EPR investigation of local paramagnetic centers in perovskite-like crystals

In ScF₃ single crystals (pure and doped) as well as in Rb₂KScF₆ and Rb₂KDyF₆ crystals with a perovskite-like structure, point nanodefects (vacancy in place of trivalent cations) have been found and studied. Electron paramagnetic resonance has been used to investigate local paramagnetic centers that are not detected using X-ray diffraction. The angular dependence of the spectra indicates a local distortion of the cubic symmetry of the crystals. An additional hyperfine structure in the observed spectra is due to the delocalization of electrons over six F^? ions forming the first coordination polyhedron around the vacancy. The crystals studied are characterized by a high electron mobility and a high electron velocity, which depends on the impurity. The high mobility of electrons of the cation center can be indirectly responsible for the structural phase transition occurring in the ScF₃ crystal under uniaxial pressure.     

Random distribution of paramagnetic centers in the nuclear spin-lattice relaxation

A numerical solution is presented for the differential equation governing the nuclear spin-lattice relaxation via paramagnetic centers for spherically symmetric spin-diffusion constantD and direct relaxation transition probabilityC in the one-paramagnetic-center approximation. An interpolation function is given which reproduces the computedT ₁ values within ±5% for both the rapid diffusion and diffusion-limited cases. The introduction of a random distribution of the paramagnetic centers over the sample causes the relaxation to occur as exp(?at ^h ) with 0.5<h<1.0 forβ=(C/D)^1/4<R _av=the average radius of the influence spheres. This differs from the experimentally observed exp(?a ₁ t) behaviour. However, the random distribution seems to explain the difference between the theoretically calculated fluorine spin-diffusion in CaF₂ and that extracted from experimental NMR data by using the uniform-distribution model.     

Special features of the EPR spectroscopy of a system of centers with different relaxation times

We found that the interaction of paramagnetic centers that have different relaxation times differs fundamentally from the interaction of centers having close relaxation times. Simulation showed that in this case there is an anomalous redistribution of the spectral-line intensity from the center to the wings with a virtually preserved distance between extremal points (super-Lorentzian shape of the line), which leads to underestimation of the total intensity recorded. The results obtained make it possible to explain a number of aspects of the radiospectroscopy of carbon materials of practical importance such as the nature of the generally accepted maximum on the curve for the dependence of the total intensity of an EPR signal on the temperature of the heat treatment of organic compounds and the degree of metamorphism of natural coals, the specific features of the effect of oxygen molecules and paramagnetic ions of metals on the EPR spectra of carbon materials, etc. Belarusian State University, 4, F. Skorina Ave., Minsk, 220080 Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 2, pp. 224–229, March–April, 1998.     

Electron paramagnetic resonance (EPR) and spin-lattice relaxation of trigonal Co2+ centers in ZnS single crystals

Single crystals with a ZnS structure are of great practical value, in which connection they are studied in detail by different methods including the spectroscopic. In particular, such crystals activated by ion impurities are investigated in detail by the EPR method. However, the spin-lattice relaxation (SLR) of such systems has hardly been studied. Papers studying the SLR of cubic Mn²⁺ centers [1] and trigonal Nd³⁺ and Yb³⁺ [2] centers in ZnS did only appear recently.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 141–143, August, 1978.The authors are grateful to A. I. Ryskin for providing the single crystals for the investigation and to R. Yu. Abdulsabirov for assistance in performing the experiments.     

Further EPR study of paramagnetic Cr3+ centers in KTiOPO4

Careful reanalysis of EPR spectra of Cr³⁺ ions in KTiOPO₄ has been carried out in order to resolve the weaker lines not considered in our previous study. Two additional centers denoted as C and D have now been identified and the respective spin Hamiltonian parameters have been determined. The reanalysis of the results for the centers A and B identified previously and those for the new centers C and D indicates that the centers A and C originate from Cr³⁺ ions at Ti(2) sites, whereas the centers B and D originate from Cr³⁺ ions at Ti(1) sites. Sixteen magnetically inequivalent Cr³⁺ sites are assigned to two crystallographically different titanium sites: Ti(1) and Ti(2). Possible charge compensation models for the two different centers arising from Cr³⁺ ions at each of the two Ti sites are also suggested.     

Specific features of the EPR line shape of dimers due to spin-lattice relaxation

The magnetization relaxation in a coupled system of two spins has been shown to induce magnetization transfer between the observed transitions. In the degeneracy region of the transitions, the magnetization transfer becomes most effective and leads to the collapse of the magnetic resonance lines. In the electron paramagnetic resonance (EPR) spectra calculated for a polycrystal, this effect manifests itself in the form of additional peaks in the absorption line. The effect of anisotropy of the g factors of the dimer spins and misalignment of the axes of the g tensor and the interaction tensor on the appearance of additional lines in the EPR spectra has been analyzed.     

Nuclear relaxation due to a paramagnetic impurity

It is shown that the contributions of two-stage resonance processes in which the state of an electron does not vary as a result of nuclear relaxation are not taken into account in the usual equations for the rate of nuclear relaxation by paramagnetic impurities. In particular, these transitions yield a contribution precisely equal to the contribution of cross-over processes. Nuclear relaxation due to a magnetic ion with singlet ground level in which these processes are particularly great is investigated.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 6, pp. 83–87, June, 1976.The author wishes to express his appreciation to B. Io Kochelaev for discussion of the present results.     

Dipolar broadening and exchange narrowing of EPR lines from the paramagnetic centers distributed on a solid surface

Experiments with char samples provide support for the theory of EPR line shape in a system of surface paramagnetic centers with dipole-dipole interactions. It is also demonstrated that, if a strong Heisenberg exchange dominates over the dipole-dipole interactions, the EPR line of a two-dimensional system narrows and acquires Lorentzian shape, similar to the three-dimensional case.     

Oxygen effect on spin relaxation of the surface paramagnetic centers in carbon chars

The original technique with the use of longitudinal electron paramagnetic resonance detection was applied to measure longitudinal and transversal electron relaxation times (T ₁ andT ₂) in specially prepared carbon chars containing paramagnetic centers (PCs) on the surface. The results not only confirmed the conclusions of our previous studies but enabled us to determine the exchange integral and clarify the mechanism of interaction between the PCs and adsorbed molecular oxygen O₂. A spin relaxation mechanism was also suggested which takes into account both the exchange and dipolar interactions in the systems combining PC and O₂. The value of the exchange integral between PCs as well as its functional dependence on the oxygen content were estimated.     

High-frequency (94.9 GHz) EPR spectroscopy of paramagnetic centers in a neutron-irradiated sapphire single-crystal fiber

Initial results are reported of an EPR study, conducted at 94.9 GHz, of a thermally annealed, neutron-irradiated white sapphire (α-Al₂O₃) single-crystal fiber. The optical centers produced in sapphire by neutron irradiation and thermal annealing are of interest for optical technologies involving the phenomenon of spectral hole-burning. While these centers have been modeled as consisting of electrons localized at anion vacancies, experimental tests of this model have been very limited. EPR spectroscopy — a choice technique for elucidating structural details of such color centers — reveals signals from numerous paramagnetic centers in this material. The predominant signals, with amplitudes a hundredfold greater than any other signals, derive from three closely related, highspin-multiplicity centers. These centers do not, however, derive from radiation-induced lattice defects: they are readily identified as Cr³⁺ (S = 3/2) and a pair of crystallographically equivalent Fe³⁺ (S=5/2) impurity ions. The advantages of high-frequency-EPR instrumentation in facilitating this identification are presented and discussed in detail. These advantages include enhanced sensitivity for this volume-limited, fiber sample. Moreover, the analysis of the spectra — entailing spectral assignments, evaluation of the spin-Hamiltonian parameters, and spin-counting — is greatly facilitated when the Zeeman interaction is dominant.     

A pulsed EPR method to determine distances between paramagnetic centers with strong spectral anisotropy and radicals: The dead-time free RIDME sequence

Methods to determine distances between paramagnetic metal centers and radicals are scarce. This is unfortunate because paramagnetic metal centers are frequent in biological systems and so far have not been employed much as distance markers. Successful pulse sequences that directly target the dipolar interactions cannot be applied to paramagnetic metal centers with fast relaxation rates and large g-anisotropy, if no echos can be detected and the excitation bandwidth is not sufficient to cover a sufficiently large part of the spectrum. The RIDME method Kulik et al. (2002) [20] circumvents this problem by making use of the T₁-induced spin-flip of the transition-metal ion. Designed to measure distance between such a fast relaxing metal center and a radical, it suffers from a dead time problem. We show that this is severe because the anisotropy of the metal center broadens the dipolar curves, which therefore, only can be analyzed if the full curve is known. Here, we introduce five-pulse RIDME (5p-RIDME) that is intrinsically dead-time free. Proper functioning of the sequence is demonstrated on a nitroxide biradical. The distance between a low-spin Fe(III) center and a spin label in spin-labeled cytochrome f shows the complete dipolar trace of a transition-metal ion center and a spin label, yielding the distance expected from the structure.     

Effect of oxygen diffusion on the relaxation parameters of paramagnetic centers on the surface of alumina

The present article is concerned with theoretical consideration of the effect of oxygen diffusion on the relaxation parameters of paramagnetic centers (PC) located on a surface. Proceeding from a model of dipole-dipole interaction between paramagnetic molecules of O₂ and PC that is modulated by translational diffusion motion of O₂ molecules in the volume and on surface of an adsorbent and in its pores, we give an explanation for the observed dependence of the intensity of the EPR signal on the pressure of O₂. In the present case, diffusion in the volume is the decisive factor. We also managed to explain the presence of an SHF field power threshold for the indicated phenomenon to be observed. In the calculations we used correlation functions obtained earlier for diffusion in a volume. Institute of Physicoorganic Chemistry, National Academy of Sciences of Belarus, 13, Surganov St., Minsk, 220072, Belarus. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 2, pp. 230–235, March–April, 1998.     

Broadening and shift of the spectral line centers of a molecule upon strong vibrational excitation

Half-widths and shifts of the line centers of the excited CO molecule caused by transitions between highly excited rovibrational states are calculated. Strong intramolecular effects caused by vibrational excitation are considered in the calculations. It is revealed that the vibrational dependence of the line broadening and shift coefficients is nonmonotonic, and the coefficients can change several times.     

COMPARE: A computer program for comparative analysis of EPR data for low-symmetry paramagnetic centers

A quantitative method has been proposed for comparative analysis of EPR (electron para-magnetic resonance) data for low-symmetry paramagnetic centers and a computer program, COMPARE, based on this method has been developed. Quantitative parametersS _A andS _D are introduced to describe the closeness of the orientation of any two-axis systems and the principal values of any two tensors, respectively. The method helps to identify the pairs of data sets, comprising the principal values as well as the orientations of the principal axes of the second-rank spin Hamiltonian terms, which may likely belong to the same physical center but have been ascribed to distinct centers. Other useful applications of the program COMPARE are briefly discussed.     

Electron paramagnetic resonance (EPR) and photo-EPR studies of aggregate centers with two iron atoms in silicon

We have investigated iron by EPR (electron paramagnetic resonance) and photo-EPR in initially boron doped silicon samples in which the iron concentration exceeded the boron content. A new EPR spectrum, showing orthorhombic-I symmetry was observed and could be fitted by an effective spin Hamiltonian with the parameters S=3/2, g=2.07, and E/D=0.68. We identify this spectrum as a new modification of a Fe₂B center which has the same symmetry but different configuration of the constituent atoms.Furthermore, we were able to determine the donor level of the old Fe₂B center to E=E_V+0.57±0.02 eV above the valence band.We have also investigated the Fe₂ donor. According to our straightforward interpretation the energy levels of the transitions from Fe₂⁺ to Fe₂⁰ and from Fe₂⁰ to Fe₂⁺ were determined as E=E_V+0.61±0.02 eV and E=E_C-0.67±0.02 eV, respectively, suggesting a lattice relaxation on electron capture, which is unusual for transition metal centers in silicon. PACS 71.55.Cn; 76.30.Fc     

Anisotropy of the EPR line shape of dislocation acceptor centers in semiconducting type Ic diamonds

The shape anisotropy of the EPR lines of broken carbon bonds in a dislocation core with an edge component in natural semiconducting type Ic diamonds has been investigated. It has been found that electrically active acceptor centers are formed at dislocation steps, jogs, or kinks. The distance between the paramagnetic centers is determined.     

Energy transfer from paramagnetic ions to a lattice through rapidly relaxing centers

Spin-lattice relaxation is analyzed in a crystal containing two types of paramagnetic centers having approximately equal resonant frequencies but markedly different spin-phonon couplings. It is assumed that for the centers having the strong spin-phonon coupling this coupling is stronger than the spin-spin coupling with other paramagnetic centers. The Green's function method is used. The spin-lattice relaxation time for the centers coupled weakly with the lattice through rapidly relaxing centers is found as a function of the difference between the splittings of their spin levels, the strength of the spin-spin coupling between these centers, the concentrations of these centers, and the strength of the spin-phonon coupling over rapidly relaxing centers.Translated from Izvestiya VUZ. Fizika, No. 3, pp. 15–20, March, 1970.In conclusion the author thanks S. A. Al'tshuler and L. K. Aminov for useful discussions of these results.     

Direct measurement of electron spin-lattice relaxation time of paramagnetic centers and non-linear modulated responses in HTSC

The report consists of two parts. 1. The method is described to measure directly the electron spin-lattice relaxation timeT ₁ in high-T _c superconductors (HTSC). The technique is based on registering the oscillating longitudinal spin magnetization while saturating the EPR line by an amplitude-modulated microwave field. TheT ₁ values of the Cu²⁺ centers in YBa₂Cu₃O_6+x (x=0.24?0.9) and Y₂BaCuO₅ materials are measured and found to be about 1.4·10^?9 s independently ofx and temperature in the range 77–300 K. Besides much longer relaxation is displayed in degradated materials. The data obtained can be considered as an argument in favor of the “green-phase” origin of the “high-temperature” EPR spectra in the YBaCuO HTSCs. 2. Non-linear responses of the microwave (10¹⁰ Hz) and rf (10⁵–10⁷ Hz) absorption in HTSC materials to low-frequency magnetic modulation are studied and found to be quite different in the two frequency ranges. It is shown that at microwaves the effect is caused by non-linear interaction with shielding supercurrents whereas at lower frequencies it is due to jumps of fluxons over potential barriers. The models developed take into account the Josephson weak links and the thermo-assisted flux creep.