Electron spin resonance studies of chlorine centres in irradiated chlorine-doped Na2SO4 single crystals

The nature of paramagnetic centres trapped in X-irradiated Na₂SO₄ single crystals containing chlorine as impurity has been determined from E.S.R. measurements both at room and liquid nitrogen temperatures. ClO₃, ClO₂, SO₄ ^- and O₃ ^- are the paramagnetic centres identified. The g and A parameters of ClO₃ change considerably between 298 K and 77 K. Thus the parameters of ClO₃ obtained at 298 K are g_xx = 2·0123, g_yy = 2·0143, g_zz = 2·0206 G and those of the A-tensor are A_xx = 97, A_yy = 100 and A_zz = 112 G. The parameters obtained for the same centre at 77 K are g_xx = 2·0132, g_yy = 2·0173, g_zz = 2·0073 G and those of the A-tensor are A_xx = 130·6, A_yy = 146·3 and A_zz = 181·2 G. The temperature variation of the principal values are determined in the temperature range between 298 K and 77 K. These data can be interpreted on the assumption that ClO₃ undergoes libration at room temperature, which is frozen out reversibly on cooling. The structure and mechanism of these defects are discussed further.     

Single crystal EPR studies of Mn(II) doped into zinc ammonium phosphate hexahydrate (ZnNH4PO4·6H2O): A case of interstitial site for bio-mineral analogue

Single crystal EPR studies of Mn(II)-doped zinc ammonium phosphate hexahydrate (ZnNH₄PC₄·6H₂O) have been reinvestigated at room temperature. Single crystal rotations along the three orthogonal axes indicate that the spin Hamiltonian parameters for the interstitial site are:g _xx = 1.966,g _yy = 1.972,g _zz = 1.976;D _xx = -12.28 mT,D _yy = -2.09 mT andD _zz = 14.37 mT;A _xx = 9.06 mT,A _yy = 9.06 mT andA _zz = 11.09 mT;a = -0.11 mT. These parameters differ considerably from the previous report of Chand and Agarwal and indicate the orthorhombic nature of the paramagnetic impurity. The impurity is found to enter the lattice interstitially, in contrast to earlier prediction of substitutional position. The percentage covalency of the Mn-0 bond has been estimated.     

ESR of X-ray induced centre in CaF2:Gd crystals

An ESR centre with orthorhombic symmetry was induced in CaF₂:Gd crystals by X-ray irradiation after heat treatment at 900°C. The observed ESR spectrum has resolved shfs in particular crystal directions. Spin Hamiltonian analysis indicates that the ESR centre is a hole trapped at an oxygen substituted for a fluorine site and interacts mainly with two nearby fluorine ions located along the crystal [001] direction. The determined spin Hamiltonian parameters are g_xx = 2.0036, g_yy = 2.0159, g_zz = 2.0306, A_xx = 22.5, A_yy = 5.6 and A_zz = 5.6 (G), respectively. A probable model is discussed by considering both the angular dependence of the ESR lines and the local charge neutrality around the ESR centre.     

Single-crystal EPR investigation of Mn(II)-doped biomineral cobalt ammonium phosphate hexahydrate: a case of multiple substitutions

Single-crystal electron paramagnetic resonance (EPR) study of Mn(II)-doped cobalt ammonium phosphate hexahydrate has been carried out at room temperature. The impurity shows more than 30 line pattern EPR spectra along the three crystallographic axes, suggesting the existence of more than one type of impurity ion in the host lattice. The spin Hamiltonian parameters, estimated from the three mutually orthogonal crystal rotations, are: site 1: g _xx =1.989, g _yy =1.994, g _zz =1.999; A _xx =?8.97, A _yy =?9.52, A _zz =?9.71 mT; D _xx =?8.09 mT, D _yy =?6.05 mT, D _zz =14.14 mT; site 2: g _xx =1.988, g _yy =2.009, g _zz =2.019; A _xx =?9.11 mT, A _yy =?9.58 mT, A _zz =?9.93 mT; D _xx =?6.61 mT, D _yy =?6.11 mT, D _zz =12.72 mT. The angular variation studies further reveal that the Mn(II) impurities enter the lattice substitutionally. The other Mn(II) sites which are at interstitial locations are difficult to follow due to their low intensity. The variation of zero-field splitting parameter with temperature indicates no phase transition. The observation of well-resolved Mn(II) spectrum at room temperature has been interpreted in terms of ‘host spin-lattice relaxation narrowing’ mechanism.     

Effect of paramagnetic doping on an inorganic polymer triaquadipotassiumbis(malonato)zincate: spectroscopic investigation

Spectroscopic investigations on Mn(II)-doped triaquadipotassiumbis(malonato)zincate [K₂(H₂O)₃] [Zn(mal)₂], an inorganic polymer, have been carried out at room temperature using single crystal electron paramagnetic resonance (EPR), ultraviolet–visible, FT-IR and powder XRD techniques. Single crystal rotations along the three orthogonal axes show more than 30 lines of patterns in EPR spectra, indicating the presence of two sites, one with a large D value and the other with a smaller D value. The calculated spin-Hamiltonian parameters are as follows. Site 1: g _xx =2.099, g _yy =2.092, g _zz =1.988, A _xx =9.77, A _yy =9.71, A _zz =8.96 mT, D _xx =?29.09, D _yy =?11.90, D _zz =40.99 mT; Site 2: g _xx =2.040, g _yy =1.995, g _zz =1.924, A _xx =9.51, A _yy =9.09, A _zz =8.80 mT, D _xx =?11.94, D _yy =?7.51 and D _zz =19.45 mT. The direction cosines of g/A/D do not match with the direction cosines of Zn–O bonds in the host lattice for either site, suggesting that both the Mn(II) sites entered the lattice interstitially. Optical results indicate a strong covalent bonding between the metal ion and ligands, with site symmetry being primarily octahedral. The FT-IR and powder XRD data confirm the retention of the crystal structure, even after incorporating a paramagnetic probe. Various admixture coefficients, bonding and optical parameters have also been calculated.     

ESR studies of Cu2+ doped in SrC4H4O6·3H2O single crystals

Electron spin resonance spectra of Cu²⁺ doped in single crystals of strontium tartrate trihydrate grown by a diffusion technique have been investigated at 77K. Copper enters the lattice substitutionally and is trapped at two magnetically inequivalent sites. ESR measurements gave the following values for the spin-Hamiltonian parameters. Cu²⁺(I): g_gg = 2.0380, g_yy = 2.1317, g_zz = 2.3918 and A_xx = 26.3 G, A_yy = 56.3 G, A_zz = 110.8 G. Cu²⁺(II): g_xx = 2.0497, g_yy = 2.1297, g_zz = 2.3706 and A_xx = 19.2 G, A_yy = 61.4 G, A_zz = 107.2 G.     

EPR and optical-absorption studies of Cu2+-doped Zn(NH2(CH2)3NHOH)2Ni(CN)4 single crystals

Electron paramagnetic resonance of Cu²⁺-doped catena-trans-bis(N-(2-hydroxyethyl)-ethylenediamine) zinc(II)-tetra-μ-cyanonicelate(II) [Zn(NH₂(CH₂)₃NHOH)₂Ni(CN)₄] single crystals and powder are examined at room temperature. The spectra show the substitution of the Zn²⁺ ion with the Cu²⁺ ion. The crystal field around the Cu²⁺ ion is nearly axial. There is a single paramagnetic site withg _xx=2.073,g _yy=2.060,g _zz=2.248,A _xx=40.5 G,A _yy=50.8 G,A _zz=172.0 G. The ground-state wave function is an admixture of d _x ²−y ² and d _z ² states. The optical-absorption studies show two bands at 320 nm (31250 cm⁻¹) and 614 nm (16286 cm⁻¹) which confirm the axial symmetry. The crystal field parameters and the wave function are determined.     

EPR and optical investigation of VO(II) in Zn(C3H3O4)2(H2O)2 single crystals: An interstitial site

EPR spectroscopic investigations on single crystals of diaquabis[malonato(1-)-κ²O,O′] zinc(II) doped with VO(II) ion have been carried out at X-band frequencies and at 300 K. The single crystal, rotated along the three mutually orthogonally axes, has yielded spin-Hamiltonian parameters g and A as: g_xx=1.980, g_yy=1.972, g_zz=1.937 and A_xx=8.4, A_yy=6.1, A_zz=18.1 mT, respectively. These spin-Hamiltonian parameters reflect a slight deviation from axial symmetry to rhombic, which is elucidated by the interstitial occupation of vanadyl ions. The isofrequency plots and powder EPR spectrum have been simulated. The percentage of metal-oxygen bond has been estimated. The optical absorption spectrum exhibits four bands at 257, 592, 720 and 764 nm suggesting a C_4v symmetry. The admixture coefficients and bonding parameters have also been calculated by collaborating EPR data with optical data.     

Impurity in three substitutional locations: Single-crystal EPR study of VO(II) in zinc maleate tetrahydrate

Single-crystal EPR study of VO(II)-doped zinc maleate tetrahydrate has been carried out at room temperature. Four types of impurities have been identified in the single-crystal spectra with intensity ratio of 11:5:2:1. However, the analysis has been done only for the three most intense resonances. The evaluated spin Hamiltonian parameters (A in units of mT) from single-crystal rotations are site I: g_xx=1.981, g_yy=1.971, g_zz=1.939; A_xx=7.43, A_yy=7.70, A_zz=18.7; site II: g_xx=1.973, g_yy=1.966, g_zz=1.939; A_xx=7.08, A_yy=7.22, A_zz=18.5; site III: g_xx=1.978, g_yy=1.977, g_zz=1.951; A_xx=7.11, A_yy=7.32, A_zz=18.3.The powder spectrum gives only one set of g and A values, confirming the presence of only one chemically equivalent site. The paramagnetic impurity, VO(II), has entered the lattice substitutionally for all the three sites. The superhyperfine structure, in the intensity ratio of 1:4:6:4:1, arising from the protons of the water ligands, has been found in one site. The admixture coefficients have been calculated and the complex is found to be fairly covalent in nature.     

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.     

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.     

Theoretical studies of the local structures and electron paramagnetic resonance parameters for Cu2+ center in Zn(C3H3O4)2(H2O)2 single crystal

The electron paramagnetic resonance (EPR) parameters (g factors g_xx, g_yy, g_zz and hyperfine structure constants A_xx, A_yy, A_zz) are interpreted by taking account of the admixture of d-orbitals in the ground state wave function of the Cu²⁺ ion in a Zn(C₃H₃O₄)₂(H₂O)₂ (DABMZ) single crystal. Based on the calculation, local structural parameters of the impurity Cu²⁺ center were obtained (i.e. R_a≈1.92 Å, R_b≈1.96 Å, R_c≈1.99 Å). The theoretical EPR parameters based on the above Cu²⁺?O^2? bond lengths in the DABMZ crystal show good agreement with the observed values and some improvements have been made as compared with those in the previous studies.     

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.     

Single-Crystal EPR Identification of a Low Hyperfine Value and Interstitial Position of Copper Impurity in Diaquabis[malonato(1-)-κ2O,O′] Zinc(II)

Electron paramagnetic resonance studies have been carried out at 300 K on the Cu(II)-doped [Zn(C₃H₂O₄)₂(H₂O)₂] system in single-crystal and powder forms in order to rationalize the low parallel ⁶³Cu hyperfine value. Angular variation of the hyperfine resonances in the three orthogonal planes shows the presence of only one magnetic site with g and A values equal to g _zz = 2.455, g _yy = 2.121, g _xx = 2.105 and A _zz = 160.9 · 10⁻⁴ cm⁻¹, A _yy = 12.5 · 10⁻⁴ cm⁻¹, A _xx = 7.35 · 10⁻⁴ cm⁻¹. The crystal structure of the host lattice is isostructural with the corresponding cobalt complex and contains two molecules per unit cell. The low magnitude of A _zz value for the complex is rationalized in terms of an admixture of the ground state with the excited state and delocalization of the unpaired spin density onto the ligands. In addition, the highest hyperfine value obtained from the single-crystal data (160.9 · 10⁻⁴ cm⁻¹) is considerably larger than that obtained from the powder spectrum (138 · 10⁻⁴ cm⁻¹). Authors' address: P. Sambasiva Rao, Department of Chemistry, Pondicherry University, Pondicherry 605014, India     

E.S.R. and ENDOR of an α-chloro radical in X-irradiated 5-chlorodeoxyuridine single crystals

The most prominent radical formed from X-irradiation of the nucleic acid constituent analogue 5-chlorodeoxyuridine at room temperature is shown to be an α-chloro radical formed by hydrogen addition to C₆. The E.S.R. analysis of the ³⁵Cl hyperfine interaction combined with theoretical simulation of the spin hamiltonian yields tensor components a_xx =46·98 MHz, a_yy =-10·98 MHz and a_zz =-17·01 MHz with a quadrupole coupling constant of eqQ=72 MHz. The principal g-tensor values are g_xx =2·0012, g_yy =2·00862 and g_zz =2·00687. Three additional hyperfine interactions in the radical are observed combining E.S.R. and ENDOR spectroscopy. Besides the two nearly equivalent β-protons on C₆ with principal values of 103·39 MHz and 110·12 MHz, there is hyperfine interaction with the ¹⁴N nucleus of nitrogen N₃ (a_xx =9·81 MHz, a_yy =a_zz ? 0 MHz) and with the proton of the hydrogen bonded to N₃. The latter interaction has tensor components of 2·65 MHz, -10·80 MHz and -8·09 MHz as obtained from ENDOR data. The chlorine hyperfine coupling parameters are related to those observed in other α-chloro radicals. The mechanism of the formation of the radical in 5-chlorodeoxyuridine is discussed briefly.     

Substitutional location for a vanadyl ion in three water coordinated triaqua(1,10-phenanthroline-k2N,N′) (sulfato-kO)magnesium(II) complex: a rare observation

Vanadyl ion substitutes for the central ion, if the doped complex has at least four water molecules. However, the present EPR, XRD, FT-IR and optical absorption studies on vanadyl-doped triaqua(1,10-phenanthroline-k²N,N′)(sulfato-kO)magnesium(II) suggest a substitutional defective nature, which is a rare observation. Single crystal EPR studies in three mutually orthogonal planes indicate two chemically non-equivalent sites with different intensities. However, the lower intensity site could not be analysed due to its weaker intensity and overlap with other sites during crystal rotations. The spin Hamiltonian parameters obtained for the major site are as follows: g _xx =1.973, g _yy =1.972, g _zz =1.930; A _xx =7.15, A _yy =6.77, A _zz =18.92 mT. The direction cosines of principal g and A values suggest that the impurity has entered the lattice substitutionally, which is a very uncommon phenomenon. Admixture coefficients, Fermi contact, dipolar interaction and covalency of metal–ligand bonds have also been evaluated. Optical, FT-IR and powder XRD techniques confirm the structure of the complex.     

Observation of three sites for vanadyl in a biomineral, zinc sodium phosphate hexahydrate: an EPR investigation

Single crystal EPR study has been carried at room temperature for VO(II) doped zinc sodium phosphate hexahydrate. Single crystal rotations in each of the three mutually orthogonal crystallographic planes namely bc, ac, and ab indicate three chemically inequivalent sites, with intensity ratios of 25:13:1. The spin Hamiltonian parameters obtained for the two intense sites are: Site I: g_xx=1.983, g_yy=1.985, g_zz=1.933; A_xx=7.39 mT, A_yy=7.15 mT, A_zz=19.03 mT; Site. II: g_xx=1.985, g_yy=1.985, g_zz=1.937; A_xx=7.36 mT, A_yy=7.25 mT, A_zz=18.67 mT. The two VO bond directions in the two sites are approximately at right angles to each other. The powder spectrum clearly indicates two chemically inequivalent sites, confirming the single crystal analysis. Admixture coefficients, Fermi contact, and dipolar interaction terms have also been evaluated.     

EPR studies of Cu2+ in anhydrous sodium oxalate

Electron paramagnetic resonance spectra of Cu²⁺ doped in single crystal of anhydrous sodium oxalate grown by slow evaporation from saturated aqueous solutions have been investigated. EPR measurements gave the following values for the spin Hamiltonian parameters: g_xx = g_yy = 2.0741, g_zz = 2.3253, A_xx = A_yy = ?14.9G and A_zz = ?147.7 G. The principal axes of the hyperfine coupling and g tensors are spatially coincident. Using the optical absorption energy values given for a similar complex we have estimated the values of the bonding parameters and orbital-reduction factors for the system under investigation.     

EPR and Optical Absorption Study of Cu2+-Doped Diammonium Hexaaqua Magnesium Sulphate Single Crystals

Electron paramagnetic resonance (EPR) study of Cu²⁺ ions doped in diammonium hexaaqua magnesium sulphate single crystal over the temperature range of 4.2–320 K is reported. Copper enters the lattice substitutionally and is trapped at two magnetically equivalent sites. The spin Hamiltonian parameters are evaluated at 320, 300, 77, and 4.2 K. The angular variations of the resonance lines in three mutually perpendicular planes ab, bc* and c*a are used to determine principal g and A values. The observed spectra are fitted to a spin Hamiltonian of rhombic symmetry with parameters of Cu²⁺ at 77 and 4.2 K: g _xx  = 2.089, g _yy  = 2.112, g _zz  = 2.437 (±0.002) and A _xx  = 38, A _yy  = 14, A _zz  = 110 (±2) × 10^?4 cm^?1. The ground state wave function of Cu²⁺ ion in this lattice is determined. The g-factor anisotropy is calculated and compared with the experimental value. The optical absorption spectra of the crystal are also recorded at room temperature. With the help of assigned bands the crystal-field parameters (Dq, Ds and Dt) are evaluated. By correlating the optical and EPR data, the nature of bonding in the complex is discussed. The temperature dependence of the g values is explained to conclude the occurrence of both static and dynamic Jahn–Teller effects over the temperature range of investigation.     

A New Paramagnetic Nitrogen Center in Natural Titanium-Containing Diamonds

A new type of nitrogen-related center in natural diamond labeled NU1 is identified as a 〈100〉 split interstitial configuration by means of electron paramagnetic resonance (EPR) and photoluminescence (PL) techniques. In PL this center is seen as an electron-vibration system with a zero-phonon line at 485 nm in the crystals containing S1/OK1 and 440.3 nm/N3 centers. NU1 EPR spectra are described by a spin-Hamiltonian with parameters: S = ½, I = 1 and A _xx (N) = 22.5 G, A _yy (N) = 19.5 G, A _zz (N) = 20.55 G, g _xx  = 2.0043, g _yy  = 2.0032, g _zz  = 2.0020. Directions of A _zz and g _zz coincide and correspond to [001]. The directions of A _xx and A _yy coincide with those of g _xx and g _yy and correspond to [110] and [?110], respectively. Analysis of the phonon structure of the NU1 center suggested that titanium can be the second atom together with nitrogen in the structure of a split interstitial.