EPR,FTIR, Powder XRD and Optical Absorption Studies of Cu(II) Ion in Triaqua(1,10-Phenanthroline-k<Superscript>2</Superscript><Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>′)(Sulfato-kO)Zinc(II)

Electron paramagnetic resonance (EPR), optical, Fourier-transform infrared (FTIR) and powder X-ray diffraction (XRD) studies have been carried out on Cu(II)-doped triaqua(1,10-phenanthroline-k² N,N′)(sulfato-kO)zinc(II) to get information about the position and spin Hamiltonian parameters of the dopant. Angular variation of copper hyperfine structure lines in EPR study shows the presence of a single site with g and A values as: g _xx  = 2.003, g _yy  = 2.001, g _zz  = 2.223; A _xx  = 4.41 mT, A _yy  = 2.39 mT and A _zz  = 12.16 mT. The slightly lower parallel value obtained for the copper nucleus has been explained by considering admixture of the ground state with the excited state. The direction cosines of principle g and A values suggest that the impurity is present interstitially. Admixture coefficients and additional parameters (P, k, α ², α′) have also been evaluated. Optical, FTIR and powder XRD techniques have been used to reconfirm the structure of the host lattice.     

Characteristics of ESEEM and HYSCORE spectra of S>1/2 centers in orientationally disordered systems

One- and two-dimensional electron-spin echo envelope modulation (ESEEM) spectra of Kramers’ multiplets in orientationally disordered systems are simulated using a simple mathematical model. A fairly general high-field spin Hamiltonian is considered with a general g-tensor and arbitrary relative orientations between all tensors involving the electron-spin S, the nuclear spin I, and their interaction. The zero field splitting (ZFS) and the nuclear quadrupole interactions are, however, approximated by their respective secular part in a way that retains all orientation dependencies and it is assumed that the nuclear quadrupole interaction is smaller than the hyperfine interaction. These approximations yield an effective sublevel nuclear Hamiltonian for each EPR transition and are sufficient to account for the most important characteristics of the ESEEM spectra of high electronic multiplets in orientationally disordered systems. Moreover, they allow to obtain some analytical expressions that for I=1/2 illuminate important aspects of 2D hyperfine sublevel correlation (HYSCORE) experiments in S=3/2, 5/2 systems. The pulses are considered as ideal and selective with respect to the different EPR transitions. The contributions of the latter to the echo intensity are weighed according to their different nutation angles and equilibrium Boltzmann populations. For simple axial cases with I=1/2, analytical expressions, analogous to the S=1/2 case, were derived for: (i) the modulation depth, (ii) the lineshapes of the HYSCORE cross-correlation ridges, and (iii) ENDOR powder pattern. Experimental results obtained from Mn(D₂O)₆²⁺ and VO(D₂O)₅²⁺ in frozen solutions are presented, compared, and analyzed in light of the theoretical part.     

Theoretical investigations of the spin Hamiltonian parameters for the CoCl(PPh3)3 molecules

The perturbation formulas of the spin Hamiltonian parameters (zero-field splitting and g factor g_// and g_⊥) are established for a 3d⁸ ion in trigonally distorted tetrahedra for the first time. In the theoretical treatments, the contributions from the Jahn–Teller effect, the ligand orbital and spin–orbit coupling interactions and configuration interactions are taken into account from the cluster approach in a uniform way. The above formulas are applied to the studies of the spin Hamiltonian parameters for the three CoCl(PPh₃)₃ molecules, and the experimental electron paramagnetic resonance spectra of all the molecules are satisfactorily explained. The significant compressions of the ligand tetrahedra with mixed chlorine and PPh₃ groups around Co⁺ are analysed for three distinct CoCl(PPh₃)₃ compounds, characterised by the Cl–Co–P bond angles θ larger than the tetrahedral angle of 109.47°. The local trigonal distortions are discussed in view of the Jahn–Teller effect.     

Observation of a Substitutional Location for VO(II) in Hexaaquazinc(diaquabismalonato)zincate in Different Crystals,in Addition to Interstitial: An Unusual Observation

Single-crystal electron paramagnetic resonance (EPR) studies of VO(II) doped in hexaaquazinc(diaquabismalonto)zincate have indicated interstitial location for vanadyl, which was a rare observation, considering the structure of the host lattice (K. Arun Prasath Lingam, S. Mithira and P. Sambasiva Rao, Appl. Magn. Reson. 38:295, 2010). However, substitutional location is noticed in a different crystal, in which interstitial resonances are almost absent. Generally, both types of resonances will be noticed in the same crystal. The spin Hamiltonian parameters calculated from the EPR spectra for substitutional location are: g _xx  = 1.981, g _yy  = 1.976, g _zz  = 1.941, A _xx  = 7.96 mT, A _yy  = 6.09 mT, and A _zz  = 17.83 mT. Crystal-field parameters, admixture and molecular orbital coefficients have been calculated from optical data, which reveal a moderately covalent metal–ligand bonding.     

A naturally induced hole center in the mineral hypersthene

In natural hypersthene a paramagnetic center in triclinic site symmetry was detected by electron paramagnetic resonance with the following spin. Hamiltonian parameters:g _xx=2.0104,g _yy=2.0256,g _zz=2.0090. This center is most likely formed as a hole on an oxygen of the SiO₄ unit. No hyperfine splitting was detected at room temperature. The EPR signal disappears after heating the crystal at 773 K for 1 h although the colour does not bleach.     

Studies on the spin Hamiltonian parameters for Mn2+ in the ZnS nanocrystals and bulks

The spin Hamiltonian parameters (zero-field splitting D, g factors and hyperfine structure constants) are theoretically studied for Mn²⁺ in the ZnS nanocrystals and bulks from the perturbation formulae of these quantities for trigonal and cubic tetrahedral 3d⁵ clusters, respectively. The trigonal Mn²⁺ centre in the ZnS nanocrystals is attributed to the impurity–ligand bond angle related to the C₃ axis about 0.39° larger than that (≈109.47°) of an ideal tetrahedron. Almost the same g factors and hyperfine structure constants for the nanocrystals and bulks can be ascribed to similar crystal-field environments (i.e. comparable cubic field parameters Dq), nearly the same covalency (i.e. the equal covalency factors N) and the Mn²⁺ 3d–3s orbital admixture (i.e. the identical core polarisation constants κ) in both systems. The ligand orbital and spin–orbit coupling contributions are found to be important and should be included in the electron paramagnetic resonance analysis in view of significant covalency.     

Thermo-optical properties of pure and Yb-doped monoclinic KY(WO4)2 crystals

The thermo-optic coefficients, dn/dT, were determined for pure and Yb(20 at.%)-doped monoclinic KY(WO₄)₂ crystals for light polarized along the optical indicatrix axes (N _p,N _m and N _g) in the wavelength range of 0.36–1.06 μm by a laser beam deviation method. The absolute values of thermo-optic coefficients satisfy the relation |dn _p/dT|>|dn _g/dT|>|dn _m/dT| and increase with the wavelength increasing. In the long-wavelength range, all the dn/dT values are negative: dn _p/dT=−14.6, dn _m/dT=−8.9, dn _g/dT=−12.4 [10⁻⁶ K⁻¹] for pure KY(WO₄)₂ at 1.06 μm. The dependency of thermo-optic coefficients on the wavelength was modeled using an approach that takes into account contribution of volumetric thermal expansion and change of electronic bandgap with temperature. Large volumetric expansion of KY(WO₄)₂ plays a key role in the observed negative dn/dT values. Electronic bandgap and its temperature coefficient were determined for KY(WO₄)₂ crystals from thermo-optic dispersion curves as E _g=4.8–5.0 eV and −dE _g/dT=0.7–1.1×10⁻⁴ eV/K. Athermal propagation directions were calculated for KY(WO₄)₂ crystals at the wavelength of 1.06 μm for light polarizations E∥N _m and N _p.     

Theoretical studies of the Spin Hamiltonian parameters and local structures for Cu2+ centers in MNB ternary glasses

ABSTRACT

The spin Hamiltonian parameters (g factors g_|| and g_⊥ and the hyperfine structure constants A_|| and A_⊥) for the doped Cu²⁺ ion (in the form of CuO) in ternary glasses (i.e. xMgO·(30-x)Na₂O·69B₂O₃·CuO, with 5?<?x < 17?mol%) are theoretically investigated based on the high-order perturbation formulas for a tetragonally elongated octahedral 3d⁹ complex. In these formulas, the required crystal-field parameters are estimated from the superposition model which enables correlation of the crystal-field parameters and hence the spin Hamiltonian parameters with the tetragonal distortion (characterized by relative tetragonal elongation δ along the C₄ axis due to the Jahn–Teller effect) of [CuO₆]^10? cluster. The concentration dependences of the spin Hamiltonian parameters are illustrated by the approximately linear increases of the cubic field parameter D_q and the covalency factor N as well as the relative elongation δ with increasing the MgO concentration x. Based on the calculation, the [CuO₆]^10? clusters in the MNB glasses are found to suffer the relative elongations of about δ (≈ 0.125?Å) along the tetragonal axis due to the Jahn–Teller effect. The theoretical results show good agreement with the experimental data. And the improvement is also achieved in present work with respect to the previous theoretical analysis based on the conventional crystal-field model formulas by including the ligand orbital and spin–orbit coupling contributions.     

Interstitial Substitution of VO(II) in Hexaaquazinc(diaquabismalonato)zincate: A Rare Observation

Single-crystal electron paramagnetic resonance (EPR) studies of VO(II) doped in hexaaquazinc(diaquabismalonto)zincate have been performed at room temperature. Single-crystal rotations along the three orthogonal axes show the presence of a single site in an interstitial position and the location has been identified from crystal data of the host lattice. The spin-Hamiltonian parameters calculated from EPR spectra are g _xx  = 1.973, g _yy  = 1.972, g _zz  = 1.930, A _xx  = 7.05 mT, A _yy  = 6.85 mT, A _zz  = 18.93 mT. Molecular orbital coefficients β ² = 0.71 and ε ² = 0.62 have been calculated and reveal a moderately covalent metal–ligand bond. Optical and EPR data have been used to obtain crystal field parameters, admixture and molecular orbital coefficients.     

Fermionic Ising glasses with BCS pairing interaction. Tricritical behaviour

We have examined the role of the BCS pairing mechanism in the formation of the magnetic moment and henceforth a spin glass (SG) phase by studying a fermionic Sherrington-Kirkpatrick model with a local BCS coupling between the fermions. This model is obtained by using perturbation theory to trace out the conduction electrons degrees of freedom in conventional superconducting alloys. The model is formulated in the path integral formalism where the spin operators are represented by bilinear combinations of Grassmann fields and it reduces to a single site problem that can be solved within the static approximation with a replica symmetric ansatz. We argue that this is a valid procedure for values of temperature above the de Almeida-Thouless instability line. The phase diagram in the T-g plane, where g is the strength of the pairing interaction, for fixed variance J ² /N of the random couplings J_ij, exhibits three regions: a normal paramagnetic (NP) phase, a spin glass (SG) phase and a pairing (PAIR) phase where there is formation of local pairs.The NP and PAIR phases are separated by a second order transition line g=g _c (T) that ends at a tricritical point T ₃ =0.9807J, g ₃ =5,8843J, from where it becomes a first order transition line that meets the line of second order transitions at T _c =0.9570J that separates the NP and the SG phases. For T<T _c the SG phase is separated from the PAIR phase by a line of first order transitions. These results agree qualitatively with experimental data in . Received 14 May 1998     

Temperature and pressure dependences of EPR spectra of Gd ion doped in the EuAl3(BO3)4 monocrystal

The ground state of Gd³⁺ ions substituting for trivalent europium in the EuAl₃(BO₃)₄ single crystal was studied by electron paramagnetic resonance (EPR) over the temperature range of 300-4.2 K and at pressures up to 9 kbar. The EPR spectra were analysed using the spin Hamiltonian of axial symmetry. The following parameters are reported: g=1.981±0.002, b₂⁰=280.18±0.12, b₄⁰=−12.95±0.08 and b₆⁰=0.61±0.12 (at Т=298 K). The distortions of the nearest environment of Gd³⁺ ion were analysed within the framework of the superposition model of crystal field.     

Structural elucidation of Cu(II) ion doped in hexaaquozincdiaquobis(malonato)zincate host by EPR spectroscopy

In order to understand the structural behaviour of Cu(II) in a variety of ligand environments, single crystal electron paramagnetic resonance studies of Cu(II) doped in hexaaquazincdiaquabis(malonato)zincate [Zn(H₂O)₆][Zn(mal)₂(H₂O)₂] are carried out at 300 K. Angular variation of copper hyperfine lines in three orthogonal planes shows the presence of single site, with spin Hamiltonian parameters as g_xx=2.034, g_yy=2.159, g_zz=2.388, A_xx=3.39 mT, A_yy=4.89 mT and A_zz=13.72 mT. The g/A tensor direction cosines are compared with various Zn-O directions in the host lattice, which confirm that Cu(II) enters substitutionally in the lattice. The low value of A_zz has been explained by considering admixture of d_²x−²y ground state with d_²z excited state. EPR powder spectra at 300 and 77 K give identical spin Hamiltonian parameters (g_∥=2.367, g_⊥=2.088, A_∥=11.47 mT, A_⊥=2.63 mT). IR, UV-vis and powder XRD data confirm the structure and symmetry of the Cu(II) ion in the host lattice.     

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)     

ESR, ENDOR, and ESEEM investigations on UV-irradiated 2,4,6-tri-tert-butyl phenol crystals

Electron spin resonance (ESR), electron nuclear double resonance (ENDOR), and electron spin echo envelope modulation (ESEEM) measurements were carried out for UV-irradiated 2,4,6-tri-tert-butyl phenol in the polycrystalline state. The radical produced in the crystal was detected by ESR and identified to be the corresponding phenoxyl radical, which is well characterized in the chemical oxidations in solutions. ENDOR and ESEEM spectra were unambiguously analyzed in terms of the hyperfine coupling constants determined from well-resolved ESR in solutions. Radical pairs in the crystals were also ascertained, and together with the single-crystal study the analysis disclosed zero-field splitting parameters in the triplet states. ESEEM time decays gave relaxation timesT ₁ = 5.94 andT ₂ = 1.12 μs at room temperature. These appropriate values permit an easy detection of the spin echoes, and therefore this radical matrix can be used as a useful standard for pulsed ESR investigations.     

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 Structure and Position of Cu(II) Ion in Aquomethylmelonatozinc(II) Host by Powder XRD, EPR, FT-IR and UV–Vis Spectroscopy

Single-crystal electron paramagnetic resonance studies of Cu(II)-doped aquomethylmelonatozinc(II) are carried out at 300 K. Angular variation of copper hyperfine lines in the three orthogonal planes shows the presence of a single site in substitutional position, with spin Hamiltonian parameters: g ₁₁ = 2.076, g ₂₂ = 2.100, g ₃₃ = 2.379; A ₁₁ = 2.40 mT, A ₂₂ = 3.22 mT and A ₃₃ = 13.47 mT. The low value for the hyperfine structure parameter value for A ₃₃ has been explained by taking into account a considerable admixture between d_{x² - y²} d_{{x^{2} - y^{2} }} ground state and d_z² d_{{z^{2} }} excited state, with admixture coefficients being a = 0.291, b = 0.951, c = 0.054, d = 0.031 and e = −0.031, where coefficients a and b correspond to coefficients for d_z² d_{{z^{2} }} and d_{x² - y²} , d_{{x^{2} - y^{2} }} , respectively. Few other calculated parameters such as κ = 0.35, P = 303 × 10⁻⁴ cm⁻¹; α = 0.88 and α′ = 0.47 indicate considerable covalency. Powder X-ray diffraction, Fourier transform infrared and ultraviolet-visible data confirm the structure of host complex and presence of the dopant.     

Anisotropy of rotation of nitroxide probes in nematogenic liquid crystals

An electron paramagnetic resonance (EPR) line shape simulation for nitroxide spin probes in the motional narrowing region was carried out assuming axially symmetricg andA tensors and with different anisotropies of rotationN (=R _∥/R _⊥) whereR _∥ andR _⊥ are, respectively, elements of the diffusion tensor along and perpendicular to its principal axisz′. In addition, it was assumed that the principal axes of the diffusion tensor coincide with the molecular axes. Each of three casesz′=x,z′=y andz′=z, which result from cyclic permutations of the molecular axesx, y andz with thez′,y′ andx′ axes of the diffusion tensor, yields its typical EPR spectrum characterized by the relative intensities of the low-, center- and high-field lines. The parameter δ defined by and calculable from the intensities of the three lines was found to vary linearly withN for thez′=x andz′=y cases and, as anticipated, to be practically constant at a value of 1 for thez′=z case. This suggested a method for estimatingN for a probe from its EPR spectrum. Experimental spectra over a narrow temperature range (1°C) in the vicinity of the nematic-to-isotropic transition (about 34.6°C) ofN-(4-n-butylbenzilidene)-4-amino-2,2,6,6-tetramethylpiperidine-1-oxide at a mole fraction of 1·10⁻³ in 4-n-pentyl-4′-cyanobiphenyl showed a pattern of peak heights characteristic of thez′=x case with δ values that gave, neglecting effects of the mean field, higher and lowerN values in the nematic and isotropic regions, respectively. Analysis of other similar systems in the literature gave similar results.     

High-Frequency ESR Measurements of <Emphasis Type="Italic">S</Emphasis> = 1/2 1-D Heisenberg Antiferromagnetic Zig-Zag Chain (VO)(SO<Subscript>4</Subscript>)(2,2-bpy)

High-frequency electron spin resonance (ESR) measurements of S = 1/2 one-dimensional Heisenberg antiferromagnetic zig-zag chain substance (VO)(SO₄)(2,2-bpy) were performed in the temperature region from 1.8 to 265 K. ESR measurements at 265 K show typical powder pattern spectra of the V⁴⁺ ion and g _|| = 1.925 ± 0.001 and g _⊥ = 1.976 ± 0.001 were obtained. Although the magnetic susceptibility shows a maximum at 3.1 K suggesting the existence of a short-range order, no g-shift and line width broadening were observed down to 1.8 K. We suggest that these temperature-independent features of ESR can be attributed to the result of frustration in (VO)(SO₄)(2,2-bpy).     

130 GHz ESEEM induced by electron–electron interaction in biradical

A three-pulse stimulated ESE at 130 GHz (D-band) of nitroxide biradical in a molecular glass shows envelope modulation (ESEEM) when the time separation between the first and the second pulses is varied. This ESEEM originates from relaxation-induced flips of the spin partner during the mixing period between the second and third pulses. These flips alternates the local dipolar field in which the resonant spins precess before second and after third pulses. The ESEEM is different for different spectral positions, due to orientation selectivity. For toluene glassy solution at 35 K Fourier transform shows pronounced peak reflecting singularity of the Pake resonance pattern. Increasing temperature up to 80 K results in appearance of a strong additional peak ascribed to alteration of the resonance field induced by methyl group reorientation.     

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.