NMRON studies of ferromagnetic hcp and fcc Co

The high resolution hyperfine spectroscopy, modulated adiabatic passage of oriented nuclei (MAPON), has been applied for the first time to high purity, elemental systems. Detailed comparisons between the electric quadrupole hyperfine interactions (EQI’s) and, in particular, their distributions, are obtained for⁶⁰CoCo where the hosts are a single crystal of hcp cobalt and a polycrystalline cobalt foil of predominantly fcc character. For hcp Co, with the electronic magnetization, M, parallel to the c-axis, the mode value P/h=3e²qQ/4I(2I−1)h=−48.5(5) kHz. This fractional distribution implies the sharpest electric field gradient (efg) measured in a metal to date, using MAPON spectroscopy, in excess of two times sharper than that of the most dilute impurity efg in a crystallographically cubic ferromagnetic host. The mode efg is V_zz=−27.3(32)×10¹⁹ Vm⁻². For the polycrystalline, predominantly fcc foil, prepared by quenching, the EQI mode value is P/h=−6.2(4) kHz with a FWHM of 12.0(7) kHz yielding a mode efg of V_zz=−3.5(5)×10¹⁹ Vm⁻².     

Anisotropic ESR parameters of Cu(II) complexes in solutions and dynamics of solvent coordination: use of “minimum linewidths”

The “minimum linewidths” seen in the ESR linewidths against temperature plots, the dependence of line widths on the⁶³Cu nuclear magnetic quantum numbers and the Hubbard relation provide sufficient number of equations to determine the anisotropic ESR parameters in the case of axially symmetric Cu(II) complexes even when unresolved hyperfine structures make contributions to linewidths. After testing the method by reanalysing the literature data on Cu(II) bis-acetylacetonate, it has been used to obtain the anisotropic ESR parameters in the case of bis-salicylaldehydate of Cu(II). Linewidth contributions from unresolved hyperfine structures associated with the¹H of coordinating CHCl₃ inferred in these studies, were confirmed by comparing the widths in CHCl₃ and CDCl₃ under ideal conditions. The temperature dependence of this contribution and the estimate of rate constant at room temperature (∼ 10¹⁰ s⁻¹) suggest that the coordinating solvent exchange is diffusion controlled.     

ESR studies on calcite encrustation on Fili neotectonic fault, Greece

Electron spin resonance (ESR) study was done on calcite encrustation on Fili neotectonic fault surface, Greece. Normally such calcite encrustations on fault surfaces are not observed. Significantly, the ESR study has detected the presence of nitrate NO₃²⁻ radical in this calcite encrustation, havingg _⊥=2.0063±0.0001 and hyperfine coupling constantA _⊥=3.44 mT, the second such detection of nitrate NO₃²⁻ radical following a sample from Scott Glacier, Antarctica. From isochronal thermal annealing measurement the NO₃²⁻ radical was found to be quite stable, only fully annealed at 475°C. This study also shows that the ESR, as a tool, can be suitably applied to date the age of formation of the calcite encrustation with SO₃⁻ as an ESR dating signal by additive γ-ray irradiation. A preliminary estimation indicates the age of formation of calcite precipitation at Fili fault, Greece to be about 5600 years.     

Genesis of supported carbon-coated Co nanoparticles with controlled magnetic properties,prepared by decomposition of chelate complexes

Following procedures formerly developed for the preparation of supported heterogeneous catalysts, carbon-coated cobalt nanoparticles dispersed on porous alumina have been prepared by impregnation of γ-Al₂O₃ with (NH₄)₂[Co(EDTA)] and thermal decomposition in inert atmosphere. Below 350 °C, Co(II) ions are complexed in a hexa-coordinated way by the EDTA ligand. The thermal treatment at 400–900 °C leads to the EDTA ligand decomposition and recovering of the support porosity, initially clogged by the impregnated salt. According to X-ray absorption spectroscopy, and due to in situ redox reactions between the organic ligand and Co(II), both oxidic and metallic cobalt phases are formed. Characterisation by transmission electron microscopy, X-ray diffraction and magnetic measurements reveals that an increase in the treatment temperature leads to an increase of the degree of cobalt reduction as well as to a growth of the cobalt metal particles. As a consequence, the samples prepared at 400–700 °C exhibit superparamagnetism and a saturation magnetisation of 1.7–6.5 emu g⁻¹ at room temperature, whilst the sample prepared at 900 °C has a weak coercivity (0.1 kOe) and a saturation magnetisation of 12 emu g⁻¹. Metal particles are homogeneously dispersed on the support and appear to be protected by carbon; its elimination by a heating in H₂ at 400 °C is demonstrated to cause sintering of the metal particles. The route investigated here can be of interest for obtaining porous magnetic adsorbents or carriers with high magnetic moments and low coercivities, in which the magnetic nanoparticles are protected from chemical aggression and sintering by their coating.     

Static and dynamic hyperfine interactions of Mn and Co impurities in the weak ferromagnet ZrZn2

We report the nuclear orientation of near-stoichiometric samples of ZrZn₂ containing trace impurities of radioactive⁵⁴Mn and⁶⁰Co in the temperature range 6.5–60 mK and in applied fields between 0.2 and 5.7 T. Analysis of the resullts indicates that in both cases, the transition-element impurities occupy two sites (the Zr and Zn sites) with about equal probablities. In one site, most probably that of Zr, both impurities have positive hyperfine fields of +12.0 T and +3.8 T for Mn and Co, respectively; in the other site, the corresponding hyperfine fields are negative, with the values −6.0 T and −6.9 T. The dynamic hyperfine interaction of Mn in ZrZn₂ has also been studied using the thermal cyclic method with a weak thermal link, and was analyzed for the two relaxation times corresponding to the different lattice sites.     

ESR and ENDOR investigations of the degenerate electron exchange reactions of various viologens in solution. Solvent dynamical effects

The temperature dependences of the rates of the degenerate electron transfer of various viologens (1,1′-di(hydrocarbyl)-4,4′-bipyridinium salts) are measured in seven different solvents by means of electron spin resonance (ESR) line broadening. Rates vary between 1.7·10⁸ and 1.1·10⁹ M⁻¹s⁻¹ at room temperature and clearly show a solvent dynamical effect, which is inferred from the dependence of the rate constants on the longitudinal relaxation time of the solvent. Activation energies ranging from 5.3 to 24.4 kJ mol⁻¹ are found. For the first time, hyperfine coupling constants are reported for the radical cations of the hydroxyethyl viologen and the amino viologen based on both continuous-wave ESR and electron-nuclear double resonance spectroscopy. Furthermore, the temperature and the solvent dependence of the hyperfine coupling constants of the methyl viologen radical cation are reported.     

Optical properties of the ternary compound Cd<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>Se:Co

We present the results of experimental studies of the optical properties of cobalt-doped Cd _x H_1−x Se (x = 0.18) single crystals with cobalt ion concentrations of NCo = 5·10¹⁸, 5·10¹⁹, and 1·10²⁰ cm⁻³ at T = 90 K and 300 K. The composition (x = 0.18) of the Cd _x Hg_1−x Se solid solution was selected so that the hypothetical resonance level is found on the bottom of the conduction band. We show that the cobalt ions in the mercury selenide can form a resonance donor level only for cobalt concentrations N_Co < 5·10¹⁸ cm⁻³. For N_Co ∼ 5·10¹⁸ cm⁻³, the cobalt ions substitute for mercury atoms, forming a solid solution and leading to an increase in the bandgap width and a change in the physical properties. The solubility of cobalt in the HgSe lattice can be greater than 5%–10%. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 74, No. 1, pp. 73–77, January–February, 2007.     

Hyperfine magnetic fields in Fe−Co alloys and their tempera ture dependences

We obtained⁵⁷Fe hyperfine field parameters from Fe₁−x-Co _x alloys (0≤x≤0.6) from 77 K to 900 K. We first discuss the origin of the low temperature hyperfine fields in terms of the 3d and 4s electrons at⁵⁷Fe atoms. The⁵⁷Fe hyperfine magnetic field (hmf) of Fe-Co alloys depends more weakly on temperature than the hmf of pure Fe. This temperature dependence occurs because the alignment of the magnetic moments at both the Fe atoms and at the Co atoms depend on temperature in the same way as the bulk magnetization of Fe-Co alloys.     

Nucleoside-copper(II) system in water and on 13X-zeolite studied by continuous-wave and pulsed-wave ESR

In order to study the effects of large-surface-area solids on the formation of biomacromolecules, copper(II)-nucleosides complexes were studied in water at high pH and after contact with the cavity walls of 13X-zeolite. The results were mainly collected by electron spin resonance in continuous (cw-ESR) and pulsed (ESE) wave. Particular attention was dedicated to adenosine as a nucleoside model for the formation of compounds which were fully characterized in alkaline water solution and after adsorption on commercial 13X-zeolite. In aqueous solution, adenosine was coordinated to copper(II) through deprotonated hydroxyl groups in the 2′ and 3′ positions of the ribose unit, with the formation of [Cu(adenosine)₂(H₂O)₂]²⁻. When adsorbed on zeolite, both cw-ESR and ESE showed that a fraction of the adsorbed complexes did not change their structure and showed high mobility in the zeolite faujasite cavity. The remaining fraction directly interacted with²⁷Al nuclei of the zeolite framework. Other copper(II)-nucleosides behaved in the same manner. Surprisingly enough, alkaline solutions of nucleosides were able to remove copper (II) from zeolite in the form of [Cu(adenosine)₂(H₂O)₂]²⁻ which freely moved in the supernatant aqueous liquid.     

Negative muon spin precession in ferromagnetic iron and the hyperfine anomaly

The hyperfine field (B _μ ^hf ) at the negative muon μ⁻ in ferromagnetic iron was investigated by means of the zero-field μ⁻ spin precession technique. In the temperature range 320–690 K,B _μ ^hf for μ⁻ Fe departs from the magnetization curve of pure iron in the same way as the hyperfine field seen by a⁵⁵Mn impurity in dilute MnFe measured by NMR. The hyperfine anomaly for μ⁻ Fe relative to dilute (1.5 at.%)⁵⁵Mn in iron is found to be −0.9(3)% and temperature independent over the temperature range investigated.     

Single-Crystal EPR Identification of a Low Hyperfine Value and Interstitial Position of Copper Impurity in Diaquabis[malonato(1-)-κ2O,O′] Zinc(II)

Electron paramagnetic resonance studies have been carried out at 300 K on the Cu(II)-doped [Zn(C₃H₂O₄)₂(H₂O)₂] system in single-crystal and powder forms in order to rationalize the low parallel ⁶³Cu hyperfine value. Angular variation of the hyperfine resonances in the three orthogonal planes shows the presence of only one magnetic site with g and A values equal to g _zz = 2.455, g _yy = 2.121, g _xx = 2.105 and A _zz = 160.9 · 10⁻⁴ cm⁻¹, A _yy = 12.5 · 10⁻⁴ cm⁻¹, A _xx = 7.35 · 10⁻⁴ cm⁻¹. The crystal structure of the host lattice is isostructural with the corresponding cobalt complex and contains two molecules per unit cell. The low magnitude of A _zz value for the complex is rationalized in terms of an admixture of the ground state with the excited state and delocalization of the unpaired spin density onto the ligands. In addition, the highest hyperfine value obtained from the single-crystal data (160.9 · 10⁻⁴ cm⁻¹) is considerably larger than that obtained from the powder spectrum (138 · 10⁻⁴ cm⁻¹). Authors' address: P. Sambasiva Rao, Department of Chemistry, Pondicherry University, Pondicherry 605014, India     

EPR spectra of γ-irradiated hydrated barium dithionate single crystals

EPR spectra of barium dithionate hydrate single crystals γ-irradiated at low (80 Gy) and high (10 kGy) doses are studied. Four lines, the strongest of which is due to the SO₃⁻ radical, are observed in the EPR spectrum of the low-irradiated samples. Another line seems to belong to SO₂⁻. The strong line and weak lines with hyperfine structure and lines for pairs of closely spaced SO₃⁻ centers with a strong angular dependence are observed at high irradiation doses. The main values of the SO₃⁻ hyperfine coupling tensor and the dipole-dipole coupling constants of the SO₃⁻ pairs are determined. A quantum-chemical calculation of the electronic structure of isolated SO₃⁻ and SO₂⁻ radicals is performed. Values of the g tensors and hyperfine couplings are calculated. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 75, No. 1, pp. 18–22, January–February, 2008.     

Quadrupolar interaction in iridium cyanide mixed-ligands complexes in alkali halide matrices by ESR spectroscopy

Quadrupolar interactions in [IR(CN)₅]³⁻, [Ir(CN)₅Cl]⁴⁻ and [Ir(CN)₄Cl₂]⁴⁻ paramagnece a complexes in alkali halide host lattices are systematically analysed by ESR spectroscopy. The measured quadrupolar interactions are shown to have no clear correlation to the measured unpaired electron populations.     

Synthesis and characterization of α-cobalt hydroxide nanobelts

α-Cobalt hydroxide was synthesized by a facile hydrothermal process from Co(Ac)₂ and NH₃·H₂O in the presence of 1,3-propanediol. The large-scale-prepared cobalt hydroxide has a uniform nanobelt morphology with a considerably high aspect-ratio more than 20 which may be advantageous for exploration of their physicochemical properties. This synthetic method is convenient, economical, and controllable. The samples were characterized by powder X-ray diffraction, energy dispersive spectrum, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, CHN element analysis, thermogravimetric and differential-thermogravimetric analysis, which revealed the compound is lamellar structural cobalt organic–inorganic hybrid with the chemical formula of Co(OH)_1.49(NH₃)_0.01(CO₃ ²⁻)_0.22(Ac⁻)_0.07(H₂O)_0.11 and single-crystalline.     

Electron paramagnetic resonance studies in photorefractive crystals I: Hyperfine interaction and photoinduced charge transfer in233U5+ and238U5+ doped LiNbO3

Electron paramagnetic resonance studies were conducted on the photoinduced charge transfer and also hyperfine interaction of U⁵⁺ stabilized in photorefractive matrix LiNbO₃. This work deals with: (i) first observation of hyperfine structure due to²³³U (I=5/2) in its pentavalent state at octahedral sites and comparison with other possible site symmetries, (ii) photoinduced charge transfer as observable by EPR and its relevance to photorefractive behaviour of LiNbO₃. The effect of chemical bonding on the hyperfine interaction of 5f ¹ configuration was also studied by converting the existing literature data on²³⁵U⁵⁺ to that of²³³U⁵⁺ by standard methods. This suggests that progressive substitution of oxygen by F⁻, in the series UO ₆ ⁷⁻ , (UO₅F)⁶⁻ and (UO₄F₂)⁵⁻ drastically decreases the hyperfine coupling constantA _∥, along the local distortion axis. This trend is explained as being due to the absence of ligand ion along the distortion axis at U⁵⁺ site in trigonal LiNbO₃. The effects of illumination by copper vapor laser (CVL) on the intensity of the U⁵⁺ signal was studied in the 10–300K region. The kinetics of decay and restoration of U⁵⁺ was also studied between 10–100K range. The decay kinetics was found to obey double exponential. The reduction of concentration of U⁵⁺ with CVL-illumination and its restoration in the absence of light show that pentavalent uranium takes part in the photorefractive effects in LiNbO₃.     

Hyperfine interactions in paramagnetic centres in beryllium oxide crystals

The hyperfine structure of ESR spectra of new paramagnetic centres in neutron exposed and X-rayed beryllium single crystals was identified. A new data on hyperfine interaction in Zn⁺ and P⁻ centres are presented. Spin-Hamiltonian parametres were obtained.     

NMR-ON measurements of187WFe,182,183,186ReNi,186ReFe and203PbFe

The magnetic hyperfine splitting frequencies of¹⁸⁷WFe,¹⁸²Re(j ^π=2⁺)Ni,¹⁸³ReNi,¹⁸⁶ReNi,¹⁸⁶ReFe and²⁰³PbFe in a zero external magnetic field have been determined by the NMR-ON method at about 7 mK as 225.56(6), 130.9(1), 98.17(4), 136.6(4), 1007.0(3) and 58.43(3) MHz, respectively. With the knowng-factors ofg(¹⁸⁶Re, 1⁻)=1.739(3) andg(²⁰³Pb, 5/2⁻)=0.27456(20), the following hyperfine fields were deduced:B _HF(¹⁸⁶ReNi)=−103.05(35) kG;B _HF(¹⁸⁶ReFe)=−759.7(13) kG;B _HF(²⁰³PbFe)=+279.18(25) kG. Taking hyperfine anomalies into account, theg-factor of¹⁸³Re was deduced as |g(¹⁸³Re, 5/2⁺)|=1.267(6). With the assumption of Knight shift factorK=0, theg-factors of¹⁸²Re and¹⁸⁷W and the hyperfine field of¹⁸⁷WFe were determined as |g(¹⁸²Re, 2⁺)|=1.63(5), |g(¹⁸⁷W, 3/2⁻)|=0.414(10) andB _HF(¹⁸⁷WFe) =−714(18) kG. The large hyperfine anomaly was deduced to be¹⁸³W Δ¹⁸⁷W =−0.124(22).     

Hyperfine structure of <Emphasis Type="Italic">S</Emphasis>- and <Emphasis Type="Italic">P</Emphasis>-wave states in muonic-helium ion

Corrections of order α ⁵ and α ⁶ to the hyperfine structure of S- and P-wave energy levels of the muonic-helium ion are calculated. Electron-vacuum-polarization effects, corrections for the nuclear structure, and recoil effects are taken into account. The numerical values obtained for respective hyperfine splitting, −1334.73 meV (1S), −166.64 meV (2S), −58 712.90 μeV (2P _1/2), and −24 290.69 μeV (2P _3/2), can be viewed as a reliable estimate for a comparison with experimental data, and the hyperfine-structure interval of Δ₁₂ = 8ΔE ^hfs(2S) − ΔE ^hfs(1S) = 1.59 meV can be used to test QED predictions.     

Paramagnetic hyperfine structure of193Ir in K2IrCl6 diluted into K2PtCl6

The paramagnetic hyperfine splitting of Ir⁴⁺ (5d⁵) in K₂IrCl₆ diluted into diamagnetic K₂PtCl₆ has been observed at 4.2 K and Ir∶Pt ratios of 1∶10 and 1∶25. In the latter case a narrow paramagnetic pattern with a hyperfine coupling constant ofA=−13.1(2) mm/s was observed, but both samples also exhibit a single Ir⁴⁺ line typical for fast relaxation, either because of macroscopic inhomogeneities in the Ir distribution or because part of the Ir spins are still coupled to nearest Ir neighbours.     

On the electronic structure of N,N′-ethylenebis(acetylacetonatiminato)Co(II), Co(II)acacen

Single crystal E.P.R. and cobalt ENDOR measurements on N,N′-ethylenebis(acetylacetonatiminato)Co(II), Coacacen, diluted in Niacacen·1/2 H₂O are reported. Forbidden Δm _Co = 1,2 transitions in the E.P.R. spectra have been observed. The g-tensor (E.P.R.) and the cobalt hyperfine and quadrupole tensors (ENDOR) have been determined. The g- and cobalt hyperfine tensors are discussed. They support the |² A ₂,yz? groundstate proposed for four coordinated low-spin Schiff base complexes of cobalt(II). The measured quadrupole coupling is also compatible with a |² A ₂,yz? groundstate, if anisotropic contraction of the cobalt 3d orbitals is taken into consideration.