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Iron-doped ZnO nanocrystals, ZnO (Zn1−xFexO), with doping atomic ratio x = 0.0–0.05, have been synthesized using the sol–gel method to obtain nanoparticles with an average crystal size of around 38.0 nm. Materials were characterized by X-ray fluorescence, Raman and Fourier transform infrared (FTIR) spectroscopies, diffuse reflectance spectroscopy and continuous-wave electron paramagnetic resonance (EPR). FTIR spectroscopies suggest minor change in the Zn–O band occurring in the ZnO environment when the ZnO is Fe doped. X-ray diffraction measurements indicate that lattice parameters of Fe-doped ZnO do not change for all doping levels x. Raman studies show that no peaks disappear when Fe concentration increases, indicating that the incorporation of Fe does not cause an important local structural disarray in the crystalline ZnO lattice. Two broad bands appear at ≈ 570 cm−1 and ≈ 650 cm−1, which are related to the Zn substitution by iron. The band gap does not seem to be influenced by the average crystal size or the cell parameter. It could be because the crystal size is large enough to prevent quantum confinement. X-band EPR spectra recorded at 10 K are similar for all doping levels, x = 0.01–0.05, and are consistent with a spin Hamiltonian composed by isotropic Zeeman and second-order axial symmetry zero-field splitting (ZFS) interactions. Due to the small size of the particles and the difference between the ionic radii of Fe3+ and Zn2+, the EPR spectra are strongly influenced by a static distribution of ZFS parameters, leading to a severe lineshape broadening. An efficient strategy for the simulation process is explained in detail. [ABSTRACT FROM AUTHOR]

This work describes an Electron Paramagnetic Resonance (EPR) and Nuclear Magnetic Resonance (NMR) study of the vanadium pentoxide xerogel V 2 O 5 :nH 2 O with n ˜ 1.8 Experiments were performed in the temperature range 65 K–310 K. The EPR spectrum at high temperatures exhibits the typical liquid-like eight lines-hyperfine structure. At low temperatures the EPR spectrum change to a V 4+ anisotropic powder spectra. These two different regimes can be delimited by a transition temperature region centered at 280 K. Numerical simulations of the EPR spectra in the two temperature limits are in good agreement with the experimental data. Proton ( 1 H) NMR lineshapes, as functions of temperature were measured in the range 150–323 K and indicate that nuclear motional narrowing is effective at temperatures above 210 K, with an activation energy of 0.14 eV. The NMR spin-lattice relaxation recovery, associated to protons in the water molecules, was found to be non-exponential throughout the temperature range and described by two different relaxation processes. The slow relaxing component is temperature-independent and was attributed to water molecules located far from V 4+ ions. The temperature dependence of the fast relaxing component shows maximum at around 260 K, suggesting a relaxation process sensitive to the temperature induced dynamic structural changes in the vanadium oxide matrix. [ABSTRACT FROM AUTHOR]

Continuous-wave (cw) and pulsed electron paramagnetic resonance (pulsed EPR) techniques were used to investigate spin dynamics in moderately and slightly doped poly(o-methoxyaniline), a polyaniline derivative. EPR line shapes, measured by standard cw EPR, and relaxation rates, measured by electron spin-echo methods, were studied as a function of temperature, in the ranges 6-300 K (cw) and 6-100 K (pulsed). Experimental results were explained by a model which considered two types of spins: isolated polarons in an insulating matrix, and delocalized ones in conductive islands (clusters) formed by dopant aggregates. Experimental data agreed with the assumption that EPR spectra are generated by isolated polarons coupled to the clusters by dipole-dipole interactions. Furthermore, some experimental evidence suggests that spin dynamic within the clusters, which play an essential role in the interpretation of the EPR results, is determined by exchange interactions. Consequences of the proposed model are discussed. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Glass structure and fluorine motion dynamics are investigated in lead-cadmium fluorgermanate glasses by means of differential scanning calorimetry, Raman scattering, x-ray absorption (EXAFS), electrical conductivity (EC), and 19F nuclear magnetic resonance (NMR) techniques. Glasses with composition 60PbGeO3-xPbF2-yCdF2 (in mol%), with x+y=40 and x=10, 20, 30, 40, are studied. Addition of metal fluorides to the base PbGeO3 glass leads to a decrease of the glass transition temperature (Tg) and to an enhancement of the ionic conductivity properties. Raman and EXAFS data analysis suggest that metagermanate chains form the basic structural feature of these glasses. The NMR study leads to the conclusion that the F-F distances are similar to those found in pure crystalline phases. Experimental results suggest the existence of a heterogeneous glass structure at the molecular scale, which can be described by fluorine rich regions permeating the metagermanate chains. The temperature dependence of the NMR line shapes and relaxation times exhibits the qualitative and quantitative features associated with the high fluorine mobility in these systems. [ABSTRACT FROM AUTHOR]

Impedance spectroscopy and nuclear magnetic resonance (NMR) were used to investigate the mobility of water molecules located in the interlayer space of H+ - exchanged bentonite clay. The conductivity obtained by ac measurements was 1.25 × 10-4 S/cm at 298 K. Proton (1H) lineshapes and spin-lattice relaxation times were measured as a function of temperature over the temperature range 130-320 K. The NMR experiments exhibit the qualitative features associated with the proton motion, namely the presence of a 1H NMR line narrowing and a well-defined spin-lattice relaxation rate maximum. The temperature dependence of the proton spin-lattice relaxation rates was analyzed with the spectral density function appropriate for proton dynamics in a two-dimensional system. The self-diffusion coefficient estimated from our NMR data, D ∼ 2 × 10-7 cm2/s at 300 K, is consistent with those reported for exchanged montmorillonite clay hydrates studied by NMR and quasi-elastic neutron scattering (QNS). [ABSTRACT FROM AUTHOR]

Measurements of 1 H and 13 C Nuclear Magnetic Resonance (NMR) for the nanocomposite materials formed by the intercalation of hexadecylamine (HAD) in metal oxides (TiO 2 , V 2 O 5 , CrO 3 and MoO 3 ), are reported. The 1 H NMR relaxation results reflects the complexity of the chain segments motions in these systems. The proton (and the carbon) mobility depends on the position in the HDA hydrocarbon chain to the restriction imposed by the anchoring of the head group. The nonexponential behavior of the proton magnetization is clearly a result of the anisotropic motion of the HDA chains. For most of the protons, the rate of motion is not fast enough to fulfill the condition for the nuclear spin-lattice relaxation (NSR) maximum in the laboratory frame but they fulfill the condition for the NSR in the rotating frame. The maximum at around 280 K reflects the relative lower mobility of the protons of the HDA chain inside the low-dimensional space of the metal oxide. The description of the segmental motion of long chains require the use of an asymmetric distribution of correlation times to describe relaxation in flexible polymer chains. [ABSTRACT FROM AUTHOR]

Describes a moving coil alternating current (ac) magnetic susceptometer. Reduction of probe coupling using a magnetic shield; Control of the probe movement and bridge operation through a personal computer; Use of the probe system with a helium flow refrigerator; Need for the ac magnetic field in the primary coils to be homogeneous.

In this work the magnetic properties of bimetallic phases M′0.2Mn0.8PS3·0.25H2O (M′ = CoII, NiII, CuII or ZnII) have been explored and compared with those of the pristine phase MnPS3. Magnetic susceptibility, high field magnetization and electron paramagnetic resonance (EPR) studies reveal that the transition temperature between the antiferromagnetic and paramagnetic order for the pristine phase is shifted to lower values in the bimetallic phases. From magnetization measurements the critical field of the spin-flop transition is found to be dependent on the nature of the added secondary transition metal ion. EPR spectra of all compounds in the temperature range of 8–300 K present a single resonance line shape. Temperature dependence of the EPR parameters, like line width, g values and double integrated area (IDIN), are obtained from the spectra and present a scenario compatible with the magnetization results. The temperature dependence of the first derivative of the product (IDINT) shows two maxima for all samples, with exception of the CoII phase, indicating two critical temperatures, while these critical temperatures could not be clearly determined by dc susceptibility. [ABSTRACT FROM AUTHOR]

The present paper focuses on the results obtained from impedance spectros-copy, thermal analysis (DSC and DMA) and electron paramagnetic resonance (EPR) of membranes of DNA plasticized with glycerol and containing either poly(cthylene dioxylhiophcne) (PEDOT) or Prussian Blue (PB) and copper perehlorale (CuClO4). The ionic conductivity values for the DNA-PEDOT-CuClO4 and DNA-PB-CuClO4 of 4.8x10-4 S/cm and 2.2x10-4 S/cm at 30 °C, respectively, were obtained. The DSC analysis of the samples showed two glass transition temperatures at about -80 and 50 °C and DMA a permanent deformation above 50 °C ascribed to a transition between elastic and plastic states. The EPR spectra evidenced that Cu2+ ions are in tetragonally distorted octahedral sites and the quantitative analysis of the EPR spin Hamiltonian g|| and A|| parameters revealed copper ions complexed by nitrogen and oxygen atoms. All these results suggest that DNA-biomembrancs are interesting materials to be used in materials engineering science. [ABSTRACT FROM AUTHOR]

Iridium-based compounds are materialsof great interest in theproduction of highly efficient organic light emitting diodes and severalother applications. However, these organometallic compounds presentrelative low stability due to photodegradation processes still notwell understood. In this work we investigated paramagnetic statesinduced by UV photoexcitation on iridium(III) bis[(4,6-fluorophenyl)-pyridinato-N,C2′]picolinate(FIrpic) and iridium(III)-tris(2-phenylpyridine) (Ir(ppy)3) complexes dispersed in different polymeric matrices by electronspin resonance (ESR). Photogenerated charged states with relativelystrong hyperfine interactions were observed and attributed to matrix/complexcharge-transfer processes. Measurements of the signal amplitude decayafter photoexcitation interruption were performed as a function oftemperature. The photoinduced centers are thermally activated withenergy barrier between 0.3 and 0.6 eV. Electronic structure calculationssuggest that the signals observed by ESR are associated with metastablenegatively charged Ir complexes distorted structures. [ABSTRACT FROM AUTHOR]