Muhammed Açıkgöz

A study of the impurity structure for 3d3 (Cr3+ and Mn4+) ions doped into rutile TiO2 crystal

The local environment around 3d(3) (Cr(3+) and Mn(4+)) ions doped into rutile TiO(2) crystals has been investigated using superposition model (SPM) analysis. The zero-field splitting (ZFS) parameters (ZFSPs) D and E are modeled for the Cr(3+) and Mn(4+) ions at both the substitutional Ti sites with local symmetry orthorhombic D(2h) and the interstitial sites (ISs) with the same symmetry. Several model parameter sets are adopted so as to acquire the best agreement between the calculated ZFSPs and those measured by electron magnetic resonance (EMR). The feasible values of the structural distortions (ΔR(Y), ΔR(XZ) and Δθ) resulting from dopant Cr(3+) and Mn(4+) ions are determined. As a result, it is confirmed that Mn(4+) ions substitute for Ti(4+) sites in rutile TiO(2) crystal; however, it is suggested that Cr(3+) ions may replace at not only Ti(4+) site but also IS.

Investigations of zero-field splitting (ZFS) and local distortion parameters of Fe3+ ions doped into three oxi-spinels (ZnAl2O4, MgAl2O4, and ZnGa2O4) by theoretical analysis.

The local environment around the paramagnetic centers formed by the Fe(3+) ions doped into three oxi-spinel crystals (ZnAl(2)O(4), MgAl(2)O(4), and ZnGa(2)O(4)) is investigated utilizing the fourth-order perturbation formula of the axial zero-field splitting parameter D on the basis of the dominant spin-orbit coupling mechanism. In order to fix a plausible cubic space group for B-sites located by Al(3+)/Ga(3+) cations, several modeling approaches are used and the results are discussed in detail.

Memory effect and new polarized state in the incommensurate phase of TlGaSe2 ferroelectric – semiconductor

The structural, electrical, and thermal properties of the ferroelectric-semiconductor TlGaSe2 have been investigated after the thermal annealing of crystals inside the incommensurate (INC) phase for a few hours at the annealing temperature (Tann). It is found that all outlined physical parameters of TlGaSe2 are significantly modified after the annealing process. Besides the well-known memory effect, thermal annealing within the INC phase leads to previously unknown anomalies of the different physical properties of TlGaSe2 crystals at temperatures quite far from the INC-phase. To explain the experimental results it is supposed that annealing leads to the formation of a new polarized state that influences most of the physical parameters of the crystals. This outstanding feature of the annealing effect in TlGaSe2 crystals allows us to reveal some new phase transitions that take place in TlGaSe2 crystals in the 140–180 K temperature range.

Electron Paramagnetic Resonance (EPR) Investigations of Iron-Doped Ferroelectric Ternary Thallium Chalcogenides

Abstract In this review article, we summarize the results of the first investigations of EPR spectra of iron-doped ternary thallium chalcogenides TlInS2, TlGaSe2, and TlGaS2 in a wide temperature interval including previously reported structural phase transitions. In these compounds, Fe ions substituted Ga and/or In sites in crystal structure and served as local probes to study ligand crystal field and its local symmetry around these paramagnetic sites. An overview of our recent EPR studies carried out at room temperature and in the temperature interval including structural phase transitions is provided. The results revealed the fine structure of EPR spectra of the paramagnetic Fe3 + ions and the local symmetry of crystal field (CF) at Fe3 + site and CF parameters. As a result of summarizing the low-temperature EPR investigations a discussion about active atomic groups, possible atomic displacements, and the local symmetry changes during the low-temperature phase transformations is presented.

Dielectric constant and EPR spectra of Fe doped TlInS2 crystal near the structural phase transitions

Dielectric constant and electron paramagnetic resonance (EPR) spectra of Fe-doped TlInS2 crystal has been investigated in the temperature range of 5–300 K. The influence of Fe impurities on dielectric properties of TlInS2 crystal has been considered in comparison with earlier observed results from pure TlInS2 compounds. Considerable decrease of the dielectric constant, as well as the change of the shape of its temperature dependence in a result of influence of doped iron atoms, has been observed. It has been established that EPR lines exhibit remarkable splitting and appearance of additional resonance lines at the temperatures lower than 200 K, which is attributed earlier as the temperature of the ferroelectric phase transition. The simulation for low-temperature EPR spectra has been performed taking into account the appearance of four additional centers in addition to four equivalent centers observed from EPR spectra at room temperature. Such transformations are considered as result of nonequivalent displacements of different groups of Tl atoms during the structural phase transitions.

Local coordination of Fe3+ in ZnO nanoparticles: multi-frequency electron paramagnetic resonance (EPR) and Newman superposition model analysis.

Iron-doped ZnO nanoparticles have been synthesized through high-energy ball milling of powders produced by the co-precipitation method. Fine particles with an average size down to 40 nm have been obtained after 15 min of milling. Fe(3+) cations have been incorporated into the ZnO lattice within the limits of the solubility. By using multi-frequency and high-field electron paramagnetic resonance (EPR) we have resolved all electronic transitions for the S = 5/2 high-spin system and have accurately determined the EPR spin-Hamiltonian parameters. By combining data from crystallographic x-ray diffraction and EPR with the semi-empirical Newman superposition model we have found the local configurational position of Fe(3+) and have confirmed the symmetry of the lattice. Results presented in this paper indicate that Fe cations most probably substitute at Zn-sites in ZnO. At nanosizes the effect of Fe(3+) cations on the surface becomes remarkable: additional size effects can be observed in the EPR spectrum, which are different from the spectrum of bulk.

Effects of MnO doping on the electronic properties of zinc oxide: 406 GHz electron paramagnetic resonance spectroscopy and Newman superposition model analysis

MnO-doped ZnO ceramics have been synthesized through the conventional ceramic processing route. Mn2+ ions have been incorporated into the ZnO lattice within the limits of solid solubility. By using X-band-frequency and high-field electron paramagnetic resonance (EPR), we have resolved some of the main electronic transitions for the S = 5/2, I = 5/2 high-spin system and have determined accurately the EPR spin-Hamiltonian parameters. By combining data from crystallographic X-ray diffraction and EPR with the semi-empirical Newman superposition model, we have found the local configurational position of Mn2+ and have confirmed the symmetry of the lattice. The results presented in this contribution indicate that Mn ions substitute at Zn sites in ZnO. The effect of Mn2+ ions on the intrinsic defects becomes remarkable, thus the vacancy related intrinsic defect signals cannot be visible in the EPR spectrum. MnO doping affects the band gap energy of ZnO system which was confirmed via UV-Vis spectroscopy.

Investigations of zero-field splitting (ZFS) and local distortion parameters of ZnAl2S4:Mn2+ by theoretical analysis.

Fourth-order perturbation formula on the basis of the dominant spin-orbit coupling mechanism is employed to investigate the local environment around Mn2+ centers in ZnAl2S4 single crystals. The zero-field splitting (ZFS) parameter D is calculated for the Mn2+ ions at the Al3+ site with local symmetry D3d using the different orbital reduction factors. Both the contributions of the lattice distortions to the crystal-field (CF) parameters and the D are examined by means of different cases. The comparison between the calculated results in this study and the previous experimental and theoretical values reveals a good agreement and reasonable distortion parameters for Mn2+ ions at Al3+ sites.

Oxovanadium(IV) complexes based on S-alkyl-thiosemicarbazidato ligands. Synthesis, characterization, electrochemical, and antioxidant studies

Reaction of VOSO4 with 2-hydroxy-napthaldeyde-S-R-thiosemicarbazones (R: methyl, ethyl, propyl or allyl) and salicyl aldehyde yielded five-coordinate oxovanadium(IV) complexes having a N1,N4-diarylidene-S-R-thiosemicarbazidato structures. The compounds were characterized by elemental analysis, magnetic measurements, electronic, infrared, 1H-NMR, and electron paramagnetic resonance (EPR) spectra. The X-band EPR signals were recorded from powder forms and also in solution. All the complexes have a single asymmetric line shape and theoretical fit studies prove the presence of axial symmetry around the paramagnetic vanadium ions. A computer simulation of the EPR spectrum of each complex was carried out to derive the related EPR parameters. Cyclic voltammograms of the complexes exhibited two metal-based reversible redox peaks around 500 and −800 mV corresponding to one electron oxidation/reduction of VIVO/VVO and VIVO/VIIIO, respectively. The reductive response in the 50–350 mV region was assigned to ligand reduction. Antioxidant activities of the compounds were determined with CUPric Reducing Antioxidant Capacity, 2,2-azino-bis-3-ethylbenzothiazoline-6-sulfonic acid, and 1,1-diphenyl-2-picrylhydrazyl assays. The test results indicated that the antioxidant capacity of the compounds increases with the carbon number of saturated hydrocarbon chain on sulfur atom. GRAPHICAL ABSTRACT

Spectroscopic study for a chromium-adsorbed montmorillonite.

Samples of purified montmorillonite with trace amounts of quartz were subjected to different concentrations of chromium sulphate solutions for one week to allow cation exchange. The chromium-bearing montmorillonites were verified and tested using powder X-ray diffractometry (XRD), X-ray fluorescence spectrometry, electron spin resonance (ESR) spectrometry and Fourier transformation infrared (FTIR) spectroscopy to explore the occupation sites of the chromium. The ESR spectra recorded before and after the chromium exchange show clear differences: a strong and broad resonance with two shoulders at the lower magnetic field side was present to start, and its intensity as well as that of the ferric iron resonance, increased with the concentration of added chromium. The signals introduced by the chromium, for example at g=1.975 and 2.510 etc., suggested that the chromium had several occupational sites. The ESR peak with g=2.510 in the second derivative spectrum suggested that Cr3+ was weakly bounded to TOT with the form of [Cr(H2O)3]3+ in hexagonal cavities. This was verified by comparing the FTIR spectra of the pure and modified montmorillonite. The main resonance centred at g=1.975 indicated that the majority of Cr3+ occupied the interlayer region as [Cr(H2O)6]3+. The substitution of Ca2+ by Cr3+ also greatly affected the vibration of the hydrogens associate to water, ranged from 3500 to 2600cm-1 in FTIR. Furthermore, the presence of two diffraction lines in the XRD results (specifically those with d-values of 1.5171 and 1.2673nm) and the calculations of the size of the interlayer space suggested the presence of two types of montmorillonite with different hydration cations in the sample exposed to 0.2M chromium sulphate. The two diffraction lines were assigned to [Cr(H2O)6]3+ and [Cr(H2O)3O3]3+, respectively. This also suggested that the species of hydration cation was constrained by the concentration of the chromium solution.