Superhyperfine Structure of EPR Spectra in LiLuF<Subscript>4</Subscript>:U<Superscript>3+</Superscript> and LiYF<Subscript>4</Subscript>:Yb<Superscript>3+</Superscript> Single Crystals

Electron paramagnetic resonance (EPR) spectra of doped paramagnetic crystals LiLuF₄:U³⁺ and LiYF₄:Yb³⁺ have been investigated at a frequency of about 9.42 GHz in the temperature range of 10–20 K. The U³⁺ ion spectrum is characterized by g-factors g _∥ = 1.228 and g _⊥ = 2.516, and contains the hyperfine structure due to the ²³⁵U isotope with nuclear spin I = 7/2 and natural abundance of 0.71%. The observed hyperfine interaction constants are A _∥ = 81 G and A _⊥ = 83.8 G. Moreover, the spectrum reveals the well-resolved superhyperfine structure (SHFS) due to two groups of four fluorine ions forming the nearest surrounding of the U³⁺ ion. This SHFS contains up to nine components with the spacing between components being about 12.7 G. The SHFS is observed also in the EPR spectrum of the LiYF₄:Yb³⁺ crystal; up to 17 components with spacing of about 3.7 G may be traced. Some parameters of the effective Hamiltonian of the SHF interaction are estimated, the contribution of covalent bonding of f-electrons with ligands into these parameters is discussed. Authors' address: Igor N. Kurkin, Kazan State University, Kremlevskaya ulitsa 18, Kazan 420008, Russian Federation     

On the <Emphasis Type="Italic">g</Emphasis>-factor value of canted antiferromagnet CoCO<Subscript>3</Subscript>

Experimental data on the magnetization of canted antiferromagnet CoCO₃ (T_N = 18.1 K) in the paramagnetic region are described by the isotropic g factor g_‖ = g_⊥ = 6.5 that differs from the anisotropic values g_‖ = 3.05 and g_⊥ = 4.95 obtained in electron paramagnetic resonance (EPR) measurements at T = 4.2 K on Co²⁺ ions in magnetically diluted crystals. The g-factor values calculated in the Abragam-Pryce and Weiss molecular field approximations using the magnetization data in the magnetic ordered region correspond to data obtained in EPR measurements. It is shown that the absence of the anisotropy of the g factor at high temperatures cannot be explained in the approximations used. Causes of the observed discrepancies are discussed.     

Theoretical study of the electron paramagnetic resonance parameters and local structure for the tetragonal Ir<Superscript>2+</Superscript> centre in NaCl

The electron paramagnetic resonance (EPR) parameters (the g factors, hyperfine structure constants and the superhyperfine parameters) for the tetragonal Ir²⁺ centre in NaCl are theoretically investigated from the perturbation formulas of these parameters for a 5d⁷ ion in tetragonally elongated octahedra. This impurity centre is attributed to the substitutional [IrCl₆]⁴⁻ cluster on host Na⁺ site, associated with the 4% relative elongation along the C ₄-axis due to the Jahn-Teller effect. Despite the ionicity of host NaCl, the [IrCl₆]⁴⁻ cluster still exhibits moderate covalency and then the ligand orbital and spin-orbit coupling contributions should be taken into account. In addition, the theoretical EPR parameters based on the Jahn-Teller elongation show good agreement with the observed values.     

Librational dynamics of nitroxide molecules in a molecular glass studied by echo-detected EPR

Nitroxides 2,2,6,6-tetramethyl-4-piperidone N-oxide (tempone), 3-carboxy-proxyl and potassium peroxylamine disulfonate (Fremy salt) in glycerol solution were studied in a wide temperature range near the glass transition temperatureT _g. The echo-detected (ED) electron paramagnetic resonance (EPR) lineshape reveals strong dependence on the time interval τ between the echo-forming microwave pulses which is readily explained by anisotropic phase relaxation. Employing a librational model of molecular motion and the Redfield relaxation theory, spectra were simulated for the τ’s varying in a large interval. The anisotropic relaxation rate increases with temperature increase and it is larger for nitroxide with a larger molecular size. The mean-squared amplitude of motion, obtained from reduced hyperfine splitting in continuous-wave EPR, near T_g linearly depends on temperature which is characteristic of harmonic solids. For tempone in a host crystal 2,2,4,4-tetramethyl-cyclobutan-1, 3-dione the anisotropic spin relaxation rate decreases with temperature increase so the found feature solely belongs to a glassy state. A new approach is proposed for modeling slow wobbling motion in a restricted angular space.     

Investigations of EPR parameters and impurity structure for Co<Superscript>2+</Superscript> in Ca(OH)<Subscript>2</Subscript> crystal

The electron paramagnetic resonance (EPR) parameters (g _‖ andg _⊥ factors and hyperfine structure constantsA _‖,A _⊥) for Co²⁺ in Ca(OH)₂ are studied from the second-order perturbation formulas on the basis of the cluster approach. In these formulas, the contributions to EPR parameters from the state interactions and covalency effects are considered and the parameters related to both effects are obtained from the optical spectra and impurity structure of the studied system. From the study, it is found that the β angle between the metal-ligand bond and the C₃ axis changes from 61° in a pure crystal to 53.68(26)° in the impurity center of a Co²⁺-doped Ca(OH)₂ crystal because of the impurity-induced local lattice relaxation. The reduction of the angle β in the impurity center is also supported by the result obtained by analyzing the EPR zero-field splitting for Mn²⁺ in the same Ca(OH)₂ crystal. The EPR parameters of Ca(OH)₂:Co²⁺ are also reasonably explained by considering the suitable local lattice relaxation.     

EPR and Optical Study of Cu<Superscript>2+</Superscript> Ions Doped in Sodium Zinc Sulfate Tetrahydrate Single Crystals

Electron paramagnetic resonance (EPR) study of Cu²⁺-doped sodium zinc sulfate tetrahydrate is done at liquid nitrogen temperature. Two magnetically equivalent sites for Cu²⁺ are observed. The spin-Hamiltonian parameters determined by fitting the EPR spectra to the rhombic-symmetry crystalline field are g _x  = 2.2356, g _y  = 2.0267, g _z  = 2.3472, A _x  = 27 × 10⁻⁴ cm⁻¹, A _y  = 54 × 10⁻⁴ cm⁻¹and A _z  = 88 × 10⁻⁴ cm⁻¹. The ground state wave function is also determined. The g-anisotropy is evaluated and compared with the experimental value. With the help of optical study, the nature of bonding in the complex is discussed.     

High-frequency EPR of Tb<Superscript>3+</Superscript>-doped KPb<Subscript>2</Subscript>Cl<Subscript>5</Subscript> crystal

High-frequency electron paramagnetic resonance (EPR) spectra of the KPb₂Cl₅:Tb³⁺ crystal have been investigated. Three types of spectra were observed in the frequency range of 74–200 GHz. The most intensive spectrum with the resolved hyperfine structure corresponded to transitions between sublevels of the¹⁵⁹Tb³⁺ ground quasi-doublet with the zero-field splitting (ZFS) close to 48 GHz. Experimental results were analyzed by the exchange charge model of the crystal field affecting terbium ions in low-symmetry Pb²⁺ positions with the chlorine sevenfold coordination and the charge compensating vacancy in the nearest potassium site. The calculated values ofg-factors and ZFS were in agreement with the experimental data. The nature of a broad EPR line with ZFS of about 180 GHz and of additional weak EPR lines observed as satellites of the main Tb³⁺ lines was discussed.     

Magnetic Properties of a New Er(III) Macrobicyclic Complex Studied by EPR

The macrobicyclic ligands of a Schiff base form complexes with various lanthanide metal cations, where the lanthanide ion can be coordinated in the mono- or binuclear systems. As a result of the Schiff-base condensation, a new erbium cryptate complex has been synthesized and investigated by electron paramagnetic resonance EPR technique. The measurements in the 3.9–300 K temperature range confirmed the presence of the 1:1 macrobicyclic Schiff base–Er³⁺ complex. Computer simulations of the registered EPR spectra have revealed the existence of low-symmetry crystal field at the Er(III) site. The complicated EPR spectra of Er(III) complex with an effective spin S = 1/2 in low magnetic fields (<200 mT) were observed only below 60 K. At 5 K the anisotropic g-factors and the integrated intensity reached local maximum values. In the 5–13 K temperature range the line width and g-factors remained constant, while the integrated intensity strongly decreased with increasing temperature. Above 13 K a strong increase of g _y -factor and ΔB _y line width was observed. The observed peak in integrated intensity indicates that the EPR signal is produced by the excited state that lies 6.3 K above the nonmagnetic ground state. Authors' address: Sławomir M. Kaczmarek, Institute of Physics, Szczecin University of Technology, Al. Piastów 48, 70-310 Szczecin, Poland     

Superhyperfine structure of the EPR spectra of Ce<Superscript>3+</Superscript> ions in Li<Emphasis Type="Italic">R</Emphasis>F<Subscript>4</Subscript> (<Emphasis Type="Italic">R</Emphasis> = Y,Lu, Tm) double fluorides

The EPR spectra of Ce³⁺ impurity ions in LiYF₄, LiLuF₄, and LiTmF₄ double-fluoride single crystals have been investigated at a frequency of ∼9.3 GHz in the temperature range 5–25 K. The effective g factors of the ground Kramers doublet of the cerium ions in three crystals are close to each other (g _‖ = 2.737, g _⊥ = 1.475 for LiYF₄:Ce³⁺). A superhyperfine structure of the EPR spectrum of Ce³⁺ ions in the LiTmF₄ Van Vleck paramagnet has been observed in the external magnetic field B oriented along the crystallographic axis c (B ‖ c). The superhyperfine structure of the EPR soectra of the Ce³⁺ ions in the LiYF₄ and LiLuF₄ diamagnetic matrices is resolved for B ⊥ c. Possible factors responsible for this pronounced difference in the properties of the systems studied have been discussed.     

Magnetic Properties of Monomer and Dimer Tetrahedral VO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> Entities Dispersed on Amorphous Silica-based Materials: Prediction of EPR Parameters from Relativistic DFT Calculations and Broken Symmetry Approach to Exchange Couplings

Molecular structures of the isolated tetrahedral oxovanadium(IV) and bridged μ-oxo-divanadium(IV) complexes hosted by the clusters mimicking surfaces of amorphous silica-based materials were investigated using density functional theory (DFT) calculations. Principal values of the g and A tensors for the monomer vanadyl species were obtained using the coupled-perturbed DFT level of theory and the spin–orbit mean-field approximation (SOMF). Magnetic exchange interaction for the μ-oxo bridged vanadium(IV) dimer was investigated within the broken symmetry approach. An antiferromagnetic coupling of the individual magnetic moments of the vanadium(IV) centers in the [VO–O–VO]²⁺ bridges was revealed and discussed in detail. The coupling explains pronounced decrease of the electron paramagnetic resonance signal (EPR) intensity, observed for the reduced VO _x /SiO₂ samples with the increasing coverage of vanadia, in terms of transformation of the paramagnetic monomer species into the dimers with S = 0 ground state.     

Hyperfine structure of <Emphasis Type="Italic">S</Emphasis>- and <Emphasis Type="Italic">P</Emphasis>-wave states in muonic-helium ion

Corrections of order α ⁵ and α ⁶ to the hyperfine structure of S- and P-wave energy levels of the muonic-helium ion are calculated. Electron-vacuum-polarization effects, corrections for the nuclear structure, and recoil effects are taken into account. The numerical values obtained for respective hyperfine splitting, −1334.73 meV (1S), −166.64 meV (2S), −58 712.90 μeV (2P _1/2), and −24 290.69 μeV (2P _3/2), can be viewed as a reliable estimate for a comparison with experimental data, and the hyperfine-structure interval of Δ₁₂ = 8ΔE ^hfs(2S) − ΔE ^hfs(1S) = 1.59 meV can be used to test QED predictions.     

A New EPR Data on the MA1 and NIRIM8 (NP1) Phosphorus-Related Centers in Synthetic 1b Diamonds

Microdiamonds grown under high-pressure and high-temperature conditions from a P–C medium at different temperatures have been studied by electron paramagnetic resonance (EPR). Two paramagnetic centers P1 and MA1 were observed in microdiamonds grown at 1,873 K. Analysis of weak lines around these centers suggests that they may be due to ¹³C hyperfine structure (HFS) of the MA1 center. The calculated s/p hybridization parameter for this carbon atom (C₁) was similar to that for the undistorted lattice. At a growth temperature of 1,973 K, in addition to MA1 and P1, a new center, labeled NP1, with HFS from nitrogen and phosphorus atoms and with EPR parameters similar to NIRIM 8 was identified. The NP1 (NIRIM8) centers have an electron spin S = 1/2. We propose that nitrogen–phosphorus defects are created through subsequent migration of nitrogen atoms towards phosphorus atoms upon increasing of the growth.     

Anisotropic hyperfine structure in the EPR spectrum of impurity ions in single crystals

Expressions are obtained for the hyperfine splitting in EPR spectra of impurity ions with electron spin 1/2 and arbitrary nuclear spin for arbitrary anisotropy of the g-factor and hyperfine structure constants. These results generalize the previously obtained results of Weil for the case of weakly anisotropic g-factors and hyperfine structure constants. Fiz. Tverd. Tela (St. Petersburg) 41, 1026–1027 (June 1999)     

Theoretical investigations of the EPR parameters and local structure for Cu2+ in BeO

The electron paramagnetic resonance (EPR) parameters (the anisotropic g factors, the hyperfine structure parameters and the quadrupole coupling constant Q) and local structure for Cu²⁺ in BeO are theoretically investigated from the perturbation formulas of these parameters for a 3d⁹ ion under trigonally distorted tetrahedra. The ligand orbital and spin-orbit coupling contributions are included in the basis of the cluster approach, in view of the strong covalency of the [CuO₄]^6? cluster. From the calculations, the impurity Cu²⁺ is suggested not to occupy exactly the ideal Be²⁺ site but to suffer a slight inward displacement (≈0.024 Å) toward the ligand triangle along the C₃ axis. The theoretical EPR parameters show good agreement with the experimental data.     

A program for simulation of powder-type EPR spectra

An algorithm of a computer program for simulation of powder type EPR spectra (S=1/2,I≠0) is described. For determination of the energy levels, the perturbation approach is used and corrections up to second order due to hyperfine, quadrupole, and nuclear Zeeman coupling are taken into account. No restrictions on the mutual orientations of the eigenvectors ofg, hyperfine, and quadrupole tensors are imposed. Simulated spectra for a model paramagnetic system (S=1/2,I=3/2) with axial symmetry and noncoinciding directions ofg _‖ andA _‖ are presented.     

Theoretical explanation of electron paramagnetic resonance and optical parameters for Cu<Superscript>2+</Superscript> ion in LiNbO<Subscript>3</Subscript> crystal

The EPR parameters, anisotropic g-factors g _x , g _y and g _z for cu²⁺ ion and hyperfine structure constants A _x , A _y and A _z for Cu²⁺ in LiNbO₃ crystal are calculated by the method of diagonalizing the full Hamiltonian matrix. The crystal-field parameters contact with the crystal structure by the aid of the superposition model. The optical transition parameters are calculated using Zhao crystal-field model. The calculated results are in good agreement with the observed values. The results are discussed.      

EPR studies of Cu2+ and V4+ ions in phosphate glasses

Two lead-phosphate glass systems doped with both copper and vanadium ions in different ratios were studied by EPR (electron paramagnetic resonance) method. EPR spectra and parameters (g_‖ = 2.44, g_⊥ = 2.08 andA _‖ = 117.6 · 10⁻⁴ cm⁻¹) obtained for x(CuO · V₂O₅)(l−x)[2P₂O₅ · PbO] glasses withx ≤ 10 mol% suggest a tetrahedral (Td) coordination of Cu²⁺ ions and not a tetragonally elongated octahedron as has been assumed in previous works. The ground state of the paramagnetic electron is thed _xy copper orbital with a 4p_z contribution of 6%. For 20 ≤x ≤ 40 mol% a broad line (ΔB = 307 G) characteristic for clustered ions appears atg = 2.18. The V⁴⁺ ions are evidenced only in the spectra of x(CuO · 2V₂O₅)(1 −x)[2P₂O₅ · PbO] glasses and the resonance parameters suggest a pentacoordinated C_4v local symmetry for these ions. The hyperfine structures characteristic for Cu²⁺ and V⁴⁺ ions disappear for 10 ≤x ≤ 40 mol% due to the mixed exchange Cu²⁺−V⁴⁺ pair formation in these glasses.     

Structural, spectroscopic characterization and EPR studies of tetramethylammonium-hydrogen fumarate-fumaric acid complex

The tetramethylammonium-hydrogen fumarate-fumaric acid (hereafter, [TMAHF-FA]) complex was synthesized and characterized by spectroscopic (IR), structural (XRD) and electron paramagnetic resonance (EPR) techniques. In the crystal structure, tetramethylammonium cation lies on a mirror plane of the space group P2₁/m. Crystal structure analysis reveals that there is a large degree of proton sharing between the fumaric acid and hydrogen fumarate, with the H atom lying almost symmetrically between the donor and acceptor sites, as evidenced by the long O-H and short H?O distances [1.19(3) Å, 1.26(3) Å], respectively. γ-Irradiation damage centers in [TMAHF-FA] single crystal have been investigated by EPR at room temperature. The spectra indicated the existence of radical. The EPR spectra recorded in the three mutually perpendicular planes have shown two magnetically distinct paramagnetic sites in monoclinic lattice. The principal values of g and hyperfine constants for both sites were calculated. IR spectrum was resolved and transitions were assigned based on the molecular structure.     

Anomalous hyperfine field and orbital moment of cobalt in RCo2; R = Pr,Nd, Sm,Gd, Tb,Dy, Ho,Er and Tm

⁵⁹Co spin echo NMR spectra in the magnetically ordered phase of the MgCu₂ type RCo₂ compounds (R = Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er and Tm) have been observed. For the RCo₂ with the easy direction of magnetication parallel to the 〈011〉 or 〈111〉 direction, the ⁵⁹Co hyperfine fields at two magnetically inequivalent Co sites are found to be antiparallel, revealing a large anisotropy in the ⁵⁹Co hyperfine field. The results are discussed in terms of a large and anisotropic orbital moment of Co. The transferred hyperfine field due to rare earth spins is estimated from well resolved satellite lines observed in Tb_1?xY_xCo₂. The nuclear quadrupole splitting in the magnetically ordered phase is found to be always larger than that in the paramagnetic phase.