Anti-phase domain structuring in oxide ferroelectrics: Physical properties and application

The results from studies of domain borders by the optical and acoustic topography methods in LiNbO₃:Fe samples are presented. The formation of Fe ion concentration gradients, a change in the refractive index, and a gradient of the photoinduced field and parameters of acoustic waves near the domain borders were observed. It is shown that main reason for the distortion of crystalline structure parameters is the formation of photoinductive field gradients of spatially separated Fe₂₊ and Fe₃₊ ions.     

Mössbauer spectra of Fe3+ and Fe2+ ions in hexagonal ferrite with w-structure

The hexagonal ferrite Fe₂W = BaFe₂²⁺Fe³⁺₁₆O₂₇ exhibits a sharp ⁵⁷Fe Mössbauer spectrum at 300 and 78 K. All seven sublattices in this complicated crystal structure are detected. Fast electron exchange between Fe²⁺ and Fe³⁺ ions gives rise to sharp lines and makes them indistinguishable. At 5 K the exchange is slow and the Fe²⁺ ions are detected from the presence of a weak subspectrum with broadened lines separated from the main spectrum of the Fe³⁺ ions. Analysis shows that the Fe²⁺ ions reside exclusively on one of the seven sublattices, which is occupied statistically by Fe²⁺ and Fe³⁺ ions in the ratio of 2 : 1. For SrFe²⁺₂Fe³⁺₁₆O₂₇ the situation is the same.     

Arrangement of ferric and ferrous ions at low temperatures in FeCl3 intercalated graphite

Previous work has shown that electrons donated by the graphite to the FeCl₃ intercalant freeze onto some Fe³⁺ ions to change them into Fe²⁺ ions at low temperatures. We show that the created Fe²⁺ ions are not randomly situated. Indirect evidence suggesting the formation of a specific chemical superlattice of Fe³⁺ and Fe²⁺ ions is presented. A method for direct observation of this superlattice is suggested.     

Assessment of role of iron ions on the physical and spectroscopic properties of multi-component Na2O−PbO−Bi2O3−SiO2 glass ceramics

Multi-component glass ceramics composition Na₂O?PbO?Bi₂O₃?SiO₂ doped with different concentrations of Fe₂O₃ as nucleating agent were characterised by XRD, SEM (scanning electron microscope) and DTA (differential thermal analysis) techniques. Optical absorption, EPR, FTIR and Raman studies are also carried out on these glass ceramics. Absorption bands observed at about 457, 489, 678 and 820 nm are the characteristics of Fe³⁺ ions whereas the band observed at about 964 nm is due to Fe²⁺ ions. EPR studies suggested that Fe³⁺ ions entered in the lattice as tetragonally distorted octahedral symmetry or rhombic sites at low concentration of Fe₂O₃, whereas at higher concentration of Fe₂O₃ (beyond 1 mol%), the super exchange type of interactions between multivalency iron ions begin to dominate. FTIR and Raman spectra have revealed the behaviour of various structural units in the glass ceramic matrix. The analysis of these spectroscopic studies indicates that iron ions do exist in Fe³⁺ and Fe²⁺ state.     

The crystal field and exchange coupling in iron group metal carbonates

The wave functions of Co²⁺ and Fe²⁺ ions near the ground state in the CaCO_3? type lattice have been calculated from EPR data in the Abragam-Pryce approximation. The orbital angular momentum contributions to the anisotropic and antisymmetric parts of exchange coupling are determined assuming that this interaction between the magnetic ions occurring in nonequivalent positions is isotropic with respect to spin orientations. It is shown that, in the given approximation, the exchange coupling components in the basal plane for such Fe²⁺?Fe²⁺ and Co²⁺? Fe²⁺ ion pairs are missing. This fact explains the uniaxial antiferromagnetic ordering in FeCO₃ and the presence of a low-lying oscillation branch for Fe²⁺ impurity ions in antiferromagnetic CoCO₃. The EPR spectra of exchange-coupled Co²⁺?Co²⁺, Fe²⁺?Fe²⁺, and Co²⁺?Fe²⁺ pairs occupying nonequivalent positions have been calculated and their parameters have been numerically estimated.     

X-ray photoelectron and mössbauer spectroscopy studies of the valence state of transition metal ions in Co1–x Fe x Cr2O4 (x = 0.1, 0.2, 0.5) ceramics

The valence state of transition metal ions in the Co_1–x Fe _x Cr₂O₄ (x = 0.1, 0.2, 0.5) system has been investigated using X-ray photoelectron and Mössbauer spectroscopy. It has been shown that, in this system, there are Fe²⁺ and Fe³⁺ ions. The relative Fe³⁺/Fe²⁺ contents have been determined by fitting the experimental Fe 2p photoelectron spectra by a superposition of theoretical spectra of the Fe²⁺ and Fe³⁺ ions, as well as using Mössbauer spectroscopy.     

Associated and single iron ions in ion-exchanged faujasite zeolites

Different forms of extraframework ions present in faujasite zeolite prepared by Fe²⁺ ion exchange at different pH values and with subsequent reduction in hydrogen have been studied. “Low-coordination” Fe_tetr ²⁺ ions can be found either in the form of single ions accessible for chemisorption, or in the form of ion associations from which centers of the catalytic processes can be formed in the CO+H₂ reaction. “High-coordination” Fe_oct ²⁺ ions are not affected either by chemisorption or by CO+H₂ treatment.     

Mössbauer spectroscopy of a semiconductive phosphate glass (10V2O5 30Fe2O3 60P2O5 at low temperature

In order to clarify the difference between the local structures of Fe³⁺ and those of Fe²⁺ ions in a semiconductive phosphate (10V₂O₅-30Fe₂O₃-60P₂O₅) glass, the temperature dependencies of the isomer shift (IS), quadrupole splitting (QS) and absorption area (AR) for Fe³⁺ and Fe²⁺ ions were measured. Debye temperatures ( _D) for Fe³⁺ and Fe²⁺ ions were determined to be 318 ± 29 K and 223 ± 18 K, respectively, from the temperature dependence of the absorption area (AR). From the temperature dependence of the quadrupole splitting(QS), B strength parameters for Fe³⁺ and Fe²⁺ ions, which are the coefficients for theT ^3/2 term, were deduced. It was found that in this phosphate glass, Debye temperatures _D(AR) were consistent with those obtained using the relation of the B parameter to Debye temperature described in the literature. Also from the temperature dependence of the isomer shift, the difference between the thermal effects except the second-order Doppler shift for Fe³⁺ ions and those for Fe²⁺ ions was compared.     

Charge disproportionation induced exchange bias in La0.5Ca0.5FeO3-δ

We observed an exchange bias effect in La0.5Ca0.5FeO3 perovskite compound.The exchange bias is associated with the charge disproportionation transition from Fe4+ions to Fe3+and Fe5+ions below 175 K.The competition between the ferromagnetic interaction of Fe3+and Fe5+ions and the antiferromagnetic one of Fe3+and Fe3+ions results in a unidirectional anisotropy in the cluster-glass system.An antiferromagnetically interfacial exchange coupling constant Ji1.95 meV at the cluster-glass region was yielded by fitting the cooling field-dependence of the exchange bias field.     

Mössbauer spectra and electrical conductivity ofxFe2O3−(40-x)V2O5−60P2O5 glasses

Mössbauer spectra and electrical conductivities were measured for the purpose of studying on the conduction mechanism of xFe₂O₃?(40-x)V₂O₅?60P₂O₅ glasses. The ratios Fe²⁺/Fe²⁺+Fe³⁺ in xFe₂O₃?(40-x)V₂O₅?60P₂O₅ glasses were determined by Mössbauer spectroscopy. On the composition dependence of D. C. conductivities in these glasses, the minimum of log σ at each temperature was obtained at 25Fe₂O₃?15V₂O₅?60P₂O₅. The conductivities of ternary glassses in Fe rich region could be explained only by the changes of carrler (Fe²⁺) concentration and the hopping conduction between Fe²⁺ ions and Fe³⁺ ions in binary glasses. In V rich region, the saturation tendency of D. C. conductivities are observed. It was suggested to be explained by increasing of V⁴⁺ ions due to the influence of Fe ions.     

Valence states of iron ions in nanostructured yttrium iron garnet Y3Fe5O12 studied by means of soft X-ray absorption spectroscopy

Nanostructured samples of yttrium iron garnet Y₃Fe₅O₁₂ obtained by plastic deformation method (high-pressure torsion) were studied with help of soft X-ray absorption spectroscopy using Fe 2p and O 1s spectra. Experimental spectra were compared with crystal field multiplet calculations for Fe ions. Some amount of Fe²⁺ ions in nanostructured Y₃Fe₅O₁₂ was found. The concentration of Fe²⁺ ions was found to be increased with the increase of the degree of plastic deformation.     

Twinned EPR spectra of Fe centers in ternary layered TlGaS2 crystal

TlGaS₂ single crystal doped by paramagnetic Fe³⁺ ions has been studied by electron paramagnetic resonance (EPR) technique. The fine structure of EPR spectra of paramagnetic Fe³⁺ ions was observed. The spectra reveal a nearly orthorhombic symmetry of the crystal field (CF) on the Fe³⁺ ions. Two groups each consisting of four equivalent Fe³⁺ centers were observed in the EPR spectra. The local symmetry of the crystal field on the Fe³⁺ centers and CF parameters were determined. Experimental results indicate that the Fe ions substitute Ga at the center of the GaS₄ tetrahedrons. The rhombic distortion of the sulfur ligand CF is attributed to the effect of Tl ions located in the trigonal cavities between the tetrahedral complexes. The observed twinning of the resonance lines indicates a presence of two non-equivalent positions of Tl ions that confirms their zigzag alignment in the TlGaS₂ crystal structure.     

Efficiency and purity control in the preparation of pure and/or aluminum-doped barium ferrites by hydrothermal methods using ferrous ions as reactants

The synthesis of hexagonal barium ferrite (BaFe₁₂O₁₉) was studied under hydrothermal conditions by a method in which a significant amount of ferrous chloride was introduced alongside ferric chloride among the starting materials. Though all of the Fe²⁺ ions in the starting material were converted to Fe³⁺ ions in the final product, Fe²⁺ was confirmed to participate differently from the Fe³⁺ used in the conventional method in the mechanism of forming barium ferrite. Indeed the efficiency of the synthesis and the quality of the product and the lack of impurities such as Fe₂O₃ and BaFe₂O₄ were improved when Fe²⁺ was included. However, the amount of ferrous ions that could be included to obtain the desired product was limited with an optimum ratio of 2:8 for FeCl₂/FeCl₃ when only 2 h of reaction time were needed. It was also found that the role of trivalent Fe³⁺ could be successfully replaced by Al³⁺. Up to 50% of the iron could be replaced by Al³⁺ in the reactants to produce Al-doped products. It was also found that the ratio of Fe²⁺/M³⁺ could be increased in the presence of Al³⁺ to produce high quality barium ferrite.     

Photomagnetic effects and disaccommodation in Co-doped YIG

Y₃Fe₅O₁₂ (YIG) doped with ≦ 0.01 Co per molecule in combination with small dopes of V or (and) Mn show a decrease of the permeability in the dark (disaccommodation, DA) as well as when irradiated (photomagnetic effect, PE) with white light. All materials investigated exhibit DA and PE at 77 K, whereas in some cases effects occur at room temperature. The origin is attributed to domain wall pinning by magnetically anisotropic Co²⁺ ions that exchange electrons with Co³⁺ or other (V⁵⁺, Mn³⁺) ions. In view of the wide separation between the cobalt ions (?30 Å) the charge transport via iron ions is supposed to play an essential part.For a sample with higher cobalt dope the shape of the hysteresis loop at 77 K changes in the dark and the change can be hastened by irradiation. This phenomenon is attributed to the growth of a uniaxial anisotropy in the bulk of the material by Co²⁺, Co³⁺ ordering.From the electric resistivity of certain V and Si doped YIG materials it is concluded that V⁵⁺ ions oxidize Fe²⁺ ions according to V⁵⁺+Fe²⁺ → V⁴⁺+Fe³⁺.     

Spectroscopic properties of Fe ions at tetragonal sites—Crystal field effects and microscopic modeling of spin Hamiltonian parameters for Fe (S=2) ions in K2FeF4 and K2ZnF4

Magnetic and spectroscopic properties of the planar antiferromagnet K₂FeF₄ are determined by the Fe²⁺ ions at tetragonal sites. The two-dimensional easy-plane anisotropy exhibited by K₂FeF₄ is due to the zero field splitting (ZFS) terms arising from the orbital singlet ground state of Fe²⁺ ions with the spin S=2. To provide insight into the single-ion magnetic anisotropy of K₂FeF₄, the crystal field theory and the microscopic spin Hamiltonian (MSH) approach based on the tensor method is adopted. Survey of available experimental data on the crystal field energy levels and free-ion parameters for Fe²⁺ ions in K₂FeF₄ and related compounds is carried out to provide input for microscopic modeling of the ZFS parameters and the Zeeman electronic ones. The ZFS parameters are expressed in the extended Stevens notation and include contributions up to the fourth-order using as perturbation the spin-orbit and electronic spin-spin couplings within the tetragonal crystal field states of the ground ⁵D multiplet. Modeling of the ZFS parameters and the Zeeman electronic ones is carried out. Variation of these parameters is studied taking into account reasonable ranges of the microscopic ones, i.e. the spin-orbit and spin-spin coupling constants, and the energy level splittings, suitable for Fe²⁺ ions in K₂FeF₄ and Fe²⁺:K₂ZnF₄. Conversions between the ZFS parameters in the extended Stevens notation and the conventional ones are considered to enable comparison with the data of others. Comparative analysis of the MSH formulas derived earlier and our more complete ones indicates the importance of terms omitted earlier as well as the fourth-order ZFS parameters and the spin-spin coupling related contributions. The results may be useful also for Fe²⁺ ions at axial symmetry sites in related systems, i.e. Fe:K₂MnF₄, Rb₂Co_1−xFe_xF₄, Fe²⁺:Rb₂CrCl₄, and Fe²⁺:Rb₂ZnCl₄.     

On the magnetic interaction betwenn Fe and Ni ions occupying octahedral sites in oxyspinels

The positive asymptotic Curie temperatures observed by Blasse on GeNi_2−2χ Fe_χ O₄ with spinel structure are attributed to the presence of Fe³⁺ ions on tetrahedral sites. From the paramagnetic susceptibility of Zn_1.4 Ni_0.4 Fe_0.4 Ti_0.8O₄ it is concluded that the interaction between Fe³⁺ and Ni²⁺ ions occupying octahedral sites is weakly negative.     

Monitoring synchrotron X‐ray‐induced radiolysis effects on metal (Fe,W) ions in high‐temperature aqueous fluids

Radiolysis‐induced effects on aqueous tungsten ions are observed to form a precipitate within seconds upon exposure to a synchrotron X‐ray micro‐beam in a WO₃ + H₂O system at 873 K and 200 MPa. In situ Fe K‐edge energy‐dispersive X‐ray absorption spectroscopy (ED‐XAS) measurements were made on Fe(II)Cl₂ aqueous solutions to 773 K in order to study the kinetics of high‐temperature reactions of Fe²⁺ and Fe³⁺ ions with transient radiolysis species. The radiolytic reactions in a fluid sample within a hydrothermal diamond anvil cell result in oxidation of the Fe²⁺ ion at 573 K and reduction of Fe³⁺ at temperatures between 673 and 773 K and of the Fe²⁺ ion at 773 K. The edge‐energy drift evident in the ED‐XAS data directly reflects the kinetics of reactions resulting in oxidation and/or reduction of the Fe²⁺ and Fe³⁺ ions in the aqueous solutions at high temperatures. The oxidation and reduction trends are found to be highly consistent, making reliable determinations of reaction kinetics possible.     

Vibrations and chemical states of iron ions in soda‐lime glasses determined by element‐specific X‐ray analyses

We systematically study medium‐range structures including more than three neighboring atoms around iron ions (Fe²⁺ and Fe³⁺) in soda‐lime glass samples with low iron oxide concentrations (M_Fe2O3) and a wide number ratio of Fe²⁺ to all iron ions (Fe²⁺/Σ_nFe). The precise medium‐range structures around iron ions in glass have not yet been revealed because of a lack of the appropriate measurement methods. To avoid this problem, we used element‐specific nuclear resonant inelastic scattering (NRIS) with synchrotron X‐rays to observe the vibrations of iron ions (⁵⁷Fe). The vibrations are related to medium‐range structures with more than three neighboring atoms and to the potential asymmetry and the coordination environment, around iron ions. The NRIS method has high sensitivity and can measure over a wide concentration range. Linear combination fitting of the X‐ray absorption fine structure spectra, which measures only the first neighbors but is a faster than using the NRIS method, was also used additionally. A systematically produced set of glasses with 0.015–5 wt% M_Fe2O3 and 0–0.85 Fe²⁺/Σ_nFe was measured with these methods. It was found that the soda‐lime glass possessed two different medium‐range structures with different iron ion valences (~2+ or ~3+), which were determined by the Fe²⁺/Σ_nFe, and that these structures were generated during production of the glass. Moreover, these medium‐range structures were the same from 0.015 to 5 wt% M_Fe2O3.     

Anisotropy of hyperfine field in diamagnetically substituted YIG

The results of the nuclear magnetic resonance of ⁵⁷Fe in Al-, Ga- and In-substituted Y₃Fe₅O₁₂ garnets are reported. When diamagnetic Ga³⁺ or Al³⁺ ions are substituted for tetrahedral Fe³⁺ ions, four satellite lines in the NMR spectrum of octahedral Fe³⁺ ions appear. The substitution of In³⁺ ions for octahedral Fe³⁺ ions leads to three satellite lines in the spectrum of tetrahedral Fe³⁺ ions. The hyperfine fields of these satellites are explained using the independent bond model for the hyperfine interaction. Suprisingly large change of the anisotropic part of the hyperfine field relative to the isotropic one was found.     

Mössbauer effect study of nonstoichiometric CoNiZn ferrites

The existence of fast electron exchange between pairs of Fe³⁺?Fe²⁺ ions was detected in Mössbauer spectra of non-stoichiometric CoNiZn ferrites containing an excess of Fe₂O₃. Analysis of spectra from a number of samples of various preparations showed correlation of bulk magnetic properties with hyperfine parameters of ferric ions at octahedral sites in the spinel structure.