Papers by Keyword: Electron Paramagnetic Resonance (EPR)

Abstract: The Spin Hamiltonian Parameters (the G Factors and the Superhyperfine Parameters) for the Tetragonal [Fe(CN)₄Cl₂]^5- Complex in Nacl Are Theoretically Investigated from the Perturbation Formulas of these Parameters for a Low Spin (S=1/2) 3d⁷ Ion in Tetragonally Elongated Octahedra. this Impurity Center Is Attributed to the Substitutional Fe²⁺ (in the Form of [Fe(CN)₆]^4–) on Host Na+ Site Reduced by Capturing One Electron during the X-Ray Irradiation Process, with the Two Axial Ligands CN– Replaced by Two Cl–. since the Crystal Fields of the Axial Ligands Cl– Are Weaker than those of the Perpendicular Ligands CN–, the [Fe(CN)₄Cl₂]^5- Complex Exhibits a Tetragonally Elongated Octahedron. this System Shows the Low Spin S=1/2 of the Strong Crystal Field Case, which Is Unlike the High Spin S=3/2 of the Weak and Intermediate Crystal Field Case for Fe⁺ in Conventional Chlorides. the Theoretical Spin Hamiltonian Parameters Are in Reasonable Agreement with the Experimental Data, and the Results Are Discussed.

Abstract: In unintentionally Nb-doped 4H-SiC grown by high-temperature chemical vapor deposition (HTCVD), an electron paramagnetic resonance (EPR) center with C_1h symmetry and an electron spin S=1/2 was observed. The spectrum shows a hyperfine structure consisting of ten equal-intensity hyperfine (hf) lines which is identified as due to the hf interaction between the electron spin and the nuclear spin of ⁹³Nb. An additional hf structure due to the interaction with two equivalent Si neighbors was also observed. Ab initio supercell calculations of Nb in 4H-SiC suggest that Nb may form complex with a C-vacancy (V_C) resulting in an asymmetric split-vacancy (ASV) defect, Nb_Si-V_C. Combining results from EPR and supercell calculations, we assign the observed Nb-related EPR center to the hexagonal-hexagonal configuration of the AVS defect in the neutral charge state, (Nb_Si-V_C)⁰.

Abstract: Relatively little is known about the transition metal defects in silicon carbide (SiC). In this study we applied highly convergent and sophisticated density functional theory (DFT) based methods to investigate important transition metal impurities including titanium (Ti), vanadium (V), niobium (Nb), chromium (Cr), molybdenum (Mo) and tungsten (W) in cubic 3C and hexagonal 4H and 6H polytypes of SiC. We found two classes among the considered transition metal impurities: Ti, V and Cr clearly prefer the Si-substituting configuration while W, Nb, and Mo may fractionally form a complex with carbon vacancy in hexagonal SiC even under thermal equilibrium. If the metal impurity is implanted into SiC or when many carbon impurities exist during the growth of SiC then complex formation between Si-substituting metal impurity and the carbon vacancy should be considered. This complex pair configuration exclusively prefers the hexagonal-hexagonal sites in hexagonal polytypes and may be absent in cubic polytype. We also studied transition metal doped nano 3C-SiC crystals in order to check the effect of the crystal field on the d-orbitals of the metal impurity.

Abstract: The defect structure for Ni3+ in ZnO crystal is theoretically investigated using the perturbation formulas of the spin Hamiltonian parameters for a 3d7 ion in trigonally distorted tetrahedra. In view of the significant covalency of the system due to the high valence state of Ni3+, the ligand orbital and spin-orbit coupling contributions are taken into account in a uniform way based on the cluster approach. The impurity Ni3+ is found not to occupy the ideal Zn2+ site in ZnO but to undergo the small axial displacement of about 0.044 Ǻ away from the oxygen triangle along the C3 axis. The theoretical spin Hamiltonian parameters based on the above impurity displacement show good agreement with the experimental data. The defect structure of this impurity center is compared with that for the similar Fe3+ in ZnO.

Abstract: The EPR parameters and the local structure for Co2+ in ZnO are deduced from the perturbation formulas of these parameters for a 3d7 ion in a trigonally distorted tetrahedron. The ligand orbital and spin-orbit coupling contributions are taken into account uniformly from the cluster approach in view of the covalency of the system. The impurity V3+ is found not to locate exactly on the Zn2+ site but to experience a small displacement of 0.04 Ǻ away from the ligand triangle, along the C3 axis. The theoretical EPR parameters based upon the above impurity displacement are in good agreement with the observed values.

Abstract: The electron paramagnetic resonance parameters (g-factors and the hyperfine structure constants) for the Cu2+ center in lanthanum magnesium nitrate (LMN) are theoretically studied from the conventional perturbation formulas of these parameters for a 3d9 ion in tetragonally elongated octahedra. The studied complex is found to exhibit the slight tetragonal elongation (characterized by the relative elongation ratio ρ  4%) due to the Jahn-Teller effect, which may entirely conceal the original trigonal distortion of the host Mg2+ site in LMN. The conventional formulas containing only the metal orbital and spin-orbit coupling contributions are proved to be valid for the Cu2+ center in view of the weak covalency and ligand spin-orbit coupling interactions. This defect is also compared with the similar Cu2+ center of the Jahn-Teller nature on the octahedral interstitial site in the CdSe nanocrystals.

Abstract: The local distortion of the impurity Ni+ center in magnesium oxide is theoretically studied by analyzing its electron paramagnetic resonance g factor from the formula of a 3d9 ion under octahedra with tetragonal elongation deformation. The defect center is suggested to exhibit the relative elongation along the four-fold axis by about 0.05 Å of the Jahn-Teller nature. The observed isotropic g factor ( 2.2391) is attributable to the dynamical average of the anisotropic g values under tetragonal elongation due to the dynamical Jahn-Teller effect.

Abstract: The electron paramagnetic resonance (EPR) parameters g factor and the hyperfine structure constant A factor for the substitutional Rh+ in NaCl are theoretically studied from the perturbation formulas of these parameters for a 4d8 ion in cubic octahedra. In these formulas, the ligand orbital and spin-orbit coupling contributions which were normally omitted in the previous studies are taken into account using the cluster approach. The calculated g and A factors are in good agreement with the experimental data. The ligand contributions to the EPR parameters are somewhat important and should be considered in the analysis of the EPR spectra for a 4d8 ion in chlorides. The local structure of this center is also discussed.

Abstract: The electron paramagnetic resonance (EPR) gyromagnetic factors for the Ni3+ center in MgO are theoretically investigated using the improved g formulas for a tetragonally elongated octahedral 3d7 complex suffering the dynamical Jahn-Teller effect. From the studies, the impurity Ni3+ center is found to undergo the relative elongation along the tetragonal axis by about 1%. The calculated g value shows good agreement with the observed result, and the improvement in the theoretical calculations is achieved in this work as compared with the previous treatments. The local structure of the impurity center is discussed.

Abstract: Electron paramagnetic resonance (EPR) and electron spin echo (ESE) at X-band (9.4 GHz) and W-band (94 GHz) have been used to study defects in natural diamond nanocrystals, detonation nanodiamond (ND) with a size of  4.5 nm and detonation ND after high-pressure high-temperature (HTHP) sintering with a size of  8.5 nm. Based on identification of atomic nitrogen centers N0 and nitrogen pairs N2+ detected by means of the high frequency EPR and ESE in natural diamond nanocrystals, atomic nitrogen centers N0 have been discovered in nanodiamond core in detonation ND and detonation ND after sintering. In addition EPR signal of multi-vacancy centers with spin 3/2 seems to be observed in diamond core of detonation ND.