Analysis of point defects in polycrystalline BaTiO3 by electron paramagnetic resonance

Several paramagnetic point defects in BaTiO₃ polycrystals were detected and identified using the electron paramagnetic resonance technique. Polycrystalline samples sintered in a reducing atmosphere showed a broad signal with a giromagnetic constant of 1.932. This signal, observed only in conducting ceramics, was assigned to Ti³⁺ and its related complexes. V_Ba and V_Ti defects were found at giromagetic constants of 1.974 and 2.004, respectively. The EPR intensity of these defects increases after oxidising at T>1000 °C. The appearance of the V_Ba and V_Ti signals in EPR spectra of oxidized ceramics correlates with the onset of the PTCR effect. At donor dopant levels ≥0.3 at.%, the concentration of V_Ba and V_Ti shows good agreement with the defect compensation mechanism of donor doped BaTiO₃. However, small amounts of V_Ba and V_Ti were also detected in undoped BaTiO₃, contrary to the generally accepted defect model.     

Monte carlo simulation of the electron paramagnetic resonance spectrum displayed by copper ceruloplasmin at 77 K

Summary Human ceruloplasmin contains several intrinsic copper ions, some of them being paramagnetic. The presence of two classes of paramagnetic Cu²⁺ ions (type I and type II) has been suggested, the two types having significantly different spectroscopic features. However, there are conflicting reports both on the ratio of the number of type-I to type-II Cu²⁺ and on their EPR spectroscopic parameters. By using a Monte Carlo method we obtained the best fit of the experimental spectrum with a synthesized composite spectrum generated by two type-I Cu²⁺ (type I _a and type I _b , possessing slightly different spin Hamiltonian parameters) and by one type-II Cu²⁺. Correspondingly, the EPR spectroscopic parameter (viz. theg-tensors, hyperfine couplings, line widths and molar fractions) together with their accuracies were determined.

---

Riassunto La ceruloplasmina umana contiene diversi ioni Cu²⁺ alcuni dei quali sono paramagnetici. Di questi ne sono stati ipotizzati due tipi (tipo I e tipo II), ognuno dei quali con caratteristiche spettroscopiche diverse. Tuttavia esistono nei lavori precedenti discordanze sia sul numero di ioni Cu²⁺ che sui loro parametri EPR. Si è usato il metodo Montecarlo per ottenere il best fit dello spettro EPR della ceruloplasmina. Lo spettro simulato è risultato la somma di tre spettri, 2 di tipo I (tipo I _a e tipo I _b ) e 1 di tipo II. Sono stati così determinati i parametri relativi all'hamiltoniana di spin del sistema e le loro deviazioni standard.

---

Резюме Человеческий церулоплазмин содержит несколяко собственных ионов меди, некоторые из которых являются парамгнитными. Предполагается наличие двух классов парамгнитных Cu²⁺ ионов (тип I и тип II). Эти два типа имеют существенно различные спектроскопические свойства. Однако имеются противо-речивые публикации об отношении числа ионов Cu²⁺ типа I к числу ионов Cu²⁺ типа II и об их ЭПР спектроскопических параметрах. Исполязуя метод Монте-Карло, мы получаем наилучшее согласие экспериментального спектра с синтесированным составным спектром, образованным спектрами Cu²⁺ типа I (тип I _a и тип I _b имеющие слегка различные параметры спинового Гамилятониана) и одного спектра Cu²⁺ типа II. Определяются ЭПР спектроскопические параметы (а именно:g-тензоры, сверхтонкие связи, ширины линий и молярные доли).

     

The gas-phase electron paramagnetic resonance spectrum of vibrationally excited SO radicals

Gas-phase E.P.R. spectra from the first six vibrational levels in X ³Σ^- SO have been detected and analysed using a Hund's case (b) formalism. The spectra can be fitted using the following molecular constants:

The first and second derivatives of with respect to the normalized internuclear distance ξ were calculated from these constants. A simple analytic formalism is described which enables the variation of with internuclear distance to be separated into contributions from the matrix elements and from the change in energy spacing between interacting states. Both effects are important in the derivatives of , but it is suggested that in SO most of the variation of with rotational and vibrational state is due to the increase in the second-order part of the parameter as the separation between the ground and the dominant perturbing state decreases. This explanation implies that the expression proposed recently by Veseth and Lofthus for the rotational variation of is not accurate, and enables the conflict between the results of their analysis and ab initio calculations to be resolved.     

Finite size effects in ZnO nanoparticles: An electron paramagnetic resonance (EPR) analysis

ZnO nanoparticles were synthesized by solid coprecipitation method with consecutive high energy ball milling procedure. By reducing the particle size of ZnO to nano dimensions strong nano‐size effects were observed. In order to characterize the ZnO defect structure, EPR has been applied. It was observed that below 50 nm the surface defects play a dominant role in the electronic properties of ZnO. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Tunable-frequency high-field electron paramagnetic resonance

A tunable-frequency methodology based on backward wave oscillator sources in high-frequency and -field EPR (HFEPR) is described. This methodology is illustrated by an application to three non-Kramers transition metal ion complexes and one Kramers ion complex. The complexes are of: Ni(II) (S=1) as found in dichlorobistriphenylphosphanenickel(II), Mn(III) (S=2) as found in mesotetrasulfonatoporphyrinatomanganese(III) chloride, Fe(II) (S=2) as found in ferrous sulfate tetrahydrate, and Co(II) (S=3/2) as found in azido(tris(3-tert-butylpyrazol-1-yl)hydroborate)cobalt(II). The above Ni(II) and Mn(III) complexes have been studied before by HFEPR using the multifrequency methodology based on Gunn oscillator sources, but not by the present method, while the Fe(II) and Co(II) complexes presented here have not been studied by any form of HFEPR. Highly accurate spin Hamiltonian parameters can be obtained by the experimental methodology described here, in combination with automated fitting procedures. This method is particularly successful in determining g-matrix parameters, which are very difficult to extract for high-spin systems from single frequency (or a very limited set of multi-frequency) HFEPR spectra, but is also able to deliver equally accurate values of the zero-field splitting tensor. The experimental methods involve either conventional magnetic field modulation or an optical modulation of the sub-THz wave beam. The relative merits of these and other experimental methods are discussed.     

MgF2:Mn2+光谱、超精细常数和局部结构的关联

基于电子顺磁共振(EPR)超精细常数As确定键长的新方法和半自洽场d轨道理论,对MgF2:Mn2+光谱和EPR超精细常数作出了统一解释.得到室温下MgF2:Mn2+晶体中杂质中心Mn-F的键长为0.2124±0.0010nm.     

多晶体二次电子的角度分布

 根据二次电子发射的主要物理过程,推导了内二次电子到达多晶表面并逸出的几率的角度分布、斜射入多晶的高能原电子产生的二次电子的角度分布和由背散射电子产生的二次电子的角度分布。同时,推导了高能原电子轰击多晶产生的二次电子的角度分布公式,该公式表明多晶的二次电子遵循余弦分布,且与原电子的入射角无关。分析结果表明: 在内二次电子最大逸出深度范围内,如果由射入多晶的原电子和背散射电子产生的内二次电子数是常数, 则多晶的二次电子的角度分布遵循余弦分布;如果由射入多晶的原电子和背散射电子产生的内二次电子数越来越少,则多晶的二次电子发射角度分布随出射角减少得比出射角的余弦值更慢;如果由射入多晶的原电子和背散射电子产生的内二次电子越来越多,则多晶的二次电子发射角度分布随出射角减少得比出射角的余弦值更快。     

Nuclear polarization by electron paramagnetic resonance in paramagnetic crystals

The possibility of the polarization of nuclei in paramagnetic salts by saturation of electron paramagnetic resonance is theoretically analyzed. The proposed method assumes saturation of the forbidden transition of the typeM=±1, m=±1, ±2, for mutually perpendicular external magnetic and high-frequency fields. The analysis is carried out for the case of a large quadrupole moment of the nucleus. The degree of orientation attained is comparable in order of magnitude with Overhauser's method. This method is particularly suitable for the polarization of nuclei of transuranium elements.

---

. M=± 1, m==±1, ±2 . . . .

---

---

In conclusion the author would like to thank J. Burget, J. ajko, M. Kolá and M. ott for helping in the laborious solution of system (16).     

多晶体二次电子的角度分布

根据二次电子发射的主要物理过程,推导了内二次电子到达多晶表面并逸出的几率的角度分布、斜射入多晶的高能原电子产生的二次电子的角度分布和由背散射电子产生的二次电子的角度分布。同时,推导了高能原电子轰击多晶产生的二次电子的角度分布公式,该公式表明多晶的二次电子遵循余弦分布,且与原电子的入射角无关。分析结果表明: 在内二次电子最大逸出深度范围内,如果由射入多晶的原电子和背散射电子产生的内二次电子数是常数, 则多晶的二次电子的角度分布遵循余弦分布;如果由射入多晶的原电子和背散射电子产生的内二次电子数越来越少,则多晶的二次电子发射角度分布随出射角减少得比出射角的余弦值更慢;如果由射入多晶的原电子和背散射电子产生的内二次电子越来越多,则多晶的二次电子发射角度分布随出射角减少得比出射角的余弦值更快。     

Low-power electron paramagnetic resonance spin-echo spectroscopy

Electron spin-echo experiments generally require microwave power levels of hundreds of watts to produce the 5–10 G of RF field to generate 90° and 180° pulses in 10 ns. A low-power (i.e., less than I W) EPR spectrometer using a loop-gap resonator can generate the full range of time-domain experiments on samples with submicrosecond recovery times; 90° pulses are generated in 40 ns, and relaxation times as short as 22 ns are measured. Appropriate time-domain experiments were performed to independently measure the spinspin relaxation time, phase memory time, and spin-lattice relaxation time; the results were compared with CW saturation. It was found that the spin-spin and spin-lattice relaxation rates do differ by about 5%. The entire CW signal of PADS is reconstructed from a pulse experiment at a single field position. Small differences in linewidths among the three lines were seen in accordance with theory.     

Multifrequency electron paramagnetic resonance of ultramarine blue

Electron paramagnetic resonance spectra of the S^3/? radical center in ultramarine blue over a factor of about 2500 in frequency (258 MHz to 670 GHz) reveal a substantially Lorentzian shape, without resolution of g anisotropy. Variable temperature measurements found that the line width is independent of temperature, within experimental uncertainty, up to about 90 K at 9.5 GHz and between ca. 5 K and room temperature at 95 and 217 GHz, as expected for an exchange-narrowed signal. Analysis of the increase in the low-temperature line width as a function of frequency above 9 GHz is consistent with an exchange interaction of about 2· 10^?2 K. The line width increases as frequency is decreased from 2.7 GHz to 258 MHz which is attributed to the contribution from nonsecular terms that has been denoted the “10/3” effect.     

Precisiong-factor measurements of electron paramagnetic resonance standards

An attempt is made to measureg-factors of electrons in EPR standard samples with the lowest obtainable uncertainty. An experimental equipment for preciseg-factor measurements is described, working at microwave frequencies in the 9 and 36 GHz region, and at room temperature. The course of the measuring process is mentioned in some detail and the sources of errors, which arise during the measurement, are discussed. As an example, the determination ofg-factors of certain EPR standards (using DPPH single crystals and powdered charred dextrose) is presented. No dispersion ofg-factor value has been observed between 9 and 36 GHz.     

Observation of helicon-excited electron paramagnetic resonance in a high-mobility semiconductor

Helicon waves can be exploited in EPR studies of localized magnetic moments in high-conductivity semiconductors when the free carriers have small effective mass and high mobility. In an electron plasma helicons have precisely the polarization required to elicit EPR, and the plasma background actually enhances the resonance intensity. Microwave transmission experiments on Hg_1-xMn_xTe, a narrow-gap semiconductor with localized Mn⁺⁺ magnetic moments, are described to illustrate this effect.     

Lattice dynamic effects in electron paramagnetic resonance

The electron-phonon interaction for a paramagnetic impurity in an insulating crystal is derived and from that an electron-electron interaction in a higher order is considered. An attempt is made to provide a unified presentation of the effect of phonons on the parameters measured in an electron-paramagnetic resonance experiment. The phonon-induced corrections to the zero-field splitting, the hyperfine field, the superhyperfine splitting and line intensities is reviewed theoretically along with experimental data. Both the point charge and covalent models are discussed. The effect of local and resonance modes is calculated and discussed in terms of isotope effect. Some related problems of the energy levels, dispersion, screening and phonon force are also included in appendices.     

Targeted-ROI imaging in electron paramagnetic resonance imaging

Electron paramagnetic resonance imaging (EPRI) is a technique that has been used for in vivo oxygen imaging of small animals. In continuous wave (CW) EPRI, the measurement can be interpreted as a sampled 4D Radon transform of the image function. The conventional filtered-backprojection (FBP) algorithm has been used widely for reconstructing images from full knowledge of the Radon transform acquired in CW EPRI. In practical applications of CW EPRI, one often is interested in information only in a region of interest (ROI) within the imaged subject. It is desirable to accurately reconstruct an ROI image only from partial knowledge of the Radon transform because acquisition of the partial data set can lead to considerable reduction of imaging time. The conventional FBP algorithm cannot, however, reconstruct accurate ROI images from partial knowledge of the Radon transform of even dimension. In this work, we describe two new algorithms, which are referred to as the backprojection filtration (BPF) and minimum-data filtered-backprojection (MDFBP) algorithms, for accurate ROI-image reconstruction from a partial Radon transform (or, truncated Radon transform) in CW EPRI. We have also performed numerical studies in the context of ROI-image reconstruction of a synthetic 2D image with density similar to that found in a small animal EPRI. This demonstrates both the inadequacy of the conventional FBP algorithm and the success of BPF and MDFBP algorithms in ROI reconstruction. The proposed ROI imaging approach promises a means to substantially reduce image acquisition time in CW EPRI.     

Carbon-based standards for electron paramagnetic resonance spectroscopy

In order to meet the need for a good new EPR intensity andg-value standard whose paramagnetic species are carbon-based radicals, several materials were investigated, including coal, fusinite (a coal maceral), and several carbohydrate chars. Of the prototypical standards prepared, a chemically-treated fusinite is recommended as most suitable because of its chemical stability, spin density, EPR signal line shape and line width, microwave power saturation characteristics, availability, and homogeneity. Effects of dilution with KBr, KCl, and polymer are negligible, although the line width is broadened in the presence of paramagnetic gases. Several model standard compounds have been prepared in a polymer matrix to minimize changes in packing density over time.