Theoretical study of local lattice structure of Fe-V cd and Fe-Li systems in RbCdF3 and CsCdF3 crystals

A theoretical method for investigating the inter-relation between the electronic and the molecular structures of a 3d⁵ ion in a tetragonal ligand-field has been established on the basis of a 252×252 complete energy matrix. By means of this method, the local structure of the Fe³⁺-V _cd and Fe³⁺-Li⁺ systems in RbCdF₃:Fe³⁺ and CsCdF₃:Fe³⁺ crystals are determined by the experimental EPR spectrum. Our calculation show that the local lattice structure around an octahedral Fe³⁺ center has a compression distortion along the crystalline axis in RbCdF₃ as well as in CsCdF₃ crystals, and that the compression magnitude of a tetragonal Fe³⁺-Li⁺ system is larger than that of the Fe³⁺-V _cd system. This may be ascribed to the fact that a Fe³⁺ ion replaces a Cd²⁺ ion and a Li⁺ ion substitutes for another Cd²⁺ ion next to the Fe³⁺ ion in the Fe³⁺-Li⁺ system, and the Li⁺ ion will shift to the Fe³⁺ ion, which pushes the F₁ ion toward the Fe³⁺ ion. Using this method, the experimental EPR parameters , and are also interpreted simultaneously.     

A rapid combustion process for the preparation of MgSrAl10O17:Eu phosphor and related luminescence and defect investigations

Blue-emitting europium-ion-doped MgSrAl₁₀O₁₇ phosphor, prepared using the combustion method, is described. An efficient phosphor can be prepared by this method in a muffle furnace maintained at 500 °C in a very short time of few minutes. The phosphor is characterized by powder X-ray diffraction, scanning electron microscopy, transmission electron microscopy and BET surface area measurements. Photoluminescence (PL) spectra revealed that europium ions were present in divalent oxidation state. The thermoluminescence (TL) glow curve shows two peaks at around 178 and at 354 °C. The defect centres formed in the phosphor are studied using electron spin resonance (ESR). The ESR spectrum indicates the presence of Fe³⁺ ions in the non-irradiated system. Irradiated MgSrAl₁₀O₁₇:Eu exhibits lines due to radiation-sensitive Fe³⁺ ion and a defect centre. The centre is characterized by an isotropic g-value of 2.0012 and is assigned to a F⁺ centre. The radiation-sensitive Fe³⁺ ion appears to correlate with the main TL peak at 178 °C. During irradiation an electron is released from Fe²⁺ and is trapped at an anion vacancy to form F⁺ centre. During heating, an electron is liberated from the defect centre and recombines with Fe³⁺ emitting light.     

Fe3+离子在钠-硼玻璃中的顺磁共振

研究了Fe³⁺离子在钠-硼玻璃中的顺磁共振。在表观g=4.27处,有一条尖锐的大致对称的共振线。实验结果表明Fe³⁺离子代替接触二个钠离子的[BO₄]四面体中的硼离子位置。     

Electronic Structure of Fe3+ at a Metal-Binding Site Introduced in Modified Bacterial Reaction Centers

The electronic structure of Fe³⁺ was studied in a mutant that has been modified to bind manganese or iron at a site corresponding to the manganese-binding site of photosystem II (Kálmán et al., Biochemistry 45:13869–13874, 2006). Using electron paramagnetic resonance spectroscopy, the presence of the oxidized state of the bacteriochlorophyll dimer, P^·+, was detected in the light when no metal was added. When iron was bound to the modified reaction centers in the presence of bicarbonate, the contribution of P^·+ was greatly reduced and a signal characteristic of Fe³⁺ was evident. To characterize the electronic structure of this ferric ion, the electron paramagnetic resonance spectrum was measured at X-band at temperatures from 4 to 200 K. The major contribution to the spectrum at 4 K is from Fe³⁺ with a spin 3/2 in a rhombic coordination and E/D ratio of 0.1914 and g _eff values of 6.0, 2.9, and 2.0. As the temperature increases from 4 to 200 K, the signal shifts, with the central g _eff value changing from 2.9 to 2.2. This change with temperature may result from alterations in the interaction with the bicarbonate coordinated to the iron as the temperature increases.     

8-Hydroxyquinoline Functionalized Graphene Oxide: an Efficient Fluorescent Nanosensor for Zn<Superscript>2+</Superscript> in Aqueous Media

A nanosensor, based on 8-hydroxyquinoline functionalized graphene oxide, was developed for the fluorescence detection of Zn²⁺. It showed high selectivity and sensitivity for Zn²⁺ion in aqueous solution over other metal ions such as Li⁺, Na⁺, Ca²⁺, Mg²⁺, Al³⁺, Cd²⁺, Co²⁺, Cu²⁺, Hg²⁺, Ni²⁺, Pb²⁺, Fe²⁺, Fe³⁺and Cr³⁺. Due to the linearity of the emission intensity toward Zn²⁺ concentration, fluorescent technique could be used for the detection of Zn²⁺ ion even at very low concentrations.     

Defect clusters in titanium-substituted spinel-related lithium ferrite

The cation distribution in spinel-related titanium-substituted lithium ferrite, Li_0.5+0.5xFe_2.5−1.5xTi_xO₄ has been explored using interatomic potential and ab initio calculations. The results suggest that the cation distribution with Ti⁴⁺ substituting for Fe³⁺ on octahedral B sites and excess Li⁺ substituting for Fe³⁺ on tetrahedral A sites is stabilised by the formation of clusters of two octahedrally coordinated Ti⁴⁺ ions and one tetrahedrally coordinated Li⁺ ion linked through a common oxygen.     

Mössbauer study of Na0.82Co0.99Fe0.01O2

We studied by Mössbauer spectroscopy the Na_0.82CoO₂ compound using 1% ⁵⁷Fe as a local probe which substitutes for the Co ions. Mössbauer spectra at T=300 K revealed two sites which correspond to Fe³⁺ and Fe⁴⁺. The existence of two distinct values of the quadrupole splitting instead of a continuous distribution should be related with the charge ordering of Co⁺³, Co⁺⁴ ions and ion ordering of Na(1) and Na(2). Below T=10 K part of the spectrum area, corresponding to Fe⁴⁺ and all of Fe³⁺, displays broad magnetically split spectra arising either from short-range magnetic correlations or from slow electronic spin relaxation.     

Optical characterization of the phosphate glasses containing pair transition ions

The paper deals with optical and electronic properties of the aluminophosphate glasses containing Fe–Mn and Fe–Cr ion pairs in different concentration. The influence of the mixed alkali ions over the electronic properties has been investigated. The optical behavior (optical transmission) of the glass samples has been studied by UV-VIS spectroscopy and the refractive index dependency on wavelength has been discussed. The transmission spectra show features specific for the doping transition ions (TM), revealing different oxidation states of iron (Fe²⁺/Fe³⁺), manganese (Mn²⁺/Mn³⁺) and chromium (Cr³⁺/Cr⁶⁺) in the vitreous network. Mössbauer spectroscopy offers information regarding the TM oxidation states, redox processes and the iron coordination symmetry in the vitreous network. In the case of Fe–Mn doped glasses, the percentage of Fe²⁺ is about 40% and a doubled iron content leads to an increasing of Fe²⁺ percentage up to 53%. The replacing of lithium ions by natrium ions (mixed alkali effect) provides an increasing of the Fe²⁺ percentage up to 56%. The occurrence of the tetrahedral or octahedral symmetry of Fe²⁺ ions bonded by O^2? ions depends on the transition ion nature and Li⁺/Na⁺ ratio. Infrared absorption spectra of the pair transition ions-doped aluminophosphate glasses reveal optical phonons specific for the phosphate glass matrix.     

ESR assessment of 3d7 transition metal impurity states in GaP,GaAs and InP

Photo-sensitive electron spin resonance of the 3d⁷-ions Fe⁺, Co²⁺, Ni³⁺ has been detected and analysed in GaP, GaAs and InP. For GaP : Ni³⁺, hyperfine interaction with the four nearest P³¹-ligands could be resolved.     

EPR,optical absorption and superposition model studies of Fe3+-doped cesium chloride single crystals: a case of substitutional as well as interstitial sites

An electron paramagnetic resonance study of Fe³⁺-doped cesium chloride single crystals was carried out at room temperature. Three sites are observed. The spin Hamiltonian parameters were determined from the angular variation of the observed resonance lines. The hyperfine structure is observed due to the presence of Fe⁵⁷ centers. At site I, Fe³⁺ enters the lattice substitutionally, replacing Cs⁺ in the cubic symmetry of the crystal, whereas at sites II and III, Fe³⁺ enters the lattice interstitially. The local site symmetry of Fe³⁺ in the host lattice is considered to be orthorhombic. An optical absorption study of the crystal was also performed at room temperature. The observed bands were assigned and the Racah inter-electronic repulsion parameters (B and C) and the cubic crystal field splitting parameter (Dq) were determined. On the basis of EPR and optical data, the nature of the metal–ligand bonding in the crystal was determined. The crystal field parameters were evaluated using the superposition model and then used in the microscopic spin Hamiltonian and perturbation equations to determine the zero-field splitting parameters (ZFSPs) theoretically for all sites observed. The theoretical ZFSPs are in good agreement with the experimental values.     

Theoretical studies of the optical and EPR spectra for Fe ion in the MF3:Fe (M=Al, Ga) systems

By analyzing the EPR spectra of Fe³⁺ ion in the fluorinde glasses, the local lattice structures around impurity Fe³⁺ ion in MF₃:Fe³⁺ (M=Al, Ga) systems have been studied by means of diagonalizing the complete energy matrices of the electron-electron repulsion, the ligand-field and the spin-orbit coupling for a d⁵ configuration ion in a trigonal ligand-field. Both the second-order and fourth-order EPR parameters D and (a−F) are taken simultaneously in the structural investigation. The results indicate that the local lattice structure around octahedral Fe³⁺ center has an expansion distortion for Fe³⁺ in MF₃:Fe³⁺ (M=Al, Ga). The expansion distortion may be ascribed to the fact that the radius of Fe³⁺ ion is larger than that of Al³⁺ ion and Ga³⁺ ion, and the Fe³⁺ ion will push the fluoride ligands upwards and downwards, respectively. The local lattice structure parameters R=1.927 A, θ=55.538° for Fe³⁺ in AlF₃:Fe³⁺ and R=1.931 A, θ=56.09° for Fe³⁺ in GaF₃:Fe³⁺ are determined, respectively, and the EPR spectra of the MF₃:Fe³⁺ (M=Al, Ga) systems are satisfactorily explained.     

EPR spectra of Fe centers in layered TlGaSe2 single crystal

A single-crystal TlGaSe₂ doped by paramagnetic Fe ions has been studied at room temperature by electron paramagnetic resonance (EPR) technique. The fine structure of EPR spectra of paramagnetic Fe³⁺ ions was observed. The spectra were interpreted to correspond to the transitions among spin multiplet (S=5/2, L=0) of Fe³⁺ ion, which are splitted by the local ligand crystal field (CF) of orthorhombic symmetry. Four equivalent Fe³⁺ centers have been observed in the EPR spectra and the local symmetry of crystal field at the Fe³⁺ site and CF parameters were determined. Experimental results indicate that the Fe ions substitute Ga at the center of GaSe₄ tetrahedrons, and the rhombic distortion of the CF is caused by the Tl ions located in the trigonal cavities between the tetrahedral complexes.     

Temperature dependence of57Fe hyperfine field in Gd: YIG

The NMR spectra of⁵⁷Fe in Gd: YTG polyeryslallaline samples were measured between liquid-helium and room temperatures. In spectra of⁵⁷Fe on tetrahedral sites, beside the main line, weaker and broader peak at higher frequencies appears. It can be decomposed in to the three satellites corresponding to Fe³⁺ ion with one Gd³⁺ ion in the nearest (two satellites) and next nearest dodecahedral site. The temperature dependence of the stellite resonance frequency indicates, that the part of the transferred hyperfine field on⁵⁷Fe with the Gd³⁺ in the nearest c-site depends on the magnetic moment of Gd³⁺ ion. The resonance frequency of the satellite corresponding to the⁵⁷Fe with Gd³⁺ in the next nearest c-site follows the temperature dependence of the dipolar field, the change of the transferred field is small.     

Photocontrolled magnetic resonance plasticity of γ-irradiated KCl:Fe crystals

A photocontrolled resonance decrease in microhardness, which is due to the application of mutually perpendicular static and microwave fields, in γ-irradiated KCl:Fe crystals has been revealed. It has been found that the magnetic plasticity of unirradiated γ-KCl:Fe crystals is due to the resonance effect of magnetic fields on two types of impurity centers: first, centers containing Fe²⁺-ν _c ion-vacancy pairs and, second, centers containing Fe⁺ ions. The illumination of γ-KCl:Fe crystals with F-light (with a wavelength of λ = 500–600 nm) is accompanied by rearrangement of the spectrum of electron paramagnetic resonance detected by a change in microhardness. The effect of F-light on the spectrum of magnetic resonance plasticity is manifested as the suppression of the spectra of Fe²⁺-ν _c ions with effective g-factors of 7.0 and 3.5 due to their recombination with F-electrons and reconstruction to Fe⁺ centers with g-factors of 2.2 and 4.1.     

Observation of high-energy electrons from a metal target irradiated by protons with a mean energy of 25 keV

Electrons with abnormally high energies of up to 16 keV are detected from an iron target irradiated by ions (H⁺, Fe⁺, Fe²⁺, Fe³⁺) with energies ranging from 20 to 100 keV from the plasma of a high-power femtosecond laser pulse with an intensity of 10¹⁶ J/(s cm²). These electrons indicate that the energy of an incident ion is almost completely transferred to an electron knocked out of the target. In a range of 6–16 keV, the spectrum of electrons knocked out of the K shell of iron atoms by protons with an energy of 22 ± 2 keV is quasi-exponential with an exponent of 4 keV. For 8-keV electrons, the double differential cross section for ionization by such protons is estimated as 10^?7 b/(eV sr).     

Prediction of defect structure in lithiated tin- and titanium-doped α-Fe2O3 using atomistic simulation

The defect structure of lithiated tin- and titanium-doped α-Fe₂O₃ has been assessed using interatomic potential calculations. Of the models considered for lithiation, a model in which Li⁺ occupies an interstitial site balanced by the reduction of Fe³⁺ to Fe²⁺ on an Fe³⁺ site was found to be more favourable than the substitution of Li⁺ on an Fe³⁺ octahedral site balanced by an O²⁻ vacancy. Insertion of lithium into the interstitial site between two adjacent M⁴⁺ ions was particularly favourable. The calculated lattice parameters decrease on lithiation as has been observed experimentally.     

On the transferred hyperfine interaction in substituted yig

NMR of⁵⁷Fe is studied in a number of (M_xY_3–x) Fe₅O₁₂ garnets for small concentrations of M (M is either trivalent RE ion –Ho 3+, Gd 3+, Nd3+, Pr 3+, La 3+ or Bi 3+ ion). Beside the main resonance lines, the satellites were observed, which correspond to those Fe, in vicinity of which the impurity M is located. After correcting for the dipolar field, the field corresponding to the change of the transferred hyperfine interaction in M³⁺–O^2–-Fe³⁺ vs. Y³⁺–O^2–-Fe³⁺ triad was deduced from the satellites splitting. The analysis of the results indicates that the observed change in the transferred hyperfine field is mainly connected with the transfer of electrons between M³⁺ and Fe³⁺ ions and not with the local deformation around the impurity.     

Ferromagnetic ordering of iron impurities in the valence-fluctuating semiconductor SmB6

The electron paramagnetic resonance (EPR) of the valence-fluctuating semiconductor SmB₆ doped by 1 at % Fe is studied. The EPR measurements are performed on a SmB₆ single crystal in a temperature range of 1.6–300.0 K. A number of resonance lines whose g factors indicate the presence of iron ions in the Fe⁰, Fe⁺, Fe²⁺, and Fe³⁺ states have been detected. The iron ions are ferromagnetically ordered below a Curie temperature T = 100 K, and this ordering can be caused by the exchange interaction of impurity ions due to matrix polarization (a similar mechanism is observed in PdFe alloys). This exchange interaction is estimated to be significantly higher than that in PdFe; this fact can result from a very high density of states in the narrow f band, which is characteristic of a valence-fluctuating material.     

Optical absorption, EPR, infrared and Raman spectral studies of clinochlore mineral

Optical absorption, EPR, Infrared and Raman spectral studies have been carried out on natural clinochlore mineral. The optical absorption spectrum exhibits bands characteristic of Fe²⁺ and Fe³⁺ ions. A band observed in the NIR region is attributed to an intervalence charge transfer (Fe²⁺-Fe³⁺) band. The room temperature EPR spectrum of single crystal of clinochlore mineral reveals the dominance of Fe³⁺ ion exhibiting resonance signals at g=2.66; 3.68 and 4.31 besides one isotropic resonance signal at g=2.0. The EPR studies have been carried out for a polycrystalline sample in the temperature range from 103 to 443 K and for a single crystal of clinochlore mineral in the temperature range 123-297 K. The number of spins (N) participating in resonance at g=4.3 signal of the single crystal of clinochlore mineral has been calculated at different temperatures. The paramagnetic susceptibility (χ) is calculated from the EPR data at different temperatures for single crystal of clinochlore mineral. The Curie constant and Curie temperature values are evaluated from 1/χ versus T graph. The infrared spectral studies reveal the formation of Fe³⁺-OH complexes due to the presence of higher amount of iron in this mineral. The Raman spectrum exhibits bands characteristic of Si-O-Si stretching and Si-O bending modes.     

Photoluminescence and atomic force microscopy studies on pre- and post-irradiated ruby with Fe3+ ion

Photoluminescence spectra measured for pristine ruby and its two irradiated samples with Fe³⁺ ion show R₁, R₂, N lines and a broad band. Decrease in intensities of these features is observed with irradiation of Fe³⁺ ion in ruby. Progressive structural changes and modifications on surface of irradiated rubies with Fe³⁺ ion have been observed by atomic force microscopy. Decrease in intensities is discussed in terms of pair formation.