EPR Zero-field Splitting Research Articles

Rapid and precise determination of zero-field splittings by terahertz time-domain electron paramagnetic resonance spectroscopy.

Zero-field splitting (ZFS) parameters are fundamentally tied to the geometries of metal ion complexes. Despite their critical importance for understanding the magnetism and spectroscopy of metal complexes, they are not routinely available through general laboratory-based techniques, and are often inferred from magnetism data. Here we demonstrate a simple tabletop experimental approach that enables direct and reliable determination of ZFS parameters in the terahertz (THz) regime. We report time-domain measurements of electron paramagnetic resonance (EPR) signals associated with THz-frequency ZFSs in molecular complexes containing high-spin transition-metal ions. We measure the temporal profiles of the free-induction decays of spin resonances in the complexes at zero and nonzero external magnetic fields, and we derive the EPR spectra via numerical Fourier transformation of the time-domain signals. In most cases, absolute values of the ZFS parameters are extracted from the measured zero-field EPR frequencies, and the signs can be determined by zero-field measurements at two different temperatures. Field-dependent EPR measurements further allow refined determination of the ZFS parameters and access to the g-factor. The results show good agreement with those obtained by other methods. The simplicity of the method portends wide applicability in chemistry, biology and material science.

Theoretical calculations of the optical band positions and zero-field splitting 2D for Cr3+ ions in fluoride garnet Na3Li3In2F12

Five optical band positions and one EPR zero-field splitting 2D for Cr3+ ions at the trigonally-distorted octahedral In3+ site in fluoride garnet Na3Li3In2F12 are calculated together from the complete diagonalization (of energy matrix) method based on the two-spin–orbit-parameter model, where the contributions from both the spin–orbit parameters of central dn ion and ligand ion are contained. The calculated results are in reasonable agreement with the experimental values. The calculations show that similar to the bonding length (i. e., metal-ligand distance) R, the bonding angle θ (between the direction of R and C3 axis) is different from the corresponding one in the host crystal Na3Li3In2F12 because of the size mismatching substitution. It appears that the complete diagonalization (of energy matrix) method based on the two-spin–orbit-parameter model is effective in the unified calculation of optical spectra and EPR zero-field splitting for d3 ions in crystals.

Theoretical investigations of the local structure distortion and relationship between the EPR parameter and spin-orbit coupling coefficients for CsCdX3:Ni2+ (X = Cl, Br) systems

The EPR zero-field splitting parameters and structural distortion of Ni2+-doped CsCdX3 (X = Cl, Br) systems have been studied on the basic of 45f × 45 complete energy matrices for a configuration ion in trigonal ligand field, in which the contributions from the spin-orbit coupling coefficients of the central ions and ligands are taken into account simultaneously. It is shown that the local structure exhibits compression distortions for CsCdX3 (X = Cl, Br):Ni2+ systems, and the contribution from the s.o. coupling coefficient of ligands to the covalent effect should not be neglected. Simultaneously, the relationship between the EPR parameter D and the spin-orbit coupling coefficients (ζ, ) as well as the average parameter ζ1() and the divergent parameter ζ2() have been discussed.

Investigation of the local structure and the effects of the orbit reduction factor on the EPR g-factors for octahedral (CrO6)9− cluster in TiO2:Cr3+ and MgO:Cr3+ systems

A theoretical method for studying the inter-relation between electronic and molecular structure has been proposed based on the complete energy matrices for a d(3) configuration ion in a tetragonal ligand-field. By means of this method, the local structure for Cr(3+) ion doped in oxides TiO(2) (anatase) and MgO has been determined. The calculated results indicate that the local structure of the (CrO(6))(9-) cluster in TiO(2):Cr(3+) and MgO:Cr(3+) systems exists as a compressed distortion relative to the regular octahedron. Meanwhile, the dependence of the EPR zero-field splitting parameter D on the local structure parameter Delta R (Delta R = R(1)-R(2)) has been revealed. In addition, the relation between the EPR g-factors (g(II),g(perpendicular to),Delta g) and the orbit reduction factor k has been discussed for the two systems, suggesting that the orbit reduction factor k is very important to understand the EPR g-factors. (C) 2009 Elsevier B.V. All rights reserved.

Theoretical study of spin singlets contributions to zero-field splitting and local lattice structure of Cr 2+ in CdGa 2S 4

A theoretical method for investigating the inter-relation between electronic and the molecular structures of a d 4 ion in a tetragonal ligand-field has been established on the basis of 210 × 210 complete energy matrix within a weak-field-representation. Using the method, the local structure parameters of CdGa 2S 4:Cr 2+ system are determined by the experimental EPR zero-field splitting (ZFS) spectra. Our results show that the local structure around Cr 2+ is a compression distortion and the local lattice structure parameters R = 2.46 Å and θ = 57.63° are determined. Moreover, the contributions of the spin singlets to ZFS parameters of Cr 2+ ions in CdGa 2S 4 crystals are investigated for the first time. The results indicate that the spin singlets contributions to ZFS parameter D are negligible, but the contributions to ZFS parameters a and F cannot be neglected.

Electron paramagnetic resonance parameters of Mn4+ ion in h-BaTiO3 crystal from a two-mechanism model

The EPR parameters (g factors g (aEuro-), g (aSyen) and zero-field splitting D) of Mn(4+) ion in h-BaTiO(3) crystal are calculated from the complete high-order perturbation formulas based on a two-mechanism model for the EPR parameters of 3d (3) ions in trigonal symmetry. In the model, not only the widely used crystal-field mechanism, but also the charge-transfer mechanism (which is not considered in crystal-field theory) are included. The calculated results are in reasonable agreement with the experimental values. The relative importance of charge-transfer mechanism to EPR parameters and the defect structure of Mn(4+) centre in h-BaTiO(3) crystal obtained from the calculations are discussed.

Theoretical study of the dependence of EPR spectra on local structure for octahedral Cr3+ centres in oxides and fluorides series

A theoretical method for investigating the inter-relation between the EPR parameters and local structure has been established on the basis of the complete energy matrices for 3d3 configuration ions in both the trigonal and tetragonal ligand fields. By means of this method, the local structure of the octahedral Cr3+ centres in double molybdates series and spinels series as well as the perovskite-type fluorides series has been studied systematically. Furthermore, the dependence of the EPR zero-field splitting parameter D on the local structure parameters in both trigonal and tetragonal ligand-fields has been revealed, simultaneously. The inter-relation between the EPR parameters D and Δg(g // − g ⊥) is also elucidated.

Local structure and EPR studies of octahedral (MnO 6) 2+ cluster in the series of zinc fluoride hexahydrate

The EPR zero-field splitting of Mn 2+ ions doped in single crystals of zinc fluoride hexahydrate series is studied by diagonalizing the complete energy matrices for d 5 configuration ions in a trigonal ligand field. By simulating the EPR parameters D and ( a − F) simultaneously, the local structure parameters R = 2.252 Å, θ = 52.6006° for ZnTiF 6 · 6H 2O:Mn 2+, R = 2.266 Å, θ = 52.5466° for ZnGeF 6 · 6H 2O:Mn 2+, and R = 2.226 Å, θ = 52.9195° for ZnNbOF 5 · 6H 2O:Mn 2+ are determined, respectively, in which R is the Mn–O bond length and θ is the angle between the Mn–O bond length and C 3 axis. From our calculations, it is found that when Mn 2+ doped in the isomorphous crystals there have a similar local distortion.

An investigation of the EPR zero-field splitting of Fe 3+ ions at tetragonal sites in Tl 2MgF 4 and Tl 2ZnF 4 crystals

By analyzing the EPR zero-field splitting parameters of Fe 3+ located at tetrahedral site in Tl 2MgF 4 and Tl 2ZnF 4 crystals, the local lattice structure for Fe 3+ doped in Tl 2MgF 4 and Tl 2ZnF 4 crystals is theoretically studied by diagonalizing the complete energy matrices for a d 5 configuration ion in a tetragonal ligand-field. From the calculation, the local lattice structure parameters R ⊥ = 1.8914 Å, R ∥ = 1.84699 Å and R ⊥ = 1.88229 Å, R ∥ = 1.84490 Å for tetragonal Fe 3+ center in Tl 2MgF 4 and Tl 2ZnF 4 crystals are determined, respectively. This result suggests that the ligand-fluorines on the C 4 axis deviate easily in both two crystals, which is consistent with the conclusion drawn by earlier workers.

Investigations of the EPR zero-field splitting and the defect structure for Mn 2+ and Fe 3+ ions in anatase crystals

The EPR zero-field splittings D of Mn 2+ and Fe 3+ ions in anatase crystals at room and low temperatures are calculated from the high-order perturbation formula of zero-field splitting D for 3d 5 ions in tetragonal symmetry based on the dominant spin-orbit coupling mechanism. The calculated results are consistent with the observed values. From the calculations, Mn 2+ and Fe 3+ ions are suggested to substitute for Ti 4+ ions in anatase (in the previous paper, Mn 2+ ion was suggested at an interstitial site rather than substitutional site) and the defect structures (characterized mainly by the local oxygen parameter u) for both tetragonal Mn 2+ and Fe 3+ impurity centers are estimated. The different zero-field splitting at room and low temperatures are due mainly to the change of local oxygen parameter u with the temperature. These results are discussed.

Studies of the defect structure for Mn 2+ in KTaO 3 crystal from the calculation of EPR zero-field splitting

There are several mistakes in the recent paper about the theoretical studies of defect structure for Mn 2+ ion at the 12-fold tetrakaidecahedral K + site in KTaO 3 by calculating the spin Hamiltonian (SH) parameters, so the calculated defect structure (which is smaller than those obtained from the local density approximation (LDA) method, density functional theory in the generalized gradient approximation (GGA) and the dipole moment study (DMS)) is doubtful. Therefore, we restudy the defect structure in this paper by using the reasonable expressions and parameters. The present result is in agreement with those based on LDA, GGA and DMS methods and can be regarded as reasonable. It appears that the reliability of the defect structure of impurity center determined from the calculation of SH parameter depends strongly upon reasonableness of the used expressions and parameters.

The EPR Zero-Field Splitting D and its Pressure and Temperature Dependence for Trigonal Mn2+ Centers in [Zn(H2O)6](BF4)2:Mn2+ Crystal

The EPR zero-field splitting D (= b0 2 ) and its pressure and temperature dependence for trigonal Mn2+ centers in low and room temperature phases in [Zn(H2O)6](BF4)2 :Mn2+ crystal are studied by a high-order perturbation formula based on the dominant spin-orbit coupling mechanism. From the studies, the local trigonal distortion angles, the local angular compressibilities and the local angular thermal expansion coefficients for Mn2+ centers in both phases of the [Zn(H2O)6](BF4)2 crystal are estimated. The results are discussed

EPR zero-field splitting parameters and structural distortion study of Mn 2+-doped (CH 3) 4NCdCl 3 crystal in high-temperature phases

The EPR zero-field splitting parameters and structural distortion of Mn 2+-doped (CH 3) 4NCdCl 3 crystal in high-temperature phases have been studied by diagonalizing the complete energy matrices. The zero-field splitting parameters D and ( a − F) are demonstrated to be negative and positive, respectively, and obtain a reasonable explanation together. Simultaneously, the distortion magnitude of local lattice structure around the Mn 2+ ion in this crystal is determined, namely, Δ R = −0.13 Å and Δ θ = 2.975° for T = 124 K; Δ R = −0.12 Å and Δ θ = 2.610° for T = 297 K; Δ R = −0.26 Å and Δ θ = 2.870° for T = 573 K. From the structural distortion, we deduce that a new structural phase transition should occur to the (CH 3) 4NCdCl 3 crystal at temperatures ranging from 297 to 573 K. This is also in agreement with recent differential scanning calorimetric measurements.

Local release factors of Cr 3+ and Ni 2+ centers in the tetragonal phase of RbCdF 3 crystal

The local release factors k l, which are introduced to stand for the release or elongation effect of the impurity-ligand bond along the C 4-axis at the cubic–tetragonal phase transition, are studied by analyzing the EPR zero-field splitting for Cr 3+ and Ni 2+ ions in RbCdF 3 crystals. The results are compared with those of other paramagnetic ions in RbCdF 3 and in the similar crystal RbCaF 3. It is found that for the same crystal, the factor k l is different from impurity to impurity and that for the same impurity, the factors k l in both RbCdF 3 and RbCaF 3 crystals are close to each other. The cause of the very small zero-field splitting for Cr 3+ in the tetragonal phase of RbCdF 3 crystal is suggested.

Zero-field Splitting and Local Lattice Distortions for Fe3+ Ions in Some Ib-IIIb-VI2 Semiconductors

The EPR zero-field splitting D for Fe3+ ions in some Ib-IIIb-VI2 semiconductors is calculated with the superposition model. The calculated D values, when using the local rotation angles τ (Fe3+) for Fe3+ in CuGaS2 and AgGaS2 crystals, are consistent with the observed values, whereas for Fe3+ in CuAlS2 crystal they are not. The calculated results are discussed. The local lattice distortions except the local rotation angles τ for Fe3+ in CuAlS2 are suggested.

Study of EPR zero-field splitting of iron–sulfur clusters in tetrahedral ZnS:Fe III system

The EPR zero-field splitting of the iron–sulfur cluster in the ZnS:Fe III system has been studied on the basis of the energy matrix for the electron–electron repulsion, the ligand–field and the spin–orbit coupling of a d 5 configuration ion with a trigonal symmetry. It is demonstrated that in order to reasonably explain the EPR zero-field splitting parameters a, D and ( a− F), a positive ligand–field strength Dq for a d 5 electron configuration ion in a tetrahedron has to be employed. This means that the effective charge of each sulfur ion in ZnS is positive. This unusual result leads to the conclusion that in ZnS each sulfur ion may occur a quadricovalent argon state S 2+ with sp 3 hybrid configuration and this conclusion is in accord with Pauling's speculation.

Formation, Structures, and Magnetic and EPR Spectroscopic Properties of Dicobalt(III)-Dicopper(II) Complexes Featuring Heterotetranuclear Cations of a Puckered Cyclic Structure with Diethanolamine and Diethanolamine(2−) as Bridging Ligands

The compounds [CuII2CoIII2(H2dea)2(dea)4]X2(Solv)n [X = Cl (1), Br (2), SCN (3), O2CMe (4), I (5); Solv = H2O or/and CH3OH, DMF, n = 1−4] were synthesised by the reaction of zero-valent copper with cobalt(II) salts in non-aqueous (CH3OH, DMF, DMSO) solutions of diethanolamine (H2dea) in air. Crystallographic investigations of 1−4 reveal that the complexes contain the centrosymmetric cation [Cu2Co2(H2dea)2(dea)4]2+ in which the four metal atoms are linked together by bridging oxygen atoms of the six ligand groups to form a parallelogram with the length of the short edge (Cu···Co) being ca. 2.83 A and a short diagonal distance (Cu···Cu) at approximately 3.23−3.29 A. The copper atom is five-coordinate with a square-pyramidal geometry; the geometry at the trivalent cobalt ion is almost octahedral. The tetranuclear units are further stabilized by intramolecular O−H···O and N−H···O hydrogen bonds. The hydrogen-bonded network that involves the OH and NH groups of the ligands, uncoordinated anions, and solvent molecules links the tetranuclear units together and results in the formation of extended two- and three-dimensional networks. Variable-temperature magnetic susceptibility measurements of 1−3 and 5 show no significant exchange coupling between the copper centres. The frozen-solution and solid-state EPR spectra at 77 K are characteristic of a triplet state with the hyperfine structure distinctly resolved in solution. On the assumption of noncoaxiality of the g and D tensors the spectra were interpreted with the zero-field-splitting parameters D = 0.0560(1) (2−5) and D = 0.0712(1), E= 0.0028(2) cm−1 (1). Dominant contributions to the observed EPR zero-field splitting are presumed to be dipole-dipole in origin. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Studies of the zero-field splitting and g-factor for the defect model of Cr 3+ in α-LiIO 3 crystals at low temperature

The EPR zero-field splitting parameters D and g-factors for Cr 3+ in α-LiIO 3 single crystal, taking into account both the effect of lattice distortion and two Li + vacancies, have been investigated using a complete diagonalization method (CDM) for 3d 3 ions in a trigonal symmetry crystal field. The theoretical results ( D=−0.60876 cm −1, g ∥=1.9641, g ⊥=1.9682) are in excellent agreement with experimental results ( D=−0.6099(3) cm −1 g ∥=1.965±0.001, g ⊥=1.971±0.002). In addition, Macfarlane's perturbation expressions lead to results almost identical with the CDM for Cr 3+ in an α-LiIO 3 single crystal.

An investigation of the EPR zero-field splitting of Cr3+ ions atthe tetragonal site and the Cd2+ vacancy in RbCdF3:Cr3+crystals

The quantificational relationship between the EPR zero-field splittingparameter D and the structure parameters of the tetragonal Cr3+centre in RbCdF3:Cr3+ crystals has been establishedaccording to the superposition model and third-order perturbation theory. Theexistence of the Cd2+ vacancy and the lattice distortion have beenverified. Meanwhile, we obtain that the F- ion moves toward thecentral Cr3+ ion by X1 = 0.0031 nm, X2 = 0.001 01 nm,X3 = 0.002 81 nm. Good agreement between the theoretical resultsand the experimental values shows that the assumption of the Cd2+vacancy and the lattice distortion is reasonable. Although the main source ofthe tetragonal crystal field comes from the Cd2+ vacancy caused bythe charge compensation, the contribution of the lattice distortion cannot beneglected.

Studies of the substitutional site of Co 2+ in AgGaS 2 crystal from EPR data

The EPR zero-field splitting D and g factors g ∥, Δ g (= g ∥− g ⊤) for Co 2+ ions at Ag + and Ga 3+ sites of surphur-annealed AgGaS 2 crystal were calculated from the high-order perturbation formulas based on the two spin–orbit coupling parameter model. The calculated results show that Co 2+ replace Ag + site in the AgGaS 2 crystal. This point is contrary to that obtained from the absorption spectrum and Raman spectroscopy studies for Co 2+ in as-grown AgGaS 2 crystal. It appears that Co 2+ can replace Ag + or Ga 3+ site in AgGaS 2 ternary semiconductor, depending on the technology of material preparation.