonded to the Keggin cluster. The low temperature ESR spectrum indicates thermal electron delocalization occurs among the Mo atoms in the compound. The result of CV shows that the title compound undergoes five two-electron reversible reductions and that [PMo 12O 40] 3− anions are active centre for electrochemical redox in the solutions, while matched cations have small effect on electrochemical redox.

Synthesis, Crystal Structure and Properties of Novel Composite Complex: [La(Nmp)4(H2o)4][HsiMo12O40]·2nmp·H2o

A new composite complex of [La(NMP)4(H2O)4][HSiMo12O40]·2NMP·H2O (NMP = N-methyl-2- pyrrolidone) was synthesized and characterized by IR, UV, ESR, TG-DTA and single crystal structural analysis. The water and organic ligands (NMP) coordinate directly with La(III) to form a dodecahedral complex and the SiMo12O4− 40 anion is the counter ion. IR and X-ray analysis show that there is strong interaction between the polyoxometalate and organic donors. The complex is strongly photosensitive under irradiation with sunlight, resulting in a charge-transfer complex by oxidation of the N-methyl-2-pyrrolidone and reduction of the polyoxometalate. Low temperature ESR spectra indicate thermal electron delocalization among the Mo atoms in the title compound.

ESR study of the ordering transformation inκ−(BEDT−TTF)2Cu[N(CN)2]Br

Ordering transformation and its effect on the electronic structure of the $\ensuremath{\kappa}\ensuremath{-}{(\mathrm{B}\mathrm{E}\mathrm{D}\mathrm{T}\ensuremath{-}\mathrm{T}\mathrm{T}\mathrm{F})}_{2}{\mathrm{Cu}[\mathrm{N}(\mathrm{CN})}_{2}]\mathrm{Br}$ organic superconductor was studied by measuring annealing induced changes in low temperature ESR spectra and spin susceptibility temperature dependence. The transformation is shown to proceed through coexistence of two phases. Besides resistivity decrease, ordering was shown to increase ESR spin susceptibility (below about 160 K) with a tendency for restoration of the temperature independent Pauli term. Gaplike spin susceptibility behavior below 60 K is shown to develop only at some stage of ordering. These effects point to a relationship between temperature dependent spin susceptibility in the salt and incomplete ordering.

Low temperature ESR spectra of nickel — doped NaCl crystals

Paramagnetic resonance of Bridgman-grown NaCl crystals nominally doped with divalent nickel was studied over the temperature range from 4 K to 300 K. The broad isotropic EPR line with g = 2.12 ÷ 2.23 and ΔHpp = 280 ÷ 780 G was detected at temperatures higher than 130 K for samples of various thermal history. The origin of these spectra was attributed to aggregated and precipitated Ni2+ ions. At temperatures lower than 40 K another ESR signal was registered. It was an anisotropic one, the intensity of which strongly depends upon the temperature. This spectrum results from substitutional Ni+ ions with tetragonally distorted surrounding along a S100T direction. The angular dependence of the ESR line positions was fitted to an appropriate spin-Hamiltonian with the g-factor components g∥ and g⊥ equal to 2.86 and 2.10, respectively. The temperature-dependence of the signal intensity at temperatures between 4 and 110 K has been interpreted in terms of a transition from the static to dynamic Jahn-Teller effect.

Low-temperature photochemistry of nitro-substituted aromatic azides; subsequent reactions of intermediates

Triplet 4-nitrophenylnitrene ( 1B) and triplet 4-nitrene-4′-nitrostilbene ( 2B) are characterized by their low-temperature UV-Vis and ESR spectra as well as by their consecutive thermal and photochemical reactions. The main thermal reaction of 1B and 2B between 77 and 90 K is a stepwise, twofold hydrogen abstraction from the methyltetrahydrofuran (2-MTHF) matrix solvent, leading to the corresponding primary amines. The activation energy of the first hydrogen abstraction of 1B and 2B is not influenced by their structure and is about 30 kJ mol −1. A similar amount of energy is required for the second hydrogen abstraction. Since the activation energy required for diffusion in a solid 2-MTHF matrix is about 87 kJ mol −1, bimolecular processes that require diffusion are less efficient between 77 and 87 K. While 2B is photochemically stable, 1B reacts after optical excitation under hydrogen abstraction from the solvent to form an imino radical 1C. The photochemically induced hydrogen abstraction after the excitation of 1B and the absence of the analogous reaction after the excitation of 2B can be explained by means of two models; either as a hot ground state reaction or as a reaction starting from an electronically excited state of 1B. In the latter model, it is assumed that the activation energy of the hydrogen abstraction is lower in the first excited state than in the electronic ground state of 1B. Both models predict that the hydrogen abstraction is more efficient if the difference in energy between the electronic ground and electronically excited states is high enough. This is the case in 1B but not in 2B. The experimental results are discussed in connection with theoretical statements arising from quantum chemical calculations.

Effect of oxygen on phthalocyanine radicals. 1. ESR study of lithium phthalocyanine spin species at different oxygen concentrations

Magnetic resonance data of LiPc . , in the temperature range 4.2-380 K, are reported and analyzed. The low-temperature ESR spectrum exhibits two distinguishable lines that we call A and C, whereas at high temperature a third line, denoted by B, develops. Detailed studies of these three line as a function of oxygen concentration, crystal orientation, and microwave power indicate that line A is associated with diffusing spins more or less localized by interaction with the spins of oxygen. Line B is also due to diffusing spins, but with those that never interact with O 2 , while line C is associated with localized spins due to crystal defects

Addition Effect of NaF on the Deodorant Efficiency of an Iron Phthalocyanine Derivative

Abstract A low temperature ESR spectrum shows that Fe(III) of an iron phthalocyanine derivative treated with NaF takes a high spin state. The deodorant efficiency of the derivative treated with NaF is three times higher than that without NaF treatment.

銅イオン担持消臭材料の消臭効率に及ぼす担持ポリマーの影響

We measured the low temperature ESR spectra of mixed aqueous solution of Cu(1T) ion and water soluble polymer and estimated the electron densities on dx2-y2. orbital from anisotropy of g value and hyperfine coupling constant. The electron density relates to the deodorant efficiency of the deodorant material consisting of Cu (11) ion and the various polymer.

銅クロロフィリンの水溶液中での存在状態

The dependency of the difference electronic spectrum of copper chlorophyllin (Cu-chin) on concentration was observed and it was found that a part of Cu-chin in aqueous solution dimerizes above 10-3 mol⋅dm-3. The center-center distance in this dimer is estmated to be 10.4 A and angle formed by Cu-chin planes to be 155°. The low temperature ESR spectrum of Cu-chin indicated that Cu-chin chagnes to polymer in aqueous solution but does not in 5% ethanol solution.

Synthesis and proton NMR studies of the electronic structure of ferric phosphine porphyrin complexes

The preparation and spectral properties of low-spin ferric phosphine complexes of a series of synthetic and natural porphyrins have been reported. The proton NMR spectra of Fe(TPP)(PMe/sub 3/)/sub 2/ClO/sub 4/ and Fe(TPP)(PMe/sub 3/)(1-Melm)ClO/sub 4/ have been analyzed. The axial ligand hyperfine shifts have been separated into their dipolar and contact contributions by using both the dominant dipolar shifts for certain porphyrin positions and the magnetic anisotropy data derived from low-temperature ESR spectra. The contact shifts are shown to arise primarily from iron ..-->.. phosphine ..pi..* charge transfer. The trimethylphosphine H peak is shown to shift characteristically upfield on going from (Fe(TPP)PMe/sub 32/)/sup +/ to (Fe(TPP)(PMe/sub 3/)(1-MeIm))/sup +/, confirming that this resonance may serve as a new probe for hemoproteins. 25 references, 2 figures, 3 tables.

ESR and conductivity studies of doped polyacetylene

Low temperature ESR spectra of cis-rich and trans-rich polyacetylene, lightly doped with pentafluorides (AsF5, SbF5) exhibit two clearly distinguishable ESR lines. Detailed studies of these two lines as a function of temperature, doping concentration and microwave power indicate that one of these lines originates from localized spins, the other from conduction electrons. The concentration of the “localized spins”, generally speaking, decreases with doping. The Pauli-like susceptibility associated with the conduction electrons changes from 1.5×10−7 to 15×10−7 emu·cm3/mole for various doping levels and correlates with the conductivity of the same samples. The longitudinal relaxation time of the localized spins in doped trans-rich was found to beT1≅100 μs at low temperatures, consistent with values of others in undoped trans rich. The temperature dependence of the homogeneous transverse relaxationT2 of the localized spins in doped trans-rich polyacetylene is similar to that of undoped trans polyacetylene. These facts show that the localized spins originate from undoped regions of the doped polyacetylene, perhaps due to inhomogenous doping. Our results are consistent with either formation of metallic islands, or, probably more, with impurity band conduction in strongly disordered semiconductors.

Magnetic investigations on some copper(II) N-acetyl- and N-benzoyl-alaninates

The magnetic properties of Cu(Ac-α-ala) 2·H 2O, Cu(Bz-α-ala) 2·H 2O and Cu(Bz-β-ala) 2 (Ac-α-ala = N-acetyl-DL-alaninate, Bz-α-ala = N-benzoyl-DL-alaninate, Bz-β-ala = N-benzoyl-β-alaninate ion) have been investigated by means of variable-temperature magnetic and ESR measurements. The magnetic behaviour of the compounds suggests binuclear structures with exchange integrals typical of copper acetate monohydrate-like complexes. Low-temperature ESR spectra of the powdered samples show triplet absorptions characteristic of axially symmetric (Cu(Ac-α-ala) 2·H 2O and Cu(Bz-α-ala 2·H 2O) or rhombically distorted (Cu(Bz-β-ala) 2) copper(II) carboxylate dimers. A polymeric structure is proposed for Cu(Bz-β-ala) 2 due to the detection of interdimer exchange between triplet states.

Low-temperature crystal structure and ESR spectra of bis(isothiocyanato)[N-(2-(diphenylphosphino)ethyl)-N',N'-diethylethylenediamine]cobalt(II), a five-coordinate complex exhibiting spin equilibrium

Low-temperature crystal structure and ESR spectra of bis(isothiocyanato)[N-(2-(diphenylphosphino)ethyl)-N',N'-diethylethylenediamine]cobalt(II), a five-coordinate complex exhibiting spin equilibrium

Jahn-Teller and Reorientation Effects in the ESR Spectrum of the Self-Trapped Holes in Mixed Crystals AgBr1-xClx. II

Electron spin resonance study shows that holes in AgBr 0.001 Cl 0.999 crystal are self-trapped as the complexes (AgBrCl 5 ) 4- besides the complexes (AgCl 6 ) 4- . The complex (AgBrCl 5 ) 4- has a rhombic symmetry due to a Jahn-Teller effect and to a tetragonal crystal field created by a Br ligand. The energy barrier between two configurations of the complex (AgBrCl 5 ) 4- is estimated to be lower than those between three configurations of the complex (AgCl 6 ) 4- , so that we can expect occurrence of a dynamic or intermediate Jahn-Teller effect for the complex (AgBrCl 5 ) 4- . The ESR line shapes have been calculated for several values of \(2\varGamma/\delta\), which represents the ratio of the tunneling splitting to the random strain splitting. The result shows that the low-temperature ESR spectrum of the complex (AgBrCl 5 ) 4- is associated with an intermediate Jahn-Teller effect.