Paramagnetic centers in boron and aluminum borides

The electron spin resonance spectra of crystalline and amorphous boron and aluminum dode-caboride samples have been recorded on a Q-band spectrometer. The parameters of the spectra have been determined, and the concentration of paramagnetic centers has been estimated. It has been found that the concentration of the paramagnetic centers in amorphous boron is high and exceeds the corresponding value in crystals by two orders of magnitude. The spectra of boron powders enriched in ¹¹B and ¹⁰B isotopes have been measured. It has been established that the difference between the hyperfine interactions of the unpaired electron with the ¹¹B and ¹⁰B nuclei manifests itself as a change in the width of the structureless electron spin resonance spectra.     

Electron spin resonance of donor centers in beryllium oxide

The ESR spectra of two deep centers found in irradiated BeO crystals and powders, respectively, are reported and analyzed. They are due to electrons trapped at substitutional boron (B⁺⁺⁺) and fluorine (F^?) ions as concluded from the hf interaction of the unpaired electron with the respective nuclei. The boron center revealed a static, the fluorine center a dynamic Jahn-Teller effect between 77°K and 300°K. In both cases, the unpaired electron was found to be localized in nonaxial orbitals, which is shown to be consistent with the direction of the polar field of the crystal structure. The strong localisation of the electron is contrasted to the behavior of donor electrons in other II–VI compounds, which are highly delocalized.     

Theoretical explanation of absorption spectra and ESR parameters of Cu in shattuckite

The optical absorption spectra and electronic spin resonance parameters (ESR g factors g_‖, g_⊥ and hyperfine structure constants A_‖, A_⊥) for Cu²⁺ in shattuckite crystal are calculated from the two spin–orbital coupling parameters model, high-order perturbation formulas and complete diagonalization (of energy matrix) method (CDM) of 3d⁹ ion in tetragonal symmetry. The calculated results are in good agreement with the observed values. Since the ESR parameters are sensitive to the local structure of a paramagnetic impurity center, the defect structure of Cu²⁺ center in shattuckite crystal is estimated. The results are discussed.     

Electron spin resonance study of a CuIr2S4 single crystal

A spinel sulphide CuIr₂S₄ single crystal, which exhibits an orbitally induced Peierls phase transition at ～230?K, is investigated by electron spin resonance (ESR) spectroscopy. The phase transition is clearly manifested on the ESR spectra. It is suggested that the ESR signals are produced by a few non-dimerized Ir⁴⁺ ions. Moreover, an extra ESR spectrum appears at low temperature in addition to the paramagnetic ESR signals of Ir⁴⁺ ions, which is suggested to be caused by the Jahn–Teller effect of the non-dimerized Ir⁴⁺ ions. From the ESR results, it is found that the Jahn–Teller splitting energy ΔE _JT is much smaller than the spin-dimerization gap.     

EPR spectra of Fe centers in layered TlGaSe2 single crystal

A single-crystal TlGaSe₂ doped by paramagnetic Fe ions has been studied at room temperature by electron paramagnetic resonance (EPR) technique. The fine structure of EPR spectra of paramagnetic Fe³⁺ ions was observed. The spectra were interpreted to correspond to the transitions among spin multiplet (S=5/2, L=0) of Fe³⁺ ion, which are splitted by the local ligand crystal field (CF) of orthorhombic symmetry. Four equivalent Fe³⁺ centers have been observed in the EPR spectra and the local symmetry of crystal field at the Fe³⁺ site and CF parameters were determined. Experimental results indicate that the Fe ions substitute Ga at the center of GaSe₄ tetrahedrons, and the rhombic distortion of the CF is caused by the Tl ions located in the trigonal cavities between the tetrahedral complexes.     

ESR spectra of V4+ ions in the semiconductor V3O7

ESR studies on the semiconductor V₃O₇ reveal the existence of intrinsic paramagnetic V⁴⁺ ions. Computer simulations of spectra suggest that the unpaired electron wavefunction extends over ca. 12–14 lattice sites. This partial delocalization of the wavefunction is assumed to arise from minor crystallographic distortions at low temperatures.     

Modelling of multifrequency ESR spectra of Fe3+ ions in crystalline and amorphous materials: A Simplified approach to determine statistical distributions of spin-spin coupling parameters

Focussed on the understanding of the complex Fe³⁺ electron spin resonance (ESR) spectra in crystalline and amorphous systems, the simulation program ESR-MAKRO-1 was developed, which allows the calculation of powder spectra of paramagnetic centers with 1/2 ≤S ≤7/2. For the reproduction of the ESR spectra of amorphous systems Gaussian distributions of the zero-field-splitting parameters of second and fourth order (D, E, a, F) were implemented into the program. Applications of ESR-MAKRO-l are given both for crystalline powders and for glassy systems. A stepwise analysis of the ESR spectra, allowing an approach to Fe³⁺ ESR spectra of amorphous matrices and showing the influence ofD, E, a, andF as well as their distributions, is given.     

Relaxation mechanism in γ-ray-irradiated L-alanine studied by transfer saturation EPR and pulse EPR

Stable paramagnetic centers in γ-ray-irradiated L-alanine dosimeters exhibit a maximum in relaxation rate in the vicinity of 190 K. The mechanism of this relaxation rate has been investigated on the first stable alanine radical center, SARI, by employing continuous-wave transfer saturation electron paramagnetic resonance and pulse electron paramagnetic resonance techniques. The detected in-phase and out-of-phase spectra as well as phase memory times,T _M, indicate that besides the well-known τ_p of the CH₃ group of SAR1 an additional correlation time, τ_l (ΔElk=2689±50 K and 0 τ₁₀ = 0.15 ± 0.03 ps), is involved in the transverse relaxation process and effects the SAR1 center. For the SAR1 center this mechanism originates from the hindered motion of undamaged CH₃ and NH ₃ ⁺ groups in the lattice. The motion of these groups additionally effects the spectrum of the SAR1 center through averaging out of the anisotropic splitting.     

Dissymmetrization in X-irradiated RbTiOPO4 crystal

Electron paramagnetic resonance has been used to study the hole and electron paramagnetic centers formed in X-irradiated RbTiOPO₄, the crystals of the KTP family. X-irradiation of RbTiOPO₄ crystals at 77 K produced an oxygen hole center and four different trivalent titanium electron centers I₁, II, III and IV. Theg-tensors, their principal values and axes for the defects were calculated and compared with those for KTiOPO₄ centers. X-irradiation at 300 K produced another two oxygen hole centers and three electron centers I₁, I₂ and II. EPR spectra of the center II revealed dissymmetrization, i.e., irregular distribution of growth defects, between the physically equivalent sites lowering the point group symmetry of the local environment of paramagnetic centers Ti³⁺.     

Blue long-lasting phosphorescence of Ti-doped BaZrO3 perovskites

Long-lasting phosphorescence was observed from Ti-doped BaZrO₃ perovskite synthesized by a solid-state reaction. The phosphorescence color is blue and the phosphorescence can still be seen with the naked eye in the dark for 30 min or more after stopping UV irradiation. By the measurements of electron spin resonance (ESR) spectra, it was confirmed that F⁺ center and Zr³⁺ exist in non-luminescent undoped BaZrO₃ and their concentrations decreased with the additive amount of TiO₂. It is expected that the luminescence center is F_A center composed of Ti³⁺ and F⁺ center.     

Leakage current and paramagnetic defects in SiCN dielectrics for copper diffusion barriers

We report the properties of paramagnetic defects in SiCN films which are used as copper diffusion barriers. Electron spin resonance (ESR) signals with a g value of 2.003 and the ΔH_PP of 1.1-1.2 mT were observed for as-grown and UV-illuminated SiCN films. These characteristics of the observed ESR signals are very similar to those of the K⁰ center in N-rich silicon nitrides. We also show that a substantial increase in the leakage current occurs by exposing the SiCN films to UV illumination. We suggest that the paramagnetic defects generated by the UV illumination are responsible for this current increase.     

EPR spectra of a GdMnO3 thin film on a SrTiO3 substrate

Electron paramagnetic resonance (EPR) spectra of a GdMnO₃/SrTiO₃ thin film in the X band have been measured in the temperature interval from 200 to 450 K. Signals from two types of paramagnetic centers have been observed in the spectra. The first paramagnetic center is a subsystem of Gd³⁺ ions, in the EPR spectrum of which the fine structure lines are resolved below 350 K. The second paramagnetic center is a system of manganese and gadolinium ions, in the EPR spectrum of which an exchange-narrowed line is observed with the width ΔH several times less than the width ΔH of an exchange-narrowed line observed in the GdMnO₃ single crystal. Unusual magnetic properties are due to the mismatch of the lattice parameters of the GdMnO₃ thin film and the SrTiO₃ substrate.     

The transition from dynamics to statics in the electron-spin-resonance spectra of impurity Mn2+ ions in strontium titanate

The electron-spin-resonance (ESR) spectra of SrTiO₃:Mn single crystals have been investigated. Results unambiguously indicate that the impurity center formed by an Mn²⁺ ion has a dynamic nature. In the high temperature range (T > 100 K), ESR spectra of Mn²⁺ ions reveal cubic symmetry; the spectrum is found to broaden significantly with a decrease in temperature. Upon cooling to T < 10 K, low-symmetry centers of Mn²⁺ ions with a strong orientational dependence emerge in the spectra. Temperature evolution of the ESR spectrum can be described within the model of a dynamic off-center Mn²⁺ ion substituting for the Sr²⁺ ion, with a transition to the static regime at low temperatures with an average localization energy of ～2.4 ± 0.4 meV for Mn²⁺ centers due to random deformations.     

Deconvolution of Powder ESR Spectra: Ge Centers in Natural Quartz

A procedure for the deconvolution of complicated electron spin resonance (ESR) powder spectra into a set of spectral contours of separate paramagnetic centers is applied for the analysis of a polycomponent system of paramagnetic Ge centers in natural silica. The changes in the Ge centers system occurring during the thermal transformation of natural quartz into cristobalite are studied. The annealing at about 1300 K transforms the [GeO₄/Li⁺]⁰ centers partly into the [GeO₄/Na⁺]⁰ form in α-quartz and the further temperature increase leads to the capture of both defects by the cristobalite structure. A lag effect of the inheritance of the isomorphic impurities present in the α-quartz by cristobalite is established. The natural metamorphic transformations of vein quartz bodies also stimulate the repartitions of alkaline ion compositions in quartz structure. This can be detected by ESR as corresponding modifications in the compositions of Ge paramagnetic centers. ESR investigations of compositions of paramagnetic Ge centers in quartz from altered bodies reveal the enrichment of the quartz lattice of strained and granulated parts of veins by interstitial sodium. This process is caused by dissolution of sodium-containing fluid inclusions in the quartz lattice. Author's address: Vladimir P. Lutoev, Institute of Geology, Komi Scientific Center, Russian Academy of Sciences, Pervomaiskaya ulitsa 54, Syktyvkar 167982, Russian Federation     

The effect of melting temperature on the glasses from B2O3Na2OCuO system studied by EPR and NMR

The influence of the preparation temperature of copper doped soda-borate glasses on their structure and properties was investigated by means of the electron paramagnetic resonance of the Cu²⁺ ions and of the nuclear magnetic resonance of the ¹¹B nuclei. The melting temperature varied in the range 900–1300°C. The shape and parameters of EPR and NMR spectra show that due to the increase of thermal agitation, there is a greater degree of disorder in the glasses melted at higher temperatures. Consequently the microenvironmental amplitudes of the two magnetic resonance centers also increase, which is reflected in the poorer resolution of the resonance lineshapes. Both the fraction of tetracoordinated boron atoms and the paramagnetic center density decrease, the phase separation process is diminished and also the covalent character of CuO bonds decreases.A mechanism based on these results is suggested, that could describe the structural modifications which take place in these glasses as a result of the increase of their preparation temperature.     

Light induced charge transfer processes in Cr doped Bi12GeO20 and Bi12SiO20 single crystals

Optical absorption and ESR spectra of Bi₁₂GeO₂₀ doped with Cr were measured before and after illumination with visible light. It was found that Cr⁴⁺ ions in tetrahedral position are responsible for light induced ESR and optical spectra. The g-factor of the center is 1.945 ± 0.002. Crystal field parameters for the Cr⁴⁺ center are D_q = 820 cm^?1 and B = 429cm^?1. The photochromic effect is explained in terms of a Cr⁵⁺?Cr⁴⁺ charge transfer process.     

Paramagnetic center Cd+(2 S 1/2) in natural carbonate apatites Ca5(PO4)3 ? n (CO3) n (F,OH)

The stable paramagnetic center Cd⁺(² S _1/2) in natural sedimentary calcium phosphates containing a cadmium impurity has been studied using electron paramagnetic resonance. The parameters of the spectrum have been determined, and the influence of different radiation types on the paramagnetic center has been investigated.     

Family of paramagnetic centers of Ce3+ ions in yttrium aluminum garnet

The electron paramagnetic resonance (EPR) spectra of Ce³⁺ ions in single crystals of yttrium aluminum garnet have been investigated. It has been found that, in addition to the usually observed EPR signals of Ce³⁺ ions located in the regular environment at dodecahedral sites of the crystal lattice, the spectra contain a group of less intense anisotropic lines with g-factors close to the corresponding parameters of Ce³⁺ ions in the regular environment. It has been concluded that the observed satellite lines belong to the family of Ce³⁺ ions in the immediate vicinity of which there are permutation defects that lead to a change in the strength and symmetry of the crystal field in the vicinity of the paramagnetic center.     

Correlation between the negative magnetoresistance effect and magnon excitations in single-crystalline CuCr1.6V0.4Se4

Structural, electrical and magnetic measurements, as well as electron spin resonance (ESR) spectra, were used to characterise the single-crystalline CuCr_1.6V_0.4Se₄ spinel and study the correlation between the negative magnetoresistance effect and magnon excitations. We established the ferromagnetic order below the Curie temperature T _C ≈ 193 K, a p-type semiconducting behaviour, the ESR change from paramagnetic to ferromagnetic resonance at T _C, a large ESR linewidth value and its temperature dependence in the paramagnetic region. Electrical studies revealed negative magnetoresistance, which can be enhanced with increasing magnetic field and decreasing temperature, while a detailed thermopower analysis showed magnon excitations at low temperatures. Spin–phonon coupling is explained within the framework of a complex model of paramagnetic relaxation processes as a several-stage relaxation process in which the V³⁺ ions, the exchange subsystem and conduction electron subsystem act as the intermediate reservoirs.     

Bleaching and annealing experiments of the (Cl2-) center in irradiated cadmium chlorate

The paramagnetic centers Cl₂^- have been identified in irradiated Cd(ClO₃)₂ single crystals by X band ESR experiments and optical absorption. By isochronal annealing and bleaching we can vary the intensity of the optical and resonance spectra. The intensity correlation of both spectra showed that the formation of the center Cl₂^- is connected with the destruction of the fragment (ClO₂Cl)^-.