An EPR investigation of room temperature radiation damage and molecular motion in the ferroelectric phosphates

Several types of paramagnetic centers have been detected in the room temperature irradiated KH₂PO₄, KD₂PO₄, KH₂PO₄KH₂AsO₄, KD₂PO₄KD₂AsO₄ and in the newly discovered ferroelectric NaTh₂(PO₄)₃. Comparative studies of temperature dependence in the EPR spectra show that as in KH₂PO₄-type crystals, the ferroelectric phase transition in NaTh₂(PO₄)₃ crystals also seems to be related to the motion of the PO₄ units. The results help to clarify some earlier observed anomalies on the effects of γ-irradiation on dielectric properties of KH₂PO₄ and in particular, show that the extent of the damage can be controlled by changing the impurity content of the unirradiated samples.     

Reasons for the line broadening in the epr spectra of copper ions in Li2-2x Zn2+x (MoO4)3 lithium-zinc molybdate crystals

An EPR study of Li_2?2x Zn_2+x (MoO₄)₃ crystals activated by copper ions shows that they occupy the M2 site, one of the three possible sites of both lithium and zinc. In the EPR spectra of Cu²⁺ copper a broadening of HFS lines and a nonequidistant splitting between them, which are unusual for the orientation H‖g _zz , A _zz , are observed. In this work possible reasons for such a broadening of HFS lines from copper ions are analyzed: a distortion of the oxygen octahedron due to the introduction of copper ions, second order perturbation theory corrections, superposition of HFS from ⁶³Cu and ⁶⁵Cu isotopes, and the effect of the charge redistribution in the oxygen octahedron because the cation vacancy providing charge compensation can be located at different distances from the copper ion. It is shown that the first three reasons do not explain the features observed in the EPR spectra. In the case of cation vacancies located in the M3 site and remote at different distances from the copper ion, the charge redistribution in the oxygen octahedron of copper should occur along with the dispersion of HFS parameters and the g-factor. The studies performed in X and Q bands confirm this assumption. The width of HFS lines from copper ions in the EPR spectra measured in the Q band is three times more compared to that measured in the X band.     

EPR and IR spectra of Ni(ClO4)2)·6H2O between 98 and 298 K

EPR measurements between 98 and 298 K on single crystal of Ni(ClO₄)₂·6H₂O have indicated the appearance of a rhombic component in the axial crystal field at Ni²⁺ sites at T_c = 224 K, confirming a phase transition first reported by Chaudhuri from magnetic susceptibility measurements. Temperature variations of g, D and E parameters were determined. IR spectra at room and liquid nitrogen temperatures are consistent with our EPR results.     

Magnetic and spectroscopic investigation of partially reduced vanadium pentoxides. I. α-CuxV2O5

The electron paramagnetic resonance (EPR) and magnetic susceptibility of both powdered and crystalline α-Cu_xV₂O₅ near the compositional limit have been studied as a function of temperature. The EPR data indicate that the paramagnetic species are isolated V⁴⁺ centers in a nearly axially symmetric ligand field. It is demonstrated that reasonably precise EPR parameters can be derived from powdered spectra. The narrowness of the EPR line suggests that the spectra are motionally narrowed via thermally activated hopping of the paramagnetic electron of V⁴⁺. Expressions are derived for the temperature-dependent magnetic moments of the octahedral ²T_2g term under the combined perturbation of spin-orbit coupling and an axially symmetric ligand field, and the susceptibility data are interpreted in terms of this model. It is found that the sixfold degenerate ²T_2g level is split into three levels with the lowest and highest levels possessing a magnetic moment and the intermediate level having no moment. This model is also consistent with the g-tensor and can account for the temperature dependence of the EPR linewidth.     

A high-frequency EPR characterization of the S = 2 linear tri-atomic chain in Cr3(dpa)4Cl2·CH2Cl2

We report the high frequency electron paramagnetic resonance (HF-EPR) study of Cr₃(dpa)₄Cl₂, a linear tri-atomic (Cr^II)₃ chain, that was reported to be EPR silent at the X-band (9.5 GHz). Higher frequencies yield well resolved spectra for this S = 2 system even at room temperature. At 30 K, our variable frequency (34-400 GHz) EPR spectra yield a large axial zero-field splitting (D) of −1.643(1) cm⁻¹ as well as a small rhombic ZFS parameter E = 0.0339(4) cm⁻¹; with g_x = 1.9978(4), g_y = 1.9972(4), and g_z = 1.9808(4). The magnetic susceptibility measurements fully support the earlier magnetic susceptibility studies and the current EPR results. The observation of the EPR spectrum of only the S = 2 state at room temperature suggests that the ground state is well isolated from the excited states.     

EPR and electronic absorption studies of Mn2+ ion in 3CdSO4 · 8H2O single crystals

EPR and electronic absorption spectra of the Mn²⁺ion in 3CdSO₄ · 8H₂O crystals have been studied at 300 K and 80 K. The EPR spectra have     

Study in Aqueous Solutions of Bioactive 2-Pyridineformamide-Derived Thiosemicarbazones and Their Iron(II) and Iron(III) Complexes

The binary systems of iron(II) and iron(III) with 2-pyridineformamide thiosemicarbazone (H2Am4DH) and its N(4)-methyl (H2Am4Me), N(4)-ethyl (H2Am4Et) and N(4)-phenyl (H2Am4Ph) derivatives were studied in aqueous solution by pH-potentiometry, ultraviolet–visible spectroscopy and EPR spectra. The formation constants of the iron(II) and iron(III) complexes were calculated from potentiometric and electronic absorption data at 25 °C and ionic strength μ = 0.1 mol·L^?1 using the HYPERQUAD program. The values of the formation constant of the FeL species decrease in the order Fe:H2Am4DH > Fe:H2Am4Me ≈ Fe:H2Am4Et > Fe:H2Am4Ph in the same way as the basicity of the ligands. The species distribution diagrams show that the species FeL₂ predominates at physiological pH in the Fe:H2Am4DH, Fe:H2Am4Me and Fe:H2Am4Et systems. The similar EPR spectra of these iron(III) binary systems indicate the same coordination spheres around the metallic center and the EPR g values suggests that the unpaired electron is in the d_xy orbital, indicating a d _xz ² d _yz ² d _xy ¹ ground state configuration for the complexes. For the Fe(III):H2Am4Ph system the EPR results indicated dimerization and antiferromagnetic interaction due to the presence of only one thiosemicarbazone ligand around the metallic center.     

Electrical,magnetic, and EPR studies of the quaternary chalcogenides Cu2AIIBIVX4 prepared by iodine transport

Electrical, magnetic, and electron paramagnetic resonance (EPR) measurements have been made on crystals and powders of several quaternary chalcogenides of the type Cu₂A^IIB^IVX₄, where A^II = Zn, Mn, Fe, or Co, B^IV = Si, Ge, or Sn, and X = S or Se. The electrical properties of these compounds are extrinsic, but their magnetic properties do not appear to be affected by impurities. The magnetic moments of the Cu₂MnBX₄ compounds decrease with increasing covalency of the MnX bond, and those of Cu₂FeGeS₄ and Cu₂CoGeS₄ reflect an orbital contribution to the moment. Both the Weiss constants and magnetic ordering temperatures in these compounds show an evolution from antiferromagnetism to ferromagnetism with increasing separation between the moments. Magnetic measurements on single crystals of Cu₂MnGeS₄, Cu₂CoGeS₄, and Cu₂FeGeS₄ indicate that only the latter is anisotropic. EPR measurements on crystals and powders of Cu₂ZnGeS₄ doped nominally with 0.1% Mn reveal that Mn²⁺ experiences an axial distortion and that the bond ionicity is the same as in ZnS.     

Double resonance study of the 17O quadrupole coupling in paraelectric and ferroelectric KH2PO4

The pure quadrupole resonance spectrum of ¹⁷O has been measured in a 10% enriched KH₂PO₄ sample at 190°K at at 77°K. In the paraelectric phase all oxygen sites are chemically equivalent with e²qQ/h = 5.16 MHz and η = 0.55, whereas there are two chemically non-equivalent ¹⁷O sites in the ferroelectric phase: (e²qQ/h)₁ = 5.96 MHz, η_t = 0.72 and (e²qQ/h)_II = 4.85 MHz, η_II = 0.18. The results can be interpreted in terms of a double minimum type O-H—O hydrogen bond potential, where the protons above T_c rapidly fluctuate between the two equilibrium sites. In contrast with the neutron scattering results in KD₂PO₄, the present data show no evidence for any anomalous oxygen ferroelectric mode displacements in KH₂PO₄, which would not condense out below T_c.     

Li and Li MAS NMR Studies on Fast Ionic Conducting Spinel-Type Li2MgCl4, Li2-xCuxMgCl4, Li2-xNaxMgCl4, and Li2ZnCl4

⁶Li and ⁷Li MAS NMR spectra including 1D-EXSY (exchange spectroscopy) and inversion recovery experiments of fast ionic conducting Li₂MgCl₄, Li_2-xCu_xMgCl₄, Li_2-xNa_xMgCl₄, and Li₂ZnCl₄ have been recorded and discussed with respect to the dynamics and local structure of the lithium ions. The chemical shifts, intensities, and half-widths of the Li MAS NMR signals of the inverse spinel-type solid solutions Li_2-xM^I_xMgCl₄ (M^I=Cu, Na) with the copper ions solely at tetrahedral sites and sodium ions at octahedral sites and the normal spinel-type zinc compound, respectively, confirm the assignment of the low-field signal to Li^tet of inverse spinel-type Li₂MgCl₄ and the high-field signal to Lioct as proposed by Nagel et al. (2000). In contrast to spinel-type Li_2-2xMg_1+xCl₄ solid solutions with clustering of the vacancies and Mg²⁺ ions, the Cu⁺ and Na⁺ ions are randomly distributed on the tetrahedral and octahedral sites, respectively. The activation energies due to the various dynamic processes of the lithium ions in inverse spinel-type chlorides obtained by the NMR experiments are E_a=6.6-6.9 and ΔG^*>79 KJ mol⁻¹ (in addition to 23, 29, and 75 kJmol-1 obtained by other techniques), respectively. The largest activation energy of >79 KJ mol⁻¹ corresponds to hopping exchange processes of Li ions between the tetrahedral 8a sites and the octahedral 16d sites. The smallest value of 6.6-6.9 KJ mol⁻¹, which was derived from the temperature dependence of both the spin-lattice relaxation times T₁ and the correlation times τ_C of Li^tet, reveals a dynamic process for the Li^tet ions inside the tetrahedral voids of the structure, probably between fourfold 32e split sites around the tetrahedral 8a site.     

NMR exchange studies on the complexes trans-[PtX2(C2H4)(Him)] (X = Cl,Br; Him = imidazole)

The ¹H-NMR spectra of the complexes trans-[PtX₂(C₂H₄)(Him)] (X = Cl or Br, Him = imidazole) are discussed. Variable temperature spectra are used to monitor the exchange processes which occur in acetone-d₆ solution. It is found, contrary to previous work, that intermolecular exchange occurs for both the chloro- and bromo- complexes. In addition, it is also found that changing the solvent has a marked effect on the rate of exchange. Using an iterative simulation program and assuming intermolecular exchange, the rate constants for the exchange process are determined by band shape analysis, and the enthalpy and entropy of activation are calculated.     

Synthesis, spectral and thermal studies of new copper (II) complexes with 1,2-di(imino-2-aminomethylpyridil)ethane

New copper (II) complexes of Schiff bases with 1,2-di(imino-2-aminomethylpyridil)ethane with the general composition CuLX _m (H₂O) _x , [L = Schiff base, X = Cl^?, Br^?, NO₃ ^?, ClO₄ ^?, CH₃COO^?, m = 2; X = SO₄ ^2?, m = 1] were prepared by template synthesis. The complexes were characterized by elemental analysis, conductivity measurements, magnetic moments, IR, UV–VIS and EPR spectra. The thermal behavior of complexes was studied using thermogravimetry (TG), differential thermal analysis (DTA) and differential scanning calorimetry (DSC). Infrared spectra of all complexes are in good agreement with the coordination of a neutral tetradentate N₄ ligand to the cooper (II) through azomethinic and pyridinic nitrogen. Magnetic, EPR and electronic spectral studies show a monomeric distorted octahedral geometry for all Cu(II) complexes. Conductance measurements suggest the non-electrolytic nature of the compounds, except for copper (II) nitrate and perchlorate complexes which are 1:2 electrolytes. Heats of decomposition, ΔH, associated with the exothermal effects were also determined.     

EPR-Untersuchungen am Kupfer(II)-Chelat einesα-Cyano-β-amino-dithioacrylsäureesters

EPR Investigations on a Copper Chelate of anα-Cyano-β-amino-dithioacryl Acid Ester EPR studies on copper(II) chelates of anα-cyano-β-amino-dithioacryl acid ester are reported. The EPR spectra were obtained from solutions, diamagnetically diluted powders, and single-crystals which are stable for a short time only. The corresponding nickel(II) chelate was used as host lattice. The ¹⁴N ligand hyperfine structure observed in the spectra is in agreement with a [CuN₂S₂] coordination sphere. In some orientations of the recorded angular dependencies the EPR spectra show a hyperfine splitting due to the interaction of the unpaired electron with the N? H protons. In addition spin flip satellite lines are observed in the single-crystal spectra. The g, A^Cu and A^N tensors obtained from the powder and single-crystal spectra have an axial symmetry within the experimental errors. The unpaired electron occupies a MO which consists mainly of the copper 3d_xy and the corresponding donor atom orbitals. The co-valency of the metal ligand bond is very high.     

V2O5/TiO2 Catalytic Xerogels Raman and EPR Studies

Raman spectroscopy and Electron Paramagnetic Resonance (EPR) studies were performed on a series of V₂O₅/TiO₂ catalysts prepared by a modified sol-gel method in order to identify the vanadium species. Two species of surface vanadium were identified by Raman measurements, monomeric vanadyls and polymeric vanadates. Monomeric vanadyls are characterized by a narrow Raman band at 1030 cm^–1 and polymeric vanadates by two broad bands in the region from 900 to 960 cm^–1 and 770 to 850 cm^–1. The Raman spectra do not exhibit characteristic peaks of crystalline V₂O₅. These results are in agreement with those of X-ray Diffractometry (XRD) and Fourier Transform Infrared (FT-IR) previously reported (C.B. Rodella et al., J. Sol-Gel Sci. Techn., submitted). At least three families of V⁴⁺ ions were identified by EPR investigations. The analysis of the EPR spectra suggests that isolated V⁴⁺ ions are located in sites with octahedral symmetry substituting for Ti⁴⁺ ions in the rutile structure. Magnetically interacting V⁴⁺ ions are also present as pairs or clusters giving rise to a broad and structureless EPR line. At higher concentration of V₂O₅, a partial oxidation of V⁴⁺ to V⁵⁺ is apparent from the EPR results.     

Synthesis and spectral characterization of some binuclear complexes designed from N4O and N2O3 donor Schiff-base ligands of 2,6-diformyl-4-methylphenol

New pentadentate binucleating ligands containing phenoxide as an endogenous bridging group, 2,6-diformyl-4-methylphenol bis(carbohydrazone) (L¹H), and 2,6-diformyl-4-methylphenol bis(semicarbazone) (L²H), and their binuclear Co(II), Ni(II), Cu(II) and Zn(II) complexes of general formula [M₂LCl₃] · nH₂O with chloride as an exogenous bridge have been synthesized. The complexes were characterized on the basis of elemental analysis, conductivity measurements, thermal analysis, IR, Far-IR, NMR, UV–Vis, EPR, FAB-mass and magnetic data. The coordination mode (N₄O, N₂O₃), as well as endogenous phenoxide bridge and an exogenous chloride bridge have been established on the basis of IR, Far-IR and ¹H-NMR spectral data. Electronic spectral data of the complexes indicate square-pyramidal geometry. EPR spectra show line broadening, which is further supported by weak antiferromagnetic interaction from the room temperature magnetic moment data. All compounds show appreciable antimicrobial activity.     

Investigation of some oxygen-containing chromium (III) complexes by epr and optical spectroscopy

EPR and electronic absorption spectra of chrome alum solutions in sulphuric and phosphoric acids have been studied. Broadening of the EPR lines and shifts in the positions of the electronic spectra absorption bands were observed at low acid concentrations. In concentrated acids the ⁴T_2g band splits into several well resolved components and the EPR spectra show two resolved areas of fine structure. Each structure may be described by a spinHamiltonian of the form

where D is the zero-field splitting parameter due to the action of the axial crystal fields. The changes in the spectra as the acid concentration is increased are explained by changes in the internal environment of the Cr^III complexes as a result of the substitution of H₂O by anions of the acid residue. The symmetry of the complexes predominating at particular acid concentrations and the parameters D, g and Dt have been determined from an analysis of the EPR and optical absorption spectra. The character of the bond of the Cr^III ion with the surrounding ligands has also been estimated.     

The features of the Cu2+-entry into the structure of tourmaline

EPR and optical absorption spectra of Cu²⁺ ion were investigated in natural elbaites from Brazil and Zambia and in synthetic olenite single crystal. In elbaite from Zambia, the content of Cu²⁺ ions was found to be about 0.006 pfu, whereas in Brazilian elbaite the amount of this ion can approach up to 0.2 pfu. The rose color of elbaite from Zambia is mainly due to optical absorption at 515 nm related to Mn³⁺ ions. The blue color of Brazilian elbaite is related to Cu²⁺ absorption bands at 695 nm and 920 nm. Spin Hamiltonian parameters of Cu²⁺ calculated from the angular dependence of the EPR spectra are: g _x = 2.054, g _y = 2.092, g _z = 2.374; A _x = 27.8·10^?4 cm^?1, A _y = 59.3·10^?4 cm^{? 1}, A _z = 133.2·10^?4 cm^?1. We propose that Cu²⁺ ions enter into Y octahedra with common edges; the symmetry of these Y octahedra is lowered because of local disorder induced by occupancy of the Y site by cations of very different size and charge, such as Li⁺, Al³⁺, and Cu²⁺.     

Exchange bias in Ni4 single-molecule magnets

The syntheses and physical properties are reported for three single-molecule magnets (SMMs) with the composition [Ni(hmp)(ROH)Cl]₄, where R is CH₃ (complex 1), CH₂CH₃ (complex 2) or CH₂CH₂C(CH₃)₃ (complex 3) and hmp⁻ is the monoanion of 2-hydroxymethylpyridine. The core of each complex is a distorted cube formed by four Ni^II ions and four alkoxide hmp⁻ oxygen atoms at alternating corners. Ferromagnetic exchange interactions give a S=4 ground state. Single crystal high-frequency EPR spectra clearly indicate that each of the complexes has a S=4 ground state and that there is negative magnetoanisotropy, where D is negative for the axial zero-field splitting DŜ_z². Magnetization versus magnetic field measurements made on single crystals with a micro-SQUID magnetometer indicate these Ni₄ complexes are SMMs. Exchange bias is seen in the magnetization hysteresis loops for complexes 1 and 2.     

A single crystal EPR study of VO ions doped in Cs2Co(SO4)2.6H2O Tutton salt

EPR spectra of VO²⁺ ions doped in single crystals of Cs₂Co(SO₄)₂.6H₂O single crystals have been studied at various temperatures (390–103 K) on X-band frequency. The detailed EPR analysis shows three vanadyl complexes with differing intensities. The g and A tensors are found to be axially symmetric. The intense vanadyl complexes in the lattice are found to occupy the Co²⁺ substitutional sites, whereas the weak vanadyl complex at the interstitial sites. The optical absorption spectrum at room temperature shows three absorption bands characteristic of VO²⁺ ions in tetragonal symmetry. By correlating the EPR and optical data, the molecular bonding coefficients and the Fermi contact interaction terms have been evaluated and discussed. The line broadening of VO²⁺ spectra on cooling the crystal is explained on the basis of spin-lattice relaxation narrowing. The spin-lattice relaxation time for the host Co²⁺ ions has been estimated at various temperatures.     

Magnetic investigation of Er(C2O4) (C2O4)·3H2O

Single crystal susceptibilities of Er(C₂O₄) (C₂O₄H)·3H₂O are reported over the 1.5–20 K interval, and EPR spectra at 4.2 K of Y (C₂O₄) (C₂O₄H·3H₂O doped with Er³⁺ are also reported. The susceptibilities follow the CurieWeiss law, with g_| = 12.97 ± 0.05, g_⊥ = 2.98 ± 0.05, θ_| = ?0.25 ± 0.05 K, and θ_⊥ = ?0.12 ± 0.05 K.