Studies of local phase transition behavior for Co2+ in CsCaCl3 crystals from EPR data

In this paper, we establish the calculation formulas ofg factorsg _∥,g _⊥ and the hyperfine structure constantsA _∥,A _⊥ for 3d⁷ ions in tetragonal octahedral field from a cluster approach. In these formulas, besides the configuration interaction, the covalency effect due to the admixture between d electrons of 3d⁷ ion and the p electrons of ligands is considered. The parameters used in these formulas (except the core polarization constantsk in the calculation ofA _i) can be obtained from optical spectra of the studied crystal. From these formulas, the local release (or elongation) factork, which is introduced to characterize the release effect of impurity-ligand bond along C₄ axis at the cubic to tetragonal phase transition, is estimated for CsCaCl₃:Co²⁺ crystal by calculating the electronic paramagnetic resonance (EPR) parametersg _i andA _i . These EPR parameters are therefore explained reasonably and the results are discussed.     

EPR investigations of Cu(II) complexes with 5′-CMP and 5′-GMP

Electron paramagnetic resonance investigation of the Cu(II) complexes with guanosine-5′-monophosphate (5′-GMP) and cytidine-5′-monophosphate (5′-CMP) shows the affinity of the Cu(II) ion to interact with the base and the phosphate group. The different modes of the coordination of the metal ion at the nucleotide and the water molecules lead to octahedral species, distorted by dynamical Jahn-Teller effect (g₀ = 2.106) for the Cu(II)-5′-CMP complex and rhombically distorted (g₁ = 2.358, g₂ = 2.126, g₃ = 2.068) or tetragonally distorted (g_∥^′ = 2.299, g_⊥^′ = 2.126) for the Cu(II)-5′ -GMP complex. The compound with 5′-CMP presents also a more stable in time square-planar species (g_∥ = 2.265, A_∥ = 162 G, g_⊥ = 2.076). The local symmetry changes in aqueous solution by coordination of water molecules.     

Coexistence of one-electron- and monoprotonated two-electron-reduced species of the K5[PMo2VW9O40] · 24H2O heteropolyoxometalate identified by EPR

The simulation of the room-temperature experimental electron paramagnetic resonance spectrum of K₅[PMo₂VW₉O₄₀] · 24 H₂O heteropolyoxometalate indicates the presence of equal amounts of a one-electron-reduced species (g _∥ = 1.922,g _⊥ = 1.972,A _∥ = 181 G,A _⊥ = 63 G) and a monoprotonated two-electron-reduced species with mixed-valence V^IV, Mo^V and Mo^VI ions (g _iso = 1.972, ΔB _iso(p-p) = 450 G). Two unprotonated one-electron-reduced isomers are identified in dimethylsulfoxide-H₂O solution of the sample atT = 100 K (g _∥¹ = 1.929,g _⊥¹ = 1.990,A _∥¹ = 179 G,A _⊥¹ = 65 G andg _∥² = 1.918,g _⊥² = 2.000,A _∥² = 187 G,A _⊥² = 80 G, respectively). The values of the in-plane π(V−O) bond π₂² coefficient for the one-electron-reduced species (0.87 at room temperature and 0.83 and 0.74 for the species in frozen solution) suggests the delocalization of the vanadium unpaired electron towards the molybdenum ions via O_b atoms.     

Optical spectra and spin-Hamiltonian parameters of trivalent ytterbium in lead tungstate

By using crystal-field theory, the optical spectra and spin-Hamiltonian parameters (abbr. SH parameters, i.e. the anisotropic g factors g _∥, g _⊥, and hyperfine structure constants A _∥, A _⊥) of ¹⁷¹Yb^3 + and ¹⁷³Yb^3 + isotopes in the tetragonal PbWO₄ are calculated. The theoretical results agree well with the experimental values. The crystal-field parameters and the signs of the hyperfine structure constants for both ¹⁷¹Yb^3 + and ¹⁷³Yb^3 + isotopes are determined. The validities of the theoretical results are discussed.     

ESR of V4+ in α-RbTiOPO4 single crystals

We have recorded and investigated the ESR spectrum of vanadium-doped α-RbTiOPO₄ single crystals in the temperature interval 77–300 K. Two types of structurally distinct centers, V1 and V2, with a 4:1 ratio of the peak intensities were observed. The angular dependences of the resonance magnetic fields are described by a spin Hamiltonian corresponding to axial symmetry with the parameters g _∥1=1.9305, g _⊥1=1.9565, A _∥1=−168.2×10⁻⁴cm⁻¹, and A _⊥1=−54.3×10⁻⁴cm⁻¹ for V1 centers and g _∥2=1.9340, g _⊥2=1.9523, A _∥2=−169.0×10⁻⁴cm⁻¹, and A _⊥2=−55.2×10⁻⁴cm⁻¹ for V2 centers. A model of a paramagnetic center is proposed: The vanadium ions replace titanium ions in two structurally distinct positions Ti1 and Ti2 (V1 and V2 centers, respectively). The possibility that a VO²⁺ ion forms when α-RbTiOPO₄ crystals and crystals of the KTP group (KTiOPO₄, NaTiOPO₄, α-and β-LiTiOPO₄), studied earlier, are doped with vanadium is discussed. Fiz. Tverd. Tela (St. Petersburg) 40, 534–536 (March 1998)     

Strontium Tetraborate Glasses Doped with Transition Metal Ions: EPR and Optical Absorption Study

Electron paramagnetic resonance (EPR) and optical studies have been carried out on Cu(II)-, VO(II)- and Cr(III)-doped strontium tetraborate glasses to understand the distortion and substitution of these ions. The EPR results of Cu(II) glass indicate that g _∥ > g _⊥, typical for the tetragonally elongated octahedral site of the Cu(II) impurity. The evaluated covalency parameter 0.788 suggests a moderate covalency for the bonding. By correlating EPR and optical results, the in-plane π-bonding β₁ ² is evaluated as 0.715. In the vanadium-doped glasses, the distortion must be a tetragonally elongated octahedron, similar to Cu(II). However, the EPR studies show that g _⊥ > g _∥ indicating the tetragonally compressed octahedral site for the ion. The site symmetry is C _4V. Supported by the optical absorption, evaluated parameters propose a moderate covalency. The EPR and optical results for Cr(III) glass indicate the distorted octahedral site symmetry in the host lattice. These results further suggest that the bonding between Cr(III) and the ligands is covalent. Authors' address: Renduchintala V. S. S. N. Ravikumar, Department of Physics, Acharya Nagarjuna University, Nagarjuna Nagar 522510, India     

Superhyperfine structure of the EPR spectra of Nd<Superscript>3+</Superscript> and U<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 superhyperfine structure of the EPR spectra of impurity Nd³⁺ and U³⁺ ions in LiYF₄, LiLuF₄, and LiTmF₄ double-fluoride single crystals has been observed and discussed. In LiYF₄: Nd (g _| = 1.987, g _⊥ = 2.554) and LiTmF₄: Nd, the superhyperfine structure is observed at the orientation of the external magnetic field B in parallel to the c axis of the crystals and consists of nine components with a splitting of ∼15.4 MHz. In LiYF₄: U (g _| = 1.149, g _⊥ = 2.508) and LiLuF₄: U, the superhyperfine structure is observed at both B | c and B ⊥ c and consists of nine and eleven components, respectively, with a splitting of ∼21.5MHz. It should be noted that the resolution of the superhyperfine structure of the EPR spectrum of LiLuF₄: U³⁺ becomes significantly higher with a deviation from the orientation B ⊥ c.     

Polymorphism of biperiodic stripe domain structures

Magnetic-field-induced polymorphism of biperiodic stripe domain structures of films with a positive anisotropy constant and easy axis parallel to the normal to the surface is studied. A symmetry classification of the observed types of domain structures is performed, and the phase diagram of the films in the (H _⊥, H _∥) plane, where H _⊥ and H _∥ are, respectively, the components of the magnetic field vector perpendicular and parallel to the normal to the surface, is determined. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 9, 701–706 (10 May 1998)     

Phase transitions in biperiodic stripe domain structures of uniaxial magnetic films with a positive anisotropy constant

New phase transitions induced by a magnetic field and accompanied by a change in the symmetry or the period of the distribution of the magnetization vector are observed in biperiodic stripe domain structures of iron garnet films with a positive anisotropy constant. A symmetry classification of the observed types of domain structures is derived, and the form of the state diagram of the films is determined in the H∥H⊥ plane, where H∥ and H⊥ are the components of the magnetic field vector perpendicular and parallel to the normal to the surface. Zh. éksp. Teor. Fiz. 114, 2089–2110 (December 1998)     

Covalent binding parameters and the ground state wavefunctions of vanadyl ion complexes

The Steven’s method of molecular orbitals for octahedral complexes containing transition metal ions has been used for estimating the binding parameters and interpreting theg factors of VO ₂ ⁺ ion in single crystals. The expressions forg factors have been given in terms ofK _‖ andK _⊥ taking into account the tetragonal crystalline field and covalent binding effects. Computations show thatK _⊥ should be less than 0.066 in order to fit the experimentalg values. The ground state wave function (GSWF) of VO ₂ ⁺ ion doped in different single crystals has been estimated using crystal field theory. The GSWF is found to be ind _xy state with slight admixture of the excited states ,d _xz andd _yz. The hyperfine interaction parameterP and Fermi contact termX have also been estimated.     

EPR study of the Jahn-Teller effect of Cu2+ ions in ZnGa2O4

The Jahn-Teller effect in the ZnGa₂O₄ spinel single crystal has been investigated using electron paramagnetic resonance of Cu²⁺ ions in the temperature range 110–560 K. It has been shown that copper ions occupy octahedral sites 16d in the ZnGa₂O₄ crystal with cubic symmetry O _h ⁷ (Fd-3m). At T < 560 K, the octahedra undergo tetragonal distortions (predominantly tension) and rotation around the fourfold axes by the angle θ ≈ 2.6°. The parameters of the spin Hamiltonian, which characterize the prolate (g _‖ = 2.355, g _⊥ = 2.077, A _‖ = 116 Oe, A _tp = 12 Oe) and oblate (g _‖ = 2.018, g _⊥ = 2.246, A _‖ = 75 Oe, A _⊥ = 44 Oe) octahedra, have been determined. At temperatures above 560 K, the static Jahn-Teller effect transforms into the dynamic effect and the spectrum of the magnetic resonance becomes isotropic with g = 2.116 (the experimental frequency corresponds to the X band).     

EPR of two Cu2+ complexes showing dynamic Jahn-Teller effects

The powder electron paramagnetic resonance spectra of ∼1% Cu doped into [Zn(bipy)₂(NO₂)]NO₃ and [Zn(bipyam)₂(NO₂)]NO₃, bipy = 2,2′-bipyridyl, bipyam = 2,2′-bipyridylamine, over the temperature range 4.2–300 K are reported. For [Zn/Cu(bipy)₂(NO₂)]NO₃ a spectrum characteristic of a tetragonally elongated octahedral complex with a slight orthorhombic distortion is observed below ∼50 K, but above this temperature the signals associated with the two higherg-values gradually converge. At low temperature a spectrum similar to that of the bipy complex is observed for [Zn/Cu(bipyam)₂(NO₂)]NO₃, but between 40 and 60 K the spectrum gradually changes to one characteristic of a tetragonalg-tensor with g_∥ < g_⊥. The spectra may be interpreted satisfactorily with a model in which the temperature dependence of theg-vainlues is estimated from the vibronic wavefunctions of a copper(II) complex under the influence of second-order Jahn-Teller coupling and a ligand field of tetragonal symmetry perturbed by crystal lattice forces. The results are consistent with the crystal structures reported for the compounds at various temperatures.     

Electron paramagnetic resonance parameters of Mn<Superscript>4+</Superscript> ion in h-BaTiO<Subscript>3</Subscript> crystal from a two-mechanism model

The EPR parameters (g factors g _‖, g _⊥ and zero-field splitting D) of Mn⁴⁺ ion in h-BaTiO₃ 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 Mn⁴⁺ centre in h-BaTiO₃ crystal obtained from the calculations are discussed.      

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     

Identification of the La6F37 cubooctahedral clusters in mixed crystals (BaF2)1 ? x(LaF3)x by the electron paramagnetic resonance method

The electron paramagnetic resonance (EPR) spectra of mixed crystals (BaF₂)_{1 − x} (LaF₃) _x (x = 0, 0.001, 0.002, 0.005, 0.010, 0.020) doped with Ce³⁺ ions (0.1%) are investigated at a frequency v ≈ 9.5 GHz in magnetic fields up to 1.45 T at temperatures T = 10 and 15 K. The EPR spectrum of “pure” barium fluoride BaF₂ (x = 0) is characterized by a single Ce³⁺-F⁻ center with tetragonal symmetry (i.e., the O center with g _‖ = 2.601 and g _⊥ = 1.555). For a lanthanum trifluoride concentration x ≠ 0, the spectrum exhibits new lines due to the presence of the clusters containing Ce³⁺ and La³⁺ ions. The intensity of EPR signals from the O centers decreases rapidly as the lanthanum trifluoride concentration x increases. The lines attributed to a paramagnetic center with tetragonal symmetry and strongly anisotropic g factors (i.e., the K center with g _‖ = 0.725 and g _⊥ = 2.52) are separated in the complex EPR spectrum with the use of the angular dependence of the EPR signal intensity measured for the samples with x ≥ 0.002. This center is identified as a cubooctahedral cluster of the La₆F₃₇ type in which one of the La³⁺ ions is replaced by the Ce³⁺ ion. Original Russian Text ? L.K. Aminov, I.N. Kurkin, S.P. Kurzin, I.A. Gromov, G.V. Mamin, R.M. Rakhmatullin, 2007, published in Fizika Tverdogo Tela, 2007, Vol. 49, No. 11, pp. 1990–1993.     

EPR of trivalent iron ions in a LiCaAlF6 crystal

The orientational dependences of the EPR spectra of Fe³⁺-doped LiCaAlF₆ single crystals (space group P31c, Z=2), grown at the Laboratory of Magnetic Radio Spectroscopy at Kazan’ State University, have been investigated in detail. The spectrum is described by a trigonal spin Hamiltonian with the following parameters: B ₂₀=40.072×10⁻⁴ cm⁻¹, B ₄₀=−5.799×10⁻⁴ cm⁻¹, B ₄₃=−4.281×10⁻⁴ cm⁻¹, A _s=24.33±1, A _p=6.13±1, g _∥=g _⊥=2.00217±0.0003. A theoretical calculation of the hyperfine structure parameters shows that they are described quite well when allowance is made for the overlapping of the wave functions of the paramagnetic center and the ligands (F⁻). Fiz. Tverd. Tela (St. Petersburg) 39, 488–490 (March 1997)     

Theoretical studies of the spin-Hamiltonian parameters for the orthorhombic Pr4+ centers in Sr2CeO4 crystals

Theoretical studies of spin-Hamiltonian (SH) parameters associated with Pr⁴⁺ in Sr₂CeO₄ single crystals have been made by using the complete diagonalizing energy matrix method (CDM) for the 4f ¹ electronic configuration. The calculated results are in excellent agreement with the experimental data. The negative signs of the anisotropic g _i -factors and hyperfine structure constants A _i (where i = ∥ or ⊥) for the orthorhombic Pr⁴⁺ ion in Sr₂CeO₄ are suggested from the calculations. By comparing the results obtained by the CDM with the experimental data, one finds it is valid to interpret the SH parameters for 4f ¹ ions in crystals. The results are discussed.      

Study of EPR parameters and defect structure for two tetragonal impurity centers in MgO:Cr<Superscript>3+</Superscript> and MgO:Mn<Superscript>4+</Superscript> crystals

The electron paramagnetic resonance (EPR) parameters (g-factors g _‖, g _⊥ and zero-field splitting D) of two tetragonal 3d³ impurity centers M_3d-V_Mg and M_3d-Li⁺ (where M_3d = Cr³⁺ or Mn⁴⁺, V_Mg is the Mg²⁺ vacancy) in M_3d-doped MgO crystals are calculated from the high-order perturbation formulas including both the crystal-field (CF) and the charge-transfer (CT) mechanisms for 3d³ ions in the tetragonal symmetry. The calculated results are in reasonable agreement with the experimental values. From the calculations, it can be found that the relative importance of the CT mechanism for EPR parameters increases with increasing valence state of the 3d³ ion. So, for the high-valence 3d ⁿ ions in crystals, a reasonable explanation of EPR parameters should take into account both CF and CT mechanisms. The defect structures (characterized by the displacement ΔR of O²⁻ in the intervening M_3d and V_Mg or Li⁺ at the Mg²⁺ site) for these tetragonal impurity centers are obtained from the calculations. The results are consistent with the expectations based on the electrostatic interactions.     

Magnetic and EPR studies of the EuFe<Subscript>3</Subscript>(BO<Subscript>3</Subscript>)<Subscript>4</Subscript> single crystal

Magnetic and electron paramagnetic resonance (EPR) properties of EuFe₃(BO₃)₄ single crystals have been studied over the temperature range of 300–4.2 K and in a magnetic field up to 5 T. The temperature, field and orientation dependences of susceptibility, magnetization and EPR spectra are presented. An antiferromagnetic ordering of the Fe subsystem occurs at about 37 K. The easy direction of magnetization perpendicular to the c axis is determined by magnetic measurements. Below 10 K, we observe an increase of susceptibility connected with the polarization of the Eu sublattice by an effective exchange field of the ordered Fe magnetic subsystem. In a magnetic field perpendicular to the c axis, we have observed an increase of magnetization at T < 10 K in the applied magnetic field, which can be attributed to the appearance of the magnetic moment induced by the magnetic field applied in the basal plane. According to EPR measurements, the distance between the maximum and minimum of derivative of absorption line of the Lorentz type is equal to 319 Gs. The anisotropy of g-factor and linewidth is due to the influence of crystalline field of trigonal symmetry. The peculiarities of temperature dependence of both intensity and linewidth are caused by the influence of excited states of europium ion (Eu³⁺). It is supposed that the difference between the g-factors from EPR and the magnetic measurements is caused by exchange interaction between rare earth and Fe subsystems via anomalous Zeeman effect.     

Field-induced phase transitions in magnetooptical films with a small positive anisotropy constant

We study phase transitions induced by a static magnetic field in magnetically uniaxial films with a small positive anisotropy constant. The phase diagram of these objects is determined in the H _∥-H _⊥ plane, where H _∥ and H _⊥ are, respectively, the components of the magnetizing field along and perpendicular to the surface normal. The stability boundary is located for all of the main types of domain configurations observed: a simple stripe domain structure, a stripe domain structure with periodic bending by surface distortions in the profile of the domain walls, and hexagonal lattices of cylindrical magnetic bubbles. Zh. éksp. Teor. Fiz. 111, 283–297 (January 1997)