Paramagnetic Impurity Centers Research Articles

Paramagnetic impurity centers of Fe3+ in the silicon positions of yttrium orthosilicate

Paramagnetic impurity centers of Fe3+ in the silicon positions of yttrium orthosilicate

The crystallization temperature of vein quartz estimated from the concentration of the titanium paramagnetic center in quartz: A case study of the Peschanka porphyry copper–molybdenum–gold deposit, Western Chukchi Peninsula, Russia

The concentration of the Al and Ti paramagnetic impurity centers in pre-ore and ore-stage quartz at the Peschanka porphyry copper–molybdenum–gold deposit in the Western Chukchi Peninsula, Russia were determined using electron paramagnetic resonance spectroscopy (EPR). The [AlO 4 - /h+]0 concentration in pre-ore and ore-stage quartz varies from 29 to 124 and from 13 to 101 at. ppm, respectively. The contents of the [TiO 4 - /Li+]0 and [TiO 4 - /H+]0 centers reach 20 and 6.3 at. ppm, respectively. Pre-ore quartz associated with the formation of biotite–potassium feldspar–quartz alteration and ore-stage quartz associated with the formation of quartz–sericite rocks followed by the ore deposition differ considerably in the Ti center content, especially the [TiO 4 - /H+]0 center. The [TiO 4 - /H+]0 concentration is much higher in the pre-ore quartz (>2 at. ppm) than that in the ore-stage quartz related to copper mineralization (<2 at. ppm). The [TiO 4 - /Li+]0 concentration also decreases from pre-ore to ore-stage quartz. Taking the data we obtained into account, the formation temperature of pre-ore and ore-stage quartz estimated from a titaniumin-quartz geothermometer is 590–470°C (weighted average 520°C) and 510–310°C (weighted average 430°C), respectively. The obtained temperature range of 590 to 310°C is similar to that determined from homogenization of fluid inclusions in quartz.

Crystallization temperature of vein quartz at Peschanka porphyry Cu-Mo-Au deposit, Western Chukchi Peninsula, Russia: Estimation from content of paramagnetic titanium centers in quartz

The Al and Ti paramagnetic impurity center concentrations in pre-ore and ore-bearing quartz at the Peschanka porphyry copper-molybdenum-gold deposit in the Western Chukchi Peninsula, Russia were determined using electron paramagnetic resonance (EPR). The [AlO4-/ h+]0 concentration in pre-ore and ore-bearing quartz varies from 29 to 124 and from 13 to- + 0- + 0101 at. ppm, respectively. The contents of [TiO4 /Li ] - and [TiO4 /H ] -centers reach 20 and6,3 at. ppm, respectively. Pre-ore stage quartz associated with the formation of biotite-potassiumfeldspar-quartz alteration and quartz of the ore stage associated with the formation of quartz-sericite rocks followed by the ore deposition are substantially in content of the titanium centers,- + 0- + 0especially [TiO4 /H ] -centers: the [TiO4 /H ]concentration is much higher in the pre-orestage quartz (>2 at. ppm) than that in quartz related to copper mineralization (<2 at. ppm).+ 0The [TiO4-/Li ]concentration also decreases from pre-ore to ore-bearing quartz. Taking intoaccount the data obtained, the formation temperature of pre-ore and ore-bearing quartz estimatedfrom a titanium-in-quartz geothermometer is 590-470 °C (average 520 °C) and 510-310 °C (average 430 °C), respectively. The obtained temperature range of 590 to 310 °C is similar to that determined from homogenization of fluid inclusions in pre-ore and ore-bearing.

Theoretical investigation of the spin-Hamiltonian parameters and optical spectra for the doped Cu2+ in ZnCdO nanopowder

The spin-Hamiltonian (SH) parameters (g factors g||, g⊥ and hyperfine structure constants A||, A⊥) and d–d transitions for ZnCdO:Cu2+ are calculated based on the perturbation formulas for a 3d9 ion in tetragonally elongated octahedra. Good agreement between the calculated results (four SH parameters and three optical absorption bands) and the experimental results can be obtained. Since the SH parameters are sensitive to the local structure of a paramagnetic impurity center, the tetragonal distortion (characterized by the relative elongation ratio ρ ≈ 3.5% along the C4 axis) of the impurity center due to the Jahn–Teller effect is also acquired from the calculations. The negative and positive signs of hyperfine structure constants A|| and A⊥ for ZnCdO:Cu2+, respectively, are also suggested in the discussions.

DFT modeling of Mn charged states in Ga1 − x Mn x As diluted ferromagnetic semiconductors: The cluster approach

Quantum chemical cluster modeling of the high-symmetry nanoclusters Ga15MnAs16H36 and Ga12MnAs16H36 simulating the bulk of a GaAs crystal with Mn paramagnetic impurity center is carried out within the Density Functional Theory. A Generalized Gradient Approximation method PBEPBE/LanL2DZ is used to study the neutral Mn0 and ionized Mn− states of a Mn atom in the nanoclusters under consideration. The change of the charge state of the impurity center from Mn0 to Mn− leads to a considerable relaxation of Mn-As bonds in the immediate surroundings of the Mn atom and to the “p-hole” recombination. It also affects the spin density localization. The components of the g-tensor for both the neutral Mn0 and the ionized Mnstates are calculated by the Gauge-Independent Atomic Orbital (GIAO) method with the hybrid density-functional mPW1PW91. The resulting values of the g-factor are in good agreement with the experimental Electron Paramagnetic Resonance data. The results obtained confirm the applicability of the cluster approach for describing charge effects in dilute ferromagnetic semiconductors.

Theoretical explanation of absorption spectra and ESR parameters of Cu2+ in shattuckite

The optical absorption spectra and electronic spin resonance parameters (ESR g factors g(parallel to), g(perpendicular to) and hyperfine structure constants A(parallel to), A(perpendicular to)) 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 3d(9) 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. (C) 2012 Elsevier B.V. All rights reserved.

INVESTIGATION OF THE DEFECT STRUCTURE, OPTICAL AND EPR SPECTRA FORCdS:Ti2+ANDCdSe:Ti2+CRYSTAL

The optical spectral band positions and spin-Hamiltonian parameters (g factors g‖, g⊥and zero-field splitting D) of CdS : Ti2+and CdSe : Ti2+crystals are calculated from the complete diagonalizaion (of energy matrix) method based on a two-spin-orbit parameter model for 3d2ions in trigonal symmetry. In the model, both the contribution to spin-Hamiltonian parameters due to the spin-orbit parameter of central 3d2ions and that of ligand ions are included. The crystal field parameters used in the calculations are obtained from the superposition model which enables correlation of the optical and EPR spectral data with the defect structure of the studied paramagnetic impurity centers in crystals. From the calculations, the defect structures of Ti2+centers in CdS : Ti2+and CdSe : Ti2+are acquired, the signs of zero-field splittings D are suggested, and the optical band positions and spin-Hamiltonian parameters are explained. The results are discussed.

The relationship between the g-shift and the local structure for the axial impurity centers in the doped LiNbO 3 crystals

The relationship between the g-shift and the local structure of paramagnetic impurity center with axial symmetry is investigated. Spectroscopic data on transitions metal and rare-earth impurities in lithium niobate crystal are used to verify the validity of this method. The distortions around the impurity ions are determined based on the g-shift and compared with experimental data as well as predictions of the superposition model (SPM) and the perturbation methods (PM). The g-shift method yields displacements of impurity ions in good agreement with experimental and SPM or PM predictions. The advantages of the proposed method are discussed.

Defect structure of Cu2+ center in K2SO4‒Na2SO4‒ZnSO4 glasses

The spin Hamiltonian parameters (g factors g //, g ⊥ and hyperfine structure constants A //, A ⊥) of Cu2+ in K2SO4‒Na2SO4‒ZnSO4 glasses are calculated from the high-order perturbation formulas of 3d9 ion in tetragonal octahedral sites. The calculated results are in agreement with the observed values. Since the EPR parameters are sensitive to the local structure of a paramagnetic impurity center, the defect structure of Cu2+ center in K2SO4‒Na2SO4‒ZnSO4 glasses is estimated. The validity of results is discussed.

Investigation of the EPR Parameters and Defect Structure of Ni2+ Ions in RbMgF3 Crystals

By means of the complete energy matrix diagonalization procedure of 3d2/3d8 ions in trigonal symmetry and using the superposition model, the electron paramagnetic resonance (EPR) parameters for Ni2+ ions in RbMgF3 crystals with C3v and D3d symmetry are studied. From the investigation, the defect structures of these paramagnetic impurity centers are obtained and the EPR parameters are explained reasonably. - PACS numbers: 76.30.Fc, 61.72.Bb, 71.70.Ch

Spin lattice relaxation of spin‐½ nuclei in solids containing diluted paramagnetic impurity centers. I. Zeeman polarization of nuclear spin system

AbstractSpin‐lattice relaxation mechanisms of nuclear spins in solids in the presence of low concentrations of paramagnetic centers are discussed. Apart from the electron spin‐lattice relaxation mechanism, three‐spin processes involving two electron spins and a nuclear spin may also play a role in the spin‐lattice relaxation of the nuclear spins. Nuclear spins that are close to a paramagnetic impurity center experience large local fields and do not contribute to the NMR signal. Only a small percentage of nuclear spins surrounding an impurity center are polarized directly. The majority of the nuclear spins are polarized by spin diffusion, which is a slow process. A brief overview of a number of paramagnetic impurity centers, which are common in natural diamond, is given and experimental 13C spin‐lattice relaxation times for a suite of natural diamonds are compared with calculated values. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson 19A: 20–35, 2003.

Spin‐lattice relaxation of spin‐½ nuclei in solids containing diluted paramagnetic impurity centers. III. Dynamic polarization of nuclear spin system by nuclear orientation via electron spin locking

AbstractDynamic nuclear polarization of nuclei by means of paramagnetic electron spin locking (Hartmann‐Hahn cross‐polarization between paramagnetic electrons and nuclei, or NOVEL) is discussed. The theory is demonstrated by experiments executed at 2.4 and 9.6 GHz on a natural type Ib diamond. It is shown that the 13C polarization rate is independent of the microwave frequency, in agreement with theory. NOVEL polarization takes place only while the spin‐locking pulse is on. The rate at which the nuclei are polarized is proportional to the electron polarization in the rotating frame. Therefore, the length of the spin‐locking pulse is limited by the value of T1ρ(e), and because T1ρ(e) ≪ T1(e) for diamond the effective NOVEL polarization rate of 13C nuclei is usually relatively low. A comparison between the relative effectiveness of 13C polarization rates between NOVEL and the solid‐state effect is made for high and low paramagnetic impurity concentrations. The dependence of the 13C polarization rate on the paramagnetic impurity concentration has been determined for a suite of natural diamonds. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson 19A: 44–49, 2003.

Spin‐lattice relaxation of spin‐½ nuclei in solids containing diluted paramagnetic impurity centers. II. Dynamic polarization of nuclear spin system by thermal mixing and solid‐state effect

AbstractDynamic nuclear polarization of nuclear spins via the solid‐state and thermal mixing effects is discussed. Continuous‐wave S‐ and X‐band microwave radiation have been employed to measure 13C signal enhancements and polarization times for 13C nuclei in a natural type Ib diamond as a function of magnetic field. It was found that thermal mixing plays an important role in the 13C signal enhancement because the central electron spin resonance (ESR) line width HL ≃ H0γC/γe, resulting in flip‐flip and flip‐flop forbidden transitions taking place simultaneously. On the other hand, the 13C spin‐lattice relaxation rate is determined to a large extent by the solid‐state effect (forbidden transitions). 13C polarization rates have also been measured for a suite of natural diamonds. It is shown that the polarization rate is proportional to the paramagnetic impurity concentration, in agreement with the theory. © 2003 Wiley Periodicals, Inc. Concepts Magn Reson 19A: 36–43, 2003.

Studies of electron paramagnetic resonance parameters and defect structures for Ti 2+ and V 3+ ions in CdS crystals

By applying the high-order perturbation formulas based on the cluster approach for the EPR parameters of 3d 2 ions in trigonal symmetry, the zero-field splitting D, g factors g //, g ⊥ and hyperfine structure constants A //, A ⊥ for Ti 2+ and V 3+ ions in CdS crystals are studied. From the studies, the defect structures of these paramagnetic impurity centers are obtained and the EPR parameters are also explained reasonably.

Magnetoplastic effect in LiF crystals and longitudinal spin relaxation

New features of the dependence of the average travel distance l of dislocations on the magnetic field B have been found in an investigation of the magnetostimulated dislocation mobility in LiF crystals: A transition has been found from ordinary proportionality l∝B2 to saturation l≈ const in high fields B. It is shown that the experimental points can be described satisfactorily by the theoretical dependence l∝ [(B0/B)2+1]−1 (B0≈0.8 T), typical for the mechanism of longitudinal spin relaxation in a system of radical pairs, which are supposedly formed when dislocation nuclei interact with paramagnetic impurity centers. According to the theory, the level of the field B0 is determined by the characteristic frequency of the oscillations of the internal fields in the lattice, which for B0∼1 T is of the order of 1011s−1, which corresponds to the typical frequency of characteristic oscillations of dislocation segments between pinning centers, which, naturally, does not depend on temperature. This in turn explains the fact that the measured values of B0 are the same at 293 K and 77 K.

Effect of a magnetic field on the yield point of NaCl crystals

A constant magnetic field is found to have a substantial effect on the macroplasticity of NaCl crystals when they are being actively strained at a constant rate \(\dot \varepsilon = const\) during magnetic treatment. We have measured the dependence of the yield point σ y on the magnetic induction B=0–0.48 T and the strain rate \(\dot \varepsilon = 1.4 \times 10^{ - 5} - 1.4 \times 10^{ - 3} s^{ - 1}\). It is shown that this magnetic effect has a threshold character and is observed only for B>B c , where B c grows with increasing \(\dot \varepsilon\) as \(\sqrt {\dot \varepsilon }\). The lower the strain rate \(\dot \varepsilon\), the larger the relative decrease in the yield point σ y (B)/σ y (0) at fixed field B>B c . At small enough strain rates \(\dot \varepsilon\) the threshold field B c ceases to depend on \(\dot \varepsilon\) and goes constant. A theoretical model is proposed which is in good agreement with the observed regularities. The model is based on the competition between thermally activated and magnetically stimulated depinning of dislocations from paramagnetic impurity centers.

Local trigonal distortions of some paramagnetic impurity centers in ZnSiF6 · 6H2O crystal

The trigonal distortions of Ni2+, V2+, Mn2+ and Co2+ ions in ZnSiF6 · 6H2O crystal have been studied by analysing their zero-field splittings and the anisotropy of g factor (characterized by Δg = g ⊥ − g ∥). From the studies, it is found that the sign of trigonal distortion (compression or elongation) of octahedral environment of impurity ion is impurity-dependent, i.e., when the ionic radius of impurity is greater than that of the replaced Zn2+ ion, the octahedral environment of impurity is compressed, whereas if the ionic radius of impurity is smaller than that of Zn2+, it is elongated. This point is similar to the case of isomorphous pure crystal where the paramagnetic ion is the host ion rather than the impurity.

Local axial distortion of paramagnetic impurity center in some axial-symmetry crystals

The local octahedral environments of paramagnetic impurity ion in some crystals with small axial (tetragonal or trigonal) distortion have been studied by analysing their EPR spectra. According to the study, an interesting trend is suggested, i.e. the sign of distortion (elongation or compression) of octahedral environment of the paramagnetic impurity is different from that of the host ion it replaces, but in agreement with the one in the isomorphous pure crystal where the paramagnetic ion is the host ion rather than the impurity.

Paramagnetic impurity centers in TeO2single crystals

Paramagnetic impurity centers in TeO₂single crystals

Thermal stabilities of paramagnetic defect and impurity centers in quartz: Basis for ESR dating of thermal history.

Thermal stabilities of E′, Al, and Ti centers in quartz from the Mannari granite were investigated by electron spin resonance (ESR) with isothermal and isochronal annealing experiments. Second order decay kinetics best explain the results. The closure temperatures at which the ESR clocks start were estimated to be 91°C, 78°C, and 31°C for E′, Al, and Ti centers, respectively, with a cooling rate of about 10°C/Ma. Closure temperatures were also estimated for rocks around an intrusive dike. The thermal history of a quartz bearing rock can be investigated using ESR ages for different centers.