Electron paramagnetic resonance and photoluminescence properties of α-Al2O3:Cr3+ phosphors

The red emitting Cr³⁺ activated α-Al₂O₃ powder phosphor has been prepared by easy combustion reactions from mixed metal nitrate reactants and urea with ignition temperatures of 500 °C. The as-synthesized powder was characterized by X-ray diffraction, scanning electron microscopy, and Fourier-transform infrared techniques. The X-ray diffraction pattern reveals that the phosphor crystallized in the hexagonal α-Al₂O₃ phase directly from the combustion reaction. The EPR spectrum exhibits an intense resonance signal with effective g value at g=3.33 along with a few weak resonance signals with effective g values at g=13.7, 2.34, 1.95, 1.49, and 1.26. The spin concentration (N) and its paramagnetic susceptibility (χ) have been evaluated. The excitation spectrum consists of two broad intense bands at 415 nm and 555 nm and are assigned to ⁴A_2g (F)→⁴T_1g (F) and ⁴A_2g (F)→⁴T_2g (F) transitions, respectively. The intense fluorescence peak around 691 nm is attributed to ²E _g →⁴A_2g transition of Cr³⁺ ion. By correlating EPR and optical data, the crystal field splitting parameter (Dq), Racah inter-electronic repulsion parameter (B) have been evaluated and discussed. The EPR and optical studies reveal that Cr³⁺ ions are occupying in Al³⁺ sites in octahedral coordination.     

Simultaneous electrochemical and electron paramagnetic resonance studies of fullerene anion radicals C 60 1− and C 60 3−

Simultaneous electrochemical and electron paramagnetic resonance experiments have been carried out on the reduced C₆₀ fullerene to examine theg-factor assignment of the radical species. C ₆₀ ^1? and C ₆₀ ^3? show the following EPR characteristics at room temperature: C ₆₀ ^1? :g _1?=2.0002±0.0001, 2ΔB _1s=0.17 mT, and C ₆₀ ^3? :g _{3?=2.0008±0.0002}, 2ΔB _1s=0.07 mT. EPR linewidths are apparently narrower compared to those in most of the spectra previously reported. Variable temperature EPR study on solution containing C ₆₀ ^1? has shown thatg _1? value is not while the linewidth is only slightly temperature dependent.     

Electron spin resonance and luminescence in Ga2−xEuxS3 single crystals

In present work electron spin resonance (ESR) and luminescence have been studied in Ga_2−xEu_xS₃ single crystals. The ESR and photoluminescence (PL) spectra of Ga_2−xEu_xS₃ were observed and the intensity of these spectra has increased linearly with the Eu concentrations in the samples. At the low temperature of 3.8 K in Ga_2−xEu_xS₃ single crystals, when the magnetic field has been applied as the perpendicular to the direction of [110], the hyperfine structure of ESR spectra has been observed. The exchange interaction of Eu²⁺ atoms in Ga_2−xEu_xS₃ single crystals have been determined as, g=4.2. The PL spectrum of pure Ga₂S₃ single crystals was recorded at 4.2 K and consisted of a narrow high-energy band at 2.53–2.38 eV and a low-energy band at 2.14–1.59 eV. Ga_2−xEu_xS₃ resulted in complete quenching of the impurity luminescence bands gave rise to a green europium band (2.38–2.14 eV) in the luminescence spectrum. Crystals of Ga_2−xEu_xS₃ emitted bright green electroluminescence (EL) spectrum when they excited with static and alternating electric fields at 77 K. We have also determined the field and frequency dependences of the EL.     

Electron paramagnetic resonance and optical spectroscopy of K3Na(CrO4)2 ferroelastics activated by MnO 4 2− molecular impurity ions

Crystals of a proper ferroelastic K₃Na(CrO₄)₂ containing molecular impurity ions MnO ₄ ^2? are studied using electron paramagnetic resonance (EPR) and optical spectroscopy. The EPR spectrum of the Mn⁶⁺ ion contained in the molecular impurity ion MnO ₄ ^2? is identified at low temperatures (T ≤ 20 K). The intensity of this spectrum decreases unusually fast as the temperature increases. A broad IR luminescence band with a vibronic structure well resolved at a temperature of 8 K is revealed. Theoretical treatment of the Mn⁶⁺ ion involved in the molecular impurity ions MnO ₄ ^2? of the K₃Na(CrO₄)₂ ferroelastic crystal suggests that an important role in this case is played by the pseudo-Jahn-Teller. The pseudo-Jahn-Teller effect offers an explanation for the appearance of a fine structure in the vibronic replicas in the luminescence spectrum, on the one hand, and accounts for the fast decrease in the intensity of the EPR spectrum of K₃Na(CrO₄)₂: MnO ₄ ^2? with increasing temperature, on the other.     

Electron paramagnetic resonance of Gd3+ ions in Ca1 − x − y Y x Gd y F2 + x + y crystals

Electron paramagnetic resonance of Ca_{1 ? x ? y} Y _x Gd _y F_{2 + x + y} single crystals has revealed spectra that are not typical of gadolinium-doped CaF₂ crystals. These spectra have a nearly tetragonal symmetry and are most probably caused by Gd³⁺ ions localized in yttrium clusters. Weak spectra of tetragonal Gd³⁺ centers, whose parameters are close to those of a cubic gadolinium center caused by an isolated Gd³⁺ ion, have been also detected. These centers are attributed to isolated Gd³⁺ ions localized near octahedral rare-earth clusters or their associations.     

A Magnetic resonance study of La1−x SrxMnO3 manganites

Single crystals of La_0.9Sr_0.1MnO₃ and La_0.8Sr_0.2MnO₃ manganites are examined using magnetic resonance in the temperature range 80–370 K. It is found that magnetostatic oscillations arise near the Curie temperature. The possible reasons for the appearance of additional lines in the ferromagnetic resonance (FMR) spectrum are considered, and the anisotropy field and the type of crystalline magnetic anisotropy in the La_0.8Sr_0.2MnO₃ compound are determined. It is shown that the crystalline magnetic anisotropy in the La_0.9Sr_0.1MnO₃ compound exhibits specific features associated with its type of crystal structure.     

Electron paramagnetic resonance of globin proteins – a successful match between spectroscopic development and protein research

At the start of the twenty-first century, the research into the haem-containing globins got a considerable impetus with the discovery of three new mammalian globins: neuroglobin, cytoglobin and androglobin. Globins are by now found in all kingdoms of life and, in many cases, their functions are still under debate. This revival in globin research increased the demand for adequate physico-chemical research tools to determine the structure-function relationships of these proteins. From early days onwards, electron paramagnetic resonance (EPR) has been used in globin research. In recent decades, the field of EPR has been revolutionised with the introduction of many new pulsed and high-field EPR techniques. In this review, we highlight how EPR has become an essential tool in globin research, and how globins equally provide ideal model systems to push technical developments in EPR.     

Local order parameters at a dipolar Gd3+-O2− center in CsSrCl3 and paramagnetic resonance

The angular dependence of the EPR spectrum of a tetragonal (in the paramagnetic phase) Gd³⁺-O²⁻ center with large initial splitting is used to determine the rotation angles of the neighboring chlorine octahedron, which are related to the components of rotation modes, in the vicinity of room temperature. Aspects of the anomalous broadening of the EPR lines near structural transformations are discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 247–251 (February 1999)     

Electron paramagnetic resonance studies on manganite Pr0.5Sr0.5Mn1−x Ga x O3 (x=0 and 0.05)

In this paper, we present the investigations of electron paramagnetic resonance on perovskite manganite Pr_0.5Sr_0.5MnO₃ and Ga-doped Pr_0.5Sr_0.5Mn_0.95Ga_0.05O₃. The temperature dependent paramagnetic resonance spectra parameters (effective g-factor, peak-to-peak linewidth ΔH _pp and double integrated intensities) have been used to study the paramagnetic spin correlations and spin dynamics. The gradual increase of effective g-factor is attributed to the presence of orbital ordering above T _C. The model fittings of temperature dependent double integrated intensities reveal Arrhenius law is appropriate for describing Pr_0.5Sr_0.5Mn_0.95Ga_0.05O₃ instead of Pr_0.5Sr_0.5MnO₃ system. As for Pr_0.5Sr_0.5MnO₃, the broadening of linewidth with the temperature increase origins from the contribution of small polaron hopping in the PM regime. However, as for Pr_0.5Sr_0.5Mn_0.95Ga_0.05O₃, the broadening of EPR linewidth can be understood with the spin-lattice relaxation mechanism.     

Electron spin resonance study of γ-irradiated single crystal and powder of LiH3(SeO3)2 and LiD3(SeO3)2

Gamma-irradiated monoclinic single crystals and powders of LiH₃(SeO₃)₂ and LiD₃(SeO₃)₂ have been investigated at room temperature by ESR in both the X and the Q band. Twelve species consisting of six pairs of inequivalent radicals have all been identified as SeO₂⁻-type radicals pairwise located in two different sites. The origin of the difference in the principal values of the g-tensors for the radicals is attributed to a difference between the SeO bond lengths, which in fact gives a lower symmetry of the radicals. The generation of the radicals has been explained as a rupture of a SeO bond, which can occur in six different ways due to the crystal structure. The orientations of the radicals in the crystal are different compared to corresponding SeO₂ groups in the host crystal, and this phenomenon presumably depends either on a reorientation of the radicals or on a radical reaction. One extra splitting of peaks originating from two of the radicals in LiH₃(SeO₃)₂ is absent in LiD₃(SeO₃)₂, thus indicating hydrogen bonding to the radicals.     

High-field electron paramagnetic resonance study of the polymerization in Na2Rb1−x Cs x C60

We report on the study of the magnetic properties of the low-temperature polymer phases in N₂Rb_1?xCs_xC₆₀ at 110 GHz microwave frequency withx varying between 0 and 1. The magnetic and structural properties of the polymer phase strongly depend on the Cs content and its electronic structure progressively becomes quasi-one-dimensional asx is increased. While the electronic properties of the polymeric Na₂RbC₆₀ appear to be close to three-dimensional metal, Na₂Rb_0.3Cs_0.7C₆₀ shows characteristics of quasi-one-dimensional metal where instability in the electronic structure was found as detected by the sudden disappearance of the ESR intensity due to the opening of the gap at the Fermi surface. The observation of an additional resonance line below 15 K, which could be attributed to antiferromagnetic resonance, suggests that the low-temperature polymeric phase in Na₂Rb_0.3Cs_0.7C₆₀ has a well-defined magnetic ground state.     

Electron paramagnetic resonance of the two-dimensional system Mn(CH3COO)2·4H2O

A single crystal Electron Paramagnetic Resonance (EPR) study of manganese acetate tetrahydrate was carried out at Q-band and room temperature. The EPR spectrum always reduces to a Lorentzian singlet with g=2.008. The maximum of linewidth observed along a¹ reveals the spin diffusion effects in this two-dimensional system. The maximum of linewidth observed along b reveals the exchange narrowed dipolar interactions in the linear trimers, as well as the contribution of the fine structure terms.     

Electron paramagnetic resonance and spinlattice relaxation of Cu2+ ions in ZnSeO4·6H2O

In this paper, we present detailed studies of the EPR spectra of Cu²⁺ ions in single crystals of ZnSeO₄·6H₂O. We describe the spectrum with a rhombic spin Hamiltonian with the following parameters: g_z=2.427; g_y=2.095; g_x=2.097; A _z ⁶⁵ =138.4·10^?4 cm^?1; A _x ⁶⁵ =22.3·10^?4 cm^?1. We studied spin-lattice relaxation in the temperature range 4–300 K at the frequency v≈9.3 GHz. The measured spin-lattice relaxation rate for the orientation H∥L₄ is described well at T<5 K by a linear dependence, while at T>5 K it is described by the sum of three exponentials: $$T_1^{ - 1} = 0.27T + 3.3 \cdot 10^{\text{s}} \exp \left( {\frac{{ - 69.5}}{T}} \right) + 2.6 \cdot 10^7 \exp \left( {\frac{{ - 140}}{T}} \right) + 1.36 \cdot 10^{10} \exp \left( {\frac{{ - 735.6}}{T}} \right){\text{ sec}}^{{\text{ - 1}}} $$ .We discuss possible reasons for the exponential dependence of T ₁ ^?1 for the Raman process.     

Electron paramagnetic resonance of Gd3+ in Nd2(SO4)3 · 8H2O single crystal

Electron paramagnetic resonance study has been carried out in Gd3+ doped single crystals of Nd₂(SO₄)₃·8H₂0 at three different temperatures. Two magnetic complexes exhibiting orthorhombic or lower symmetry are found and the results have been fitted to a suitable spin-Hamiltonian. The ten “best fit” parameters are obtained from two computer programs. Zero-field splittings of Gd3+ have been deduced from spin-Hamiltonian parameters and are compared with those observed directly by Bogle and Symmons. Fine forbidden transitions ∥ΔM_s∥ I > 1 are analyzed using numerical solution of the Hamiltonian matrix.     

Electron spin resonance and structure of the ionic radical, ·PO3 =

It is shown that the ionic radical, ·PO₃ ⁼, is formed by the action of γ-rays on disodium ortho-phosphite pentahydrate. The hyperfine coupling to the ³¹P nucleus has principal values of 1967, 1514 and 1513 Mc/s and is consistent with a pyramidal ion having OPO angles of 110°.     

Electron spin resonance study of the dynamic magnetic susceptibility of CuO, Cu1−x Zn x O, and Cu1−x Li x O single crystals

Results are presented of studies of the dynamic magnetic susceptibility of CuO, Cu_1?x Zn _x O (x ≈ 1.5%), and Cu_1?x Li _x O (x ≈ 1%) single crystals. The orientational dependence of the ESR spectra was investigated at room temperature. The results for CuO are analyzed using a model of a quasi-one-dimensional antiferromagnet (S = 1/2) with anisotropic exchange interaction between Cu²⁺ spins in the chains and exchange coupling between the chains allowing for one-dimensional spin diffusion and spinon excitations. The estimated line width is of the same order of magnitude as the experimental data. Substituting Cu with Zn scarcely alters the spin dynamics of the Cu²⁺ ions, as in weakly diluted magnets. Lithium doping substantially increases the ESR line width and this is attributed to excess holes forming rapidly relaxing spin complexes with copper ions.