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.     

利用电子顺磁共振研究介电陶瓷的杂质及点缺陷

采用电子顺磁共振(EPR)技术研究了基于钛酸钡的介电陶瓷材料中的杂质及点缺陷.伴随温度的结构相变,概括了由杂质和点缺陷所诱致的EPR信号对于一级相变和扩散相变的特征.     

Electron paramagnetic resonance study of impurities and point defects in oxide crystals

In this topic review the results of the X-band electron paramagnetic resonance (EPR) measurements of Mn, Co, Cr, Fe ions in YAlO₃ (YAP) crystals and Fe ions in LiNbO₃ (LNO) crystals and of chromium doped Bi₁₂GeO₂₀ (BGO) and Ca₄GdO(BO₃)₃ single crystals, are presented. It is well known that the oxide crystals (for example:YAP, LNO, BGO) are one of the most widely used host materials for different optoelectronic applications. The nature of point defect of impurities and produced in the oxide crystal after irradiation by bismuth ions and after irradiation by the ²³⁵U ions with energy 9.47 MeV/u and fluency 5?×?10¹¹?cm^?1 is discussed. The latter is important for applications of these oxide crystal as laser materials.     

Reduction of intensities of paramagnetic resonance lines by lattice vibrations

Using a simple model of spin-phonon interaction we find that the intensities of paramagnetic resonance lines are reduced by lattice vibrations in solids. The intensity missing from the paramagnetic resonance line goes into phonon sidebands. If the phonon spectrum is a continuum, which it is in the frequency region of interest, the sidebands appear as a broad background.     

Electron paramagnetic resonance of defects with metastable properties in crystalline GaN

An EPR study of GaN revealed the presence of defects exhibiting metastable properties. EPR spectra of two centers (ii1a and ii1b) with axial symmetry along the hexagonal axis of the crystal, which have strongly anisotropic g factors, were observed. The anisotropy of the spectra decreases, and the line shape changes, with increasing temperature. The spectra of the ii1a and ii1b centers disappear at 25 and 50 K, respectively. Subsequent cooling of the samples does not restore the EPR signals, which implies that one observes here phenomena inherent in defects with metastable states. To restore EPR signals, one has to warm the samples to room temperature under very specific conditions. The possible microstructure of the discovered defects is discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 1818–1823 (October 1998)     

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.

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. M=± 1, m==±1, ±2 . . . .

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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).     

The lowest triplet state of luminescent scheelites: a study of the MoO24- ion by electron paramagnetic resonance with optical detection

The fine-structure and g tensors of the luminescent triplet state of the molybdate anion in single-crystals of CaWO₄:MoO₄, CdMoO₄ and PbMoO₄, all with the scheelite structure, are reported. In all three hosts the MoO24- ion is distorted on excitation. The directions of the principal axes of the fine-structure and g tensors support the idea that the effect results from the combined action of a Jahn-Teller instability and the crystal field. The hyperfine coupling of the ⁹⁵Mo and ⁹⁷Mo nuclei points towards excitation to an orbital with a large contribution of Mo 4d_3z2-2r.     

Electron paramagnetic resonance study of hole trapping at defects in silver halides

Abstract

Hole-trapping at cation vacancies and iodide impurity in silver halides was studied by means of electron paramagnetic resonance (EPR). In AgCl, the hole was found to be preferentially self-trapped on a Ag ion next to a cation vacancy, forming a perturbed self-trapped hole center. The position of the vacancy relative to the center of the STH was determined, by comparison of computer simulations to the experimental EPR spectra, to be in either the nearest-neighbor or next-nearest-neighbor equatorial position. The thermal trap depths of the hole in these centers are estimated to be 0.31 and 0.24 eV, respectively. For lightly doped (several tens of ppm) AgBr:I, an interesting spectrum has been observed, consisting of six equidistant and approximately equi-intense hyperfine EPR lines, consistent with a trapped hole localized at an iodide ion (nuclear spin 5/2), but not with an iodide-perturbed STH.     

A novel use of hyperfine structure in the electron paramagnetic resonance of interacting pairs of paramagnetic defects in diamond

It is the hyperfine structure of ¹⁴N and ¹³C in the electron paramagnetic resonance (EPR) spectrum which indicates that the unpaired electron of a single substitutional nitrogen atom in diamond is in one of the four anti-bonding N-C orbitals. We show that, for diamonds containing a very high concentration of nitrogen, the hyperfine structure of interacting pairs of nitrogen atoms indicates that for close neighbours there are unique orientations of the constituent N-C bonds, while at larger distances the orientations are random. This revised version was published online in August 2006 with corrections to the Cover Date.     

Two-channel resonance scattering of waves and particles by point and planar defects

The shattering of a wave (quasiparticle) with a dispersion curve consisting of two quadratic branches by a planar defect is discussed. The analog of such a process is the scattering of a similar wave (quasiparticle) in a one-dimensional system by a point defect. It is shown that even when the defect is passive, i.e., has no internal degrees of freedom, scattering may become resonant. The physical explanation of this effect is that a wave with a lower-lying spectrum scattered by the defect is in resonance with a localized (bound) state emerging because of the interaction between the defect and a wave with a higher-lying spectrum. Zh. éksp. Teor. Fiz. 115, 306–317 (January 1999)     

Broadening of resonance profiles

There are many cases in experimental physics where narrow resonances are observed under conditions in which they are broadening by an apparatus function. In the present work we give data to help in determining parameters for resonances under these circumstances. Two separate apparatus functions are assumed, namely those with Gaussian and triangular shapes, and the results are presented for the convolution of these shapes with various resonance profiles.     

Stochastic resonance in paramagnetic resonance systems

Experimental evidence of the stochastic resonance phenomenon in an electron paramagnetic resonance (EPR) system is reported. The amplitude and phase response of the EPR system operating in bistable conditions are measured for increasing values of the noise intensity. Theoretical predictions based on a simple dynamical model for the relevant system observables are shown to be in good agreement with experimental results.     

Broadening and shifts of molecular spectral lines by predissociation

The theory of predissociation is discussed for the broadening and shifts of molecular spectral lines. The application of the theory to the analysis of ultraviolet bands is based on the comparison between different isotopes of the same molecule. The theory is applied to absorption spectra of NO and good agreement is obtained with the experimental results.     

Nickel in silicon studied by electron paramagnetic resonance

We have investigated nickel in silicon samples with a wide range of initial doping concentrations by EPR, DLTS and photo-EPR techniques. Our results show that the two different Ni-centers which were observed previously by EPR, but whose structure could not be interpreted unambiguously, are both associated with Ni in a substitutional position. They are distinguished by their charges and by slightly different displacements from the ideal substitutional site. A model for the Ni⁺_s-center is suggested which explains the symmetry of this center. PACS 71.55.Cn; 76.30.Fc     

Creation of paramagnetic defects by gamma irradiation in amorphous silica

An electron spin resonance (ESR) study of the defects induced by γ-rays in various types of natural and synthetic silica is reported. Three main structures were identified: the E′ center and two doublets with field splitting of 7.4 and 11.8 mT, respectively, both centered around the E′ center signal. Another structure partially overlapping the E′ center line was also detected, consisting in three peaks with a maximum field splitting of 1.36 mT. We have investigated the growth kinetics of these centers on increasing the y-ray accumulated dose. In all investigated materials the growth of E′ centers can be interpreted as caused by γ-activated conversion of one or more precursors. The 1.36 mT structure can be considered as a hyperfine structure of the E′ centers generated by the same precursors but interacting with a nuclear spin. The 7.4 mT doublet features a sublinear growth with the γ dose in the whole investigated range (from 0.01 to 1000 Mrad) and a strict correlation with the γ-induced photoluminescence band at 4.4 eV. Finally the 11.8 mT doublet is generated only in natural silica samples and can be related to the γ-induced bleaching of the emission bands at 3.1 and 4.2 eV.     

Broadening of biphonon lines caused by interaction with acoustic phonons

A Green function technique is used to study a two-phonon spectrum. For the first time was calculated beyond the RPA the damping containing a term due to the quartic anharmonicity and a term connected with the acoustic phonon interaction. The last term causes larger line broadening, especially at room temperature.