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    Electronic and crystal structure analyses of boron doped LiFeO2 cathode material by the XAFS spectroscopy
    (Elsevier, 2022) Gunaydin, Selen; Harfouche, Messaoud; Ozkendir, O. Murat
    The influence of boron substitution in LiFeO2 (LFO) material on crystal and electronic structure properties has been investigated by x-ray absorption fine structure (XAFS) spectroscopy with the general formula LiFe1_xBxO2 where x has values of 0.00, 0.05, and 0.10. The study revealed that boron has a key role in oxide materials due to its high effect on the oxygen regions and provides better performance values by reducing the excess oxygen in the material. The electronic structure properties were investigated by x-ray absorption near-edge spectroscopy (XANES). In this study, a new analysis method has also been applied for crystal properties determination like the Rietveld method for the first time and named Inverse EXAFS Fitting . For the crystal structure study, extended XAFS (EXAFS) analyses were processed in reverse mode to determine the crystal lattice properties as an alter-native way to the x-ray diffraction pattern (XRD) study and also to find out the atomic distances from the source Fe atoms. Due to the smaller ionic radii of B3+ cations than the Fe3+ cations, EXAFS data analysis has revealed that boron atoms did not sit in iron coordination, but formed the LiBO2 crystals and caused minor perturbations around iron atoms by loosening the Fe-O bonds.
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    Electronic structure and electrochemical analysis of the Li2Mn1-xSexO3 materials
    (Elsevier, 2020) Ozkendir, O. Murat; Celik, Gultekin; Ates, Sule; Aktas, Sevda; Gunaydin, Selen; Harfouche, Messaoud; Bondino, Federica
    With the aim of probing the influence of the highly oxidizable selenium in the electrochemically inactive cathode Li2MnO3 material, samples were prepared with selenium substitution in the manganese coordination according to the general formula of Li2Mn1-xSexO3. In Li-ion batteries, oxygen instabilities are one of the major problems confronted that effect the performances of the cathode materials. The crystal and electronic structure properties of the materials were studied with x-ray absorption techniques. Selenium atoms were determined to build Li2SeO4 crystal and due to the oxygen removal during sample preparation mechanisms were determined to cause lower ionic conductivity than the parent Li2MnO3 oxide. The atomic distances in the materials were determined by the fits performed by the commercial code FEFF 8.2. Li2SeO4 crystal was determined as stacked between manganese and lithium atoms and isolated with each other.
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    Probing local distortion around structural defects in half-Heusler thermoelectric NiZrSn alloy
    (Nature Portfolio, 2020) Miyazaki, Hidetoshi; Ozkendir, Osman Murat; Gunaydin, Selen; Watanabe, Kosuke; Soda, Kazuo; Nishino, Yoichi
    The half-Heusler NiZrSn (NZS) alloy is particularly interesting owing to its excellent thermoelectric properties, mechanical strength, and oxidation resistance. However, the experimentally investigated thermal conductivity of half-Heusler NZS alloys shows discrepancies when compared to the theoretical predictions. This study investigates the crystal structure around atomic defects by comparing experimental and theoretical X-ray absorption fine structure (XAFS) spectra of the crystal structure of a half-Heusler NZS alloy. The results of both Zr and Ni K-edge XAFS spectra verified the existence of atomic defects at the vacancy sites distorting the C1(b)-type crystal structure. We concluded that the distortion of the atoms around the interstitial Ni disorder could be the probable reason for the observed lower thermal conductivity values compared to that predicted theoretically in half-Heusler alloys. Our study makes a significant contribution to the literature because the detailed investigation of the lattice distortion around atomic defects will pave the way to further reduce the thermal conductivity by controlling this distortion.
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    Study on crystallographic and electronic structure of micrometre-scale ZnO and ZnO:B rods via X-ray absorption fine-structure spectroscopy
    (Int Union Crystallography, 2021) Erat, Selma; Ozkendir, Osman Murat; Yildirimcan, Saadet; Gunaydin, Selen; Harfouche, Messaoud; Demir, Bunyamin; Braun, Artur
    X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) spectra were recorded to investigate the electronic structure and local crystal structure of ZnO and ZnO:B powders produced via hydrothermal synthesis. ZnO and ZnO:B grow as micrometre-scale rods with hexagonal shape, as confirmed by scanning electron microscopy micrographs. The number of broken ZnO:B rods increases with increasing B concentration, as observed in the images, due to B atoms locating in between the Zn and O atoms which weakens and/or breaks the Zn-O bonds. However, no disorder within the crystallographic structure of ZnO upon B doping is observed from X-ray diffraction results, which were supported by EXAFS results. To determine the atomic locations of boron atoms in the crystal structure and their influence on the zinc atoms, EXAFS data were fitted with calculated spectra using the crystal structure parameters obtained from the crystallographic analysis of the samples. EXAFS data fitting and complementary k-weight analysis revealed the positions of the B atoms - their positions were determined to be in between the Zn and O atoms.
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    The Effect of CrFe2O4 Addition on the Ionic Conductivity Properties of Manganese-Substituted LiFeO2 Material
    (Springer, 2024) Gunaydin, Selen; Miyazaki, Hidetoshi; Saran, Sevda; Baveghar, Hadi; Celik, Gultekin; Harfouche, Messaoud; Abdellatief, Mahmoud
    The influence of Mn substitution on the iron lattice sites in LiFeO2 material was investigated with respect to the electronic, crystalline, and electrochemical properties of the material, using the LiFe1-xMnxO2 (x = 0.0, 0.05, and 0.10) series. The electronic structure study was conducted with the acquisition of x-ray absorption fine structure spectroscopy data, while the crystal structure properties of the studied materials were investigated using x-ray diffraction patterns. The data collected for the ionic conductivity properties of the samples by electrochemical impedance spectroscopy under increasing temperature conditions around and above room temperature aided the crystal and electronic structure studies on cathode materials. Furthermore, studies were conducted with the addition of CrFe2O4 material in varying molar concentrations into LiFeO2 material, as CrFe2O4 is known to have thermoelectric properties well above the room temperature of 400 K (127 degrees C). Encouraging results for next-generation battery cathodes were obtained.
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    XAFS study on the boron substituted LiGaO2 semiconductor material
    (Elsevier, 2020) Gundogmus, Hakan; Gunaydin, Selen; Klysubun, Wantana; Ozkendir, Osman Murat
    An XAFS study was performed on the electronic and crystal structure properties of LiGa1-xBxO2 (LGO) material in where boron atoms were substituted at the gallium sites. Studies were carried out by the X-ray powder diffraction (XRD) patterns and supported by the Extended-XAFS data. The analysis on the substituted materials revealed interesting mechanisms at the boron sites that preserved the crystal symmetry in the entire bulk. It was determined that, boron atoms do not lie in the Ga sites due to inequivalent ionic radii and formed a crystal LiB3O5, which has the same crystal geometry and space group of the parent LGO. With the presence of the boron atoms on the vicinity of the gallium atoms, tiny shifts on the main edge spectra resulted from the change in the oxidation of the gallium atoms.