Types Of Paramagnetic Centers Research Articles

Radiation-induced centers in the Li 2B 4O 7 single crystals

The X-band ( ν≅9.4 GHz) electron spin resonance (ESR) spectra of the X-ray-, γ- and neutron-irradiated lithium tetraborate (LTB) single crystals were investigated at room and liquid nitrogen temperatures. Neutron irradiation (fluence Φ n=1.7×10 19 cm −2) of the LTB single crystals with natural isotope abundance leads to the generation of four types of paramagnetic centers, stable at room temperature. The observed ESR spectra were described by spin Hamiltonian formalism. Thermal stability of the neutron-induced centers was investigated in the 300–800 K range. The models and new mechanism of formation of the neutron-induced defects in the LTB crystals are suggested.

Influence of iron concentration on its structural position in KNbO 3 lattice

In this paper variations of structural positions of Fe 3+ ions in KNbO 3 crystals depending on iron content in growth components have been studied by the ESR method. Crystals of KNbO 3 have been grown by the Czochralski method. In addition to finding two EPR spectra published by Siegel we have found two novel types of paramagnetic centers (I, IV) caused by incorporation of iron impurity. One of them is observed at the low concentration of iron impurity. This center (I) has C 3V symmetry, S=5/2 and is due to incorporation of Fe 3+ ions into the potassium positions of regular lattice. Increasing iron content in growth mixture up to 200 ppm resulted in disappearance of this center and two ESR spectra (II, III) described by Siegel arose. Limited content of (II) and (III) iron-containing defects is due to a restricted solubility of iron in KNbO 3 crystal. An increase of iron content in growth mixture over 400 ppm produces new a type of defect—(IV). This center has C 3V symmetry, electronic spin S=1/2 and is due to 3d 5 electronic state of Fe 3+ ion in strong crystalline field. The structural models of observed centers are discussed.

Structure and formation mechanism of the Eα′ center in amorphous SiO2

We provide a possible formation mechanism for one of the Si-related paramagnetic centers in amorphous silica, Eα′, which is stable only below 200 K, on the basis of the quantum-chemical calculations. We show that the divalent Si defect can trap a hole, resulting in two different types of paramagnetic centers that are consistent with the experimental spectral features for Eα′. The highly anisotropic symmetry and the isotropic hyperfine coupling constants observed for one of the Eα′- center variants are reproduced by the present model.

An electron paramagnetic resonance study of defects in PECVD silicon oxides

Electron paramagnetic resonance (EPR) has been used to determine how the type and population of paramagnetic defects are altered firstly by changes to the composition of oxides grown on silicon by plasma enhanced chemical vapor deposition (PECVD) and secondly by subsequent anneal treatments of the various oxide films. Silicon oxide films with refractive index, n, of 1.464, 1.487, 1.508 and 1.536 and thickness of 1 μm to 3 μm were studied; those of higher index have a higher silicon content. Only E′ centers are detected in films with n=1.464 and 1.487. Raising n to 1.508 leads to the introduction of other types of paramagnetic centers and a large increase in the average total spin concentration. Increasing n further to 1.536 produces a further small increase in average spin concentration. Identities are proposed for the additional types of defect and the dependence of their population on anneal temperature is also reported.

Electron spin resonance of MO5+ in KTiOPO4

The ESR spectra from Mo5+ ions in KTiOPO4 single crystal were studied at 77 K. The ESR spectra reveal two different types of paramagnetic centers, one due to Mo5+ which occupy the Ti(1) position in the crystal and the other, due to Ti3+, formed at position Ti(2) in the crystal. The principal values and direction cosines of the g tensor are reported for the former center.

Genesis of V4+ in heteropoly compounds CsxH4-xPVMo11O40 during thermal treatment, rehydration and oxidation of methanol studied by EPR spectroscopy

The development and structural changes of V4+ and Mo5+ centers in caesium salts and the acid form of molybdovanadophosphorus heteropoly compounds CsxH4−xPVMo11O40 ·nH2O (x=0, 3, 4) of Keggin structure were investigated by electron paramagnetic resonance (EPR) after preparation, thermal treatment in oxygen, reduction by methanol and catalytic oxidation of methanol. The various types of paramagnetic centers are found to characterize the different states and structural transitions of the heteropoly anion. In addition, the EPR spectra reflect well the different reducibility, thermal stability and mobility of vanadium and Keggin (“ lattice”) oxygen of the Cs4, the Cs3H and the H4 compounds. The V4+ centers in the Cs4 salt are different in structure, lower in concentration and thermally more stable than those in the acid form. The reduction of vanadium during thermal treatment due to the loss and oxidation of coordinated (“ lattice”) oxygen to O2 is observed in the acid and the Cs3H salt only, but not in the Cs4 compound. As deduced from quantitative EPR measurements and manganometric redox titrations, between 0.5 and 8% of the vanadium is present as V4+ after preparation and thermal treatment in oxygen, whereas the majority of vanadium is reduced by reaction with methanol. V4+ substitutes Mo atoms in the Keggin unit in the fully exchanged Cs salt (Cs4). The presence of the bulky caesium counter-cations prevents the removal of vanadium from the Keggin unit to the secondary structure and thus the formation of vanadyl bridged polymers.

Application of EPR spectroscopy to the characterization of magnetic interactions in thermally decomposed coal

The complex lineshape of the X-band EPR spectra of natural coal heated at 300–650 °C was numerically analysed. The experimental spectra were approximated by superpositions of one Gauss and three Lorentz lines. Linewidths (ΔHpp), g-factors and abundances of the components of the total spectrum were evaluated. The total concentration and concentrations of the four types of paramagnetic centres in the coal samples were determined. Two groups of unpaired electrons with strong dipolar interactions responsible for broad Gauss (ΔHpp = 0.68–0.92 mT) and broad Lorentz (ΔHpp = 0.54–0.64 mT) EPR components and two groups of unpaired electrons with exchange narrowed Lorentz lines (ΔHpp = 0.17–0.24 and 0.06–0.08 mT) were detected. Dipolar interactions of unpaired electrons with broad lines changed considerably during the thermal decomposition of coal. A strong influence of atmospheric oxygen molecules on magnetic interactions of unpaired electrons with the broad Gauss and the two narrow Lorentz lines was observed. Copyright © 1999 John Wiley & Sons, Ltd.

Paramagnetic Defects Related to Positive Charges in Silicon Oxynitride Films

ABSTRACTElectron spin resonance (ESR) spectroscopy is used to explore the origin of positive charges in silicon oxynitride thin films formed in a N2O ambient. A new type of paramagnetic center in as-grown oxynitrides appeared as a single peak in the ESR spectrum with a zero crossing g value of 2.0006. This value suggests that the paramragnetic center is a kind of E' center. We investigated the quantitative behavior of the positive charge center and the paramagnetic center through the density changes induced by vacuum ultraviolet irradiation and hydrogen annealing. Based on the similar behavior of both types of center, we argue that the paramagnetic centers can account for a significant part of the positive charges specific to oxynitrides. Since the spin density increased with the oxynitridation temperature, the paramagnetic center is closely related to nitrogen incorporation processes.

Electron spin resonance and microwave photoconductivity in carbynoid films

First ESR studies are reported on oriented carbynoid samples prepared by chemical dehydrofluorination of poly(vinylidene fluoride) films. Two types of paramagnetic centres (PC), denoted as C-M1 and C-M2, and an anisotropic broad line were detected. The spectra of the PC C-M1 are isotropic with a g-value of . The absorption by given PC is well saturated at room temperature and the PC C-M1 can be attributed either to free radical states or pinned quasi-particles similar to topological Su-Schrieffer-Heeger solitons. The ESR and microwave photoconductivity results obtained confirm the quasi-1D nature of the carbon chains, being the structural elements of the samples studied, and agree with the assumption that the ground state of carbon chains is a state with a spin density wave (SDW) realized at room temperature. The ESR spectrum of the PC C-M2 consisting of a series of unsaturated lines is anisotropic both in relation to the static magnetic field and in relation to the magnetic component of the microwave field. The study of the interaction dynamics of the microwave field with PC C-M2, the absorption dependence with increase of the microwave field and the temperature dependence allow us to attribute the PC C-M2 to mobile quasi-particles, being domain walls in the alternated spin density distribution, tentatively solitons in the SDW. It has been found that spin waves associated with the PC C-M2 can be observed in samples treated thermally at for 2 hours (the samples contained fluorine with atomic ratio F/C = 3/7 and oxygen at 20 at.%). It has been proven that the spin waves' excitation and the formation of quasi-particles like solitons can represent the same process.

Magnetoplastic effect in irradiated NaCl and LiF crystals

The effect of low x-ray irradiation doses (≈102 rad) on the magnetoplastic effect — the detachment of dislocations from paramagnetic centers under the action of an external magnetic field B — in alkali-halide crystals has been investigated. The measurements were performed on LiF crystals and three types of NaCl crystals, differing in impurity content. The dependence of the mean free path l of the dislocations on the rotational frequency ν of a sample in a magnetic field was especially sensitive to low irradiation doses. In unirradiated crystals this dependence is a single-step dependence and is characterized by a critical frequency νc ∝B2 above which the magnetoplastic effect is not observed. The frequency νc depends only on the type of paramagnetic centers, and not on their density. Even the lowest irradiation dose employed (<100 rad) leads to a sharp restructuring of the dependence l(ν), converting it into a two-step dependence (for edge dislocations) with an additional critical frequency νc2, that is insensitive to the irradiation dose, and that corresponds to the appearance of magnetically sensitive stoppers of a new type under irradiation. The initial critical frequency νc1, as a rule, also varies with the dose, reflecting the change in state of the impurity complexes (Ca in NaCl and Mg in LiF). Specifically, it is shown for NaCl(Ca) crystals that as the irradiation dose increases, the frequency νc1 increases, gradually approaching the value νc2, so that by the time the dose is ≈300 rad, the dependence l(ν) once again becomes a single-step dependence, dropping sharply only for ν⩾νc2. It is shown that the addition of a small number of Ni atoms to a NaCl crystal makes the Ca complexes radiation resistant, and the critical frequency νc1 corresponding to them initially equals νc2 for crystals with no Ni. The recombination kinetics of radiation defects in the case in which the samples are irradiated under a tungsten lamp was investigated. A possible physical model of the observed dependences is discussed.

E.p.r. studies of interactions between paramagnetic centres of exinite, vitrinite and inertinite during thermal decomposition

Electron paramagnetic resonance in the X-band (9.3 GHz) was used for investigations of petrographically complex coal samples (densities 1.24–1.28 and 1.30–1.36 g cm −3) from Polish medium-rank coal (85.6 wt% C) heated in an inert atmosphere at 300–650°C. A strong increase in total concentration of paramagnetic centres in the sample of lower density at 300–350 and 450–550°C was measured. Relatively smaller changes in the total amount of paramagnetic centres in the heavier sample at 300–650°C were observed. There are three different types of paramagnetic centres in both samples, and their e.p.r. spectra are a superposition of one broad Gauss and two Lorentz (broad and narrow) lines. The most active group in the thermal decomposition of the two coal samples is formed by paramagnetic centres responsible for the broad Lorentz lines. Mainly dipolar interactions of unpaired electrons influence the linewidths of the e.p.r. signals. The microwave saturation power of all the e.p.r. lines detected increases with temperature. According to the e.p.r. results for pure macerals and the complex coal samples, the hypothesis of independent progress of reactions in individual macerals during the thermal decomposition of coal can be rejected.

Electron spin-lattice relaxation in oxide superconductors and fullerides

Using the original technique of measuring very short spin-lattice relaxation times T1, the electron-spin relaxation has been investigated in a number of HTSC materials and related phases. The relaxation of Gd3+ ions in GdBa2Cu3O6+x is shown to be affected by both quasiparticle and antiferromagnetic fluctuations, the latter being increased at lower oxigen content. The pseudo-spin gap of about 200 K is found for both contributions. For Cu2+ EPR in YBaCuO, the T1 values reveal several types of paramagnetic centers including the “green phase” Y2BaCuO5. In the fulleride RbC60, the measuring of T1 and T2 yielded additional support for 1D conductivity and allowed to monitor the metal-insulator phase.

Numerical analysis of EPR spectra of coal, macerals and extraction products

The multi-component structure of the EPR spectra of Polish medium metamorphosed coal, its macerals and their pyridine extractrion products were analyzed. The bes numerical approximation of the EPR spectra for exinite, vitrinite and their extraction residues was obtained when two Lorentz and one Gauss curve were chosen for calculation. The EPR spectra of inertinite and its residue are superposition of two Lorentz lines. The EPR spectra of the extracts are fitted by single Lorentz lines. Four groups of paramagnetic centres which give the EPR signals with different ranges of linewidths are present in the macerals and the extraction products. Paramagnetic centres with the narrow EPR lines are present not only in inertinites, but also in exinites and vitrinites. Each of the four types of paramagnetic centres is present in macromolecular and molecular part of the macerals. As was expected, four component lines were detected for the EPR spectra of the coal and its residue. Paramagnetic centres responsible for the broad lines are located on structures consisting of a few aromatic rings. Paramagnetic centres of large aromatic structures are responsible for the narrow lines.

FE3+EPR investigation of PLZT 1/65/35 and 8/65/35

Abstract X-band EPR spectra of PLZT 1/65/35 and PLZT 8/65/35 doped with 1% FE3+ were recorded at temperature range −175°C to 200°C. Three types of paramagnetic centers were found. Two of them are in strong axial and rhombic crystal field due to neighboring oxygen and lead vacancies. The other center has symmetry determined by polarization and oxygen octahedra tilt. EPR spectra of PLZT 8/65/35 are in agreement with its glassy behavior.

Application of Q-band electron spin echo spectrometer to investigation of glasses doped with rare earth ions

The Q-band electron spin echo (ESE) spectrometer which was created using modern microwave components is described. This simple incoherent apparatus was used with the X-band one for the study of phosphate and silicate glasses doped with non-Kramers rare earth Tb3+ ions. the EPR spectra measured by the ESE method have frequency independent peaks. The experimental results presumably show the existence of several types of paramagnetic centers in studied systems.

EPR and piezooptic studies in doped and irradiated tartrates

Abstract Stress optical and EPR measurements are made on doped and X-irradiated single crystals of Rochelle salt in the paraelectric phase. With the nickel impurity no EPR signal is detected upto liquid nitrogen temperature. Manganese impurity forms two distinct types of paramagnetic centres. The spin Hamiltonian parameters are calculated and discussed for the Mn2+ defect centres. The stress optical constant is found to change with the introduction of impurities and on X-irradiation of the crystals.

Distribution of paramagnetic defects formed in silicon by MeV ion implantations

Two different types of paramagnetic centers in silicon, Si-P3 (neutral {110} planar tetravacancies) in a well defined crystalline structure and point defects with poorly defined local structure in substantially damaged crystalline environments, formed by 3 MeV phosphorus and silicon ion implantations up to a dose of 1 × 10 14 cm −2 have been compared both in the dose dependence of area densities and in depth profiles. When the dose reaches to 1 × 10 14 cm −2, the area density of Si-P3 starts to saturate while that of “indefinite” point defects keeps increasing. Also at this dose, the mean concentration of Si-P3 decreases as a function of depth from the surface whereas that of “indefinite” point defects increases. These results are discussed in terms of a damage overlap model in conjunction with a Monte Carlo simulation of lattice disorder.

Ge xS 1− x glasses. Defects, density and structure

The density, X-ray scattering and ESR ofGe x S 1− x glasses within0.10 ⩽ x ⩽ 0.44 have been measured. The region of compositions in question is subdivided into three intervals: 0.10 ⩽ x ⩽ 0.325, 0.325 ⩽ x ⩽ 0.30and0.30 ⩽ 0.44 according to the types of paramagnetic centers (PC) and the specific features of the concentration dependences of the experimental quantities under study. It is found that the density, the PC concentration and the features of the radial distribution functions are correlated in each interval. The concentration dependence of the relative amount of two types of PC is the first interval is obtained. The influence of γ-irradiation at 300 K upon the structure and concentration of PC is studied. It is found that under γ-irradiation the amount of PC either increases (first and second interval) or decreases (third interval) and the unpaired spin density is redistributed among different types of structural defects. New types of PC do not arise under γ-irradiation. In the framework of the structural chemical analysis the models of structural units and their transformation with glass composition are suggested. Based on the proposed model for the glass network, the experimental data are analyzed.

ESR and ESE studies on polymer semiconductors of weakly doped poly(tetrathiafulvalene)

AbstractIn the studied poly(tetrathiafulvalenes), PTTF, the TTF units are linked via phenyl groups or via tetrahydroanthracene bridges. The PTTF are doped with oxygen and with iodine respectively. The electrical conductivity strongly depends on temperature as it is typical of semiconducting materials.The existence of various types of paramagnetic centres is confirmed by ESR saturation experiments, by spectra simulation and by electron spin echo (ESE) measurements in the temperature region of 2 K to 400 K. At room temperature the doped PTTF have a dc conductivity between 10−9 Scm−1 and 10−4 Scm−1.Relations between the concentration of various radicals and the electrical conductivity of the polymers are discussed by means of a model based on mobile and immobile radicals, polarons and bipolarons. There are experimental indications for temperature‐dependent formation and break‐up of bipolarons which are discussed in the paper.

Estimation of the influence of experimental conditions on ESR dating results

Rapid progress has recently been made in ESR dating, a new dating technique. Much attention has been paid to the accuracy of its results, as errors may be caused by various factors. This paper probes into the influence of the applied microwave power and the absorbed dose rate of additional radiation. A few measurements we made with carbonate and quartz samples reveal that the microwave power saturation point depends on the type of paramagnetic centers and the type of samples. The microwave power and the dose rate of additional radiation have an apparent effect on the total radiation dose assessment in such samples. Extrapolating on a logarithmic scale, we determine that the ESR age of coral sample 87-04 is (3.64 ± 0.55) ka as microwave power is zero. This is in accord with its 14C age (3.75 ± 0.19) ka.