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    Electronic and magnetic properties of boron substituted CuFeO2 delafossite oxide
    (Taylor & Francis Ltd, 2024) Ezircan, Ali; Aslan, M. Selim; Miyazaki, Hidetoshi; Akyol, Mustafa; Ozkendir, O. Murat; Ekicibil, Ahmet; Ozturk, Hakan
    Synchrotron x-ray diffraction (SR-XRD) and X-ray absorption fine structure spectroscopy (XAFS) were used to investigate the crystal and electronic properties of boron-substituted CuFeO2 material at room temperature. Without boron substitution, the polycrystalline structures of the trigonal (rhombohedral) Rm' over bar m' CuFeO2 (87.7%) and hexagonal 'P63/mmc' (12.3%), which were also present in each sample but in different proportions, were utilised to identify the base material. XRD patterns revealed that, beyond 10% boron substitution, the metal-oxygen bonds (Fe-O and Cu-O) weakened, resulting in the formation of new tetragonal 'I41/amd' CuFe2O4 crystals. Although the CuFeO2 structure was preserved, it is conceivable that the presence of other crystal structures could lead to the formation of new features. This state arose as a result of CuFe2O4 crystallization and the impact of boron activity on the surrounding oxygen structures. By measuring magnetisation at both swept temperatures (10-300 K) and applied magnetic fields (+/- 30 kOe), the magnetic properties of the samples were investigated. In the 10-300 K temperature range, the polycrystalline samples exhibit a ferromagnetic property without a magnetic phase transition. This suggests that replacing B with Fe in CuFe(1-x)BxO(2 )does not influence the primary magnetic property of CuFeO2. The samples' saturation magnetisation (Ms) values gradually fall as the B substitution content increases with Fe. This is because there's a chance that the non-transition metal B in CuFe1-xBxO(2) will boost antiferromagnetic superexchange Cu-O interactions while lowering the p-d exchange interaction.
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    Local Structure of Heusler-Type Fe2V1-XTaXAl Thermoelectric Materials Studied by X-Ray Absorption Fine-Structure Spectroscopy
    (Wiley-V C H Verlag Gmbh, 2022) Takahashi, Kouki; Miyazaki, Hidetoshi; Kimura, Koji; Ozkendir, Osman Murat; Nishino, Yoichi; Hayashi, Kouichi
    The local structure around doping Ta atoms in Fe2V1-xTaxAl alloys is investigated using X-ray absorption fine-structure (XAFS) and synchrotron radiation X-ray diffraction (SR-XRD) measurements to elucidate the origin of the reduction in their thermal conductivity. XAFS and SR-XRD results show that with the substitution of Ta atoms at the V site, local strain exists around the doped Ta atoms. The reduction in the thermal conductivity due to Ta doping in the Fe2V1-xTaxAl alloys is attributed to the increase in the average atomic mass substituted with the heavy element Ta as well as the existence of the local strain.
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    Machine learning based prediction of lattice thermal conductivity for half-Heusler compounds using atomic information
    (Nature Portfolio, 2021) Miyazaki, Hidetoshi; Tamura, Tomoyuki; Mikami, Masashi; Watanabe, Kosuke; Ide, Naoki; Ozkendir, Osman Murat; Nishino, Yoichi
    Half-Heusler compound has drawn attention in a variety of fields as a candidate material for thermoelectric energy conversion and spintronics technology. When the half-Heusler compound is incorporated into the device, the control of high lattice thermal conductivity owing to high crystal symmetry is a challenge for the thermal manager of the device. The calculation for the prediction of lattice thermal conductivity is an important physical parameter for controlling the thermal management of the device. We examined whether lattice thermal conductivity prediction by machine learning was possible on the basis of only the atomic information of constituent elements for thermal conductivity calculated by the density functional theory in various half-Heusler compounds. Consequently, we constructed a machine learning model, which can predict the lattice thermal conductivity with high accuracy from the information of only atomic radius and atomic mass of each site in the half-Heusler type crystal structure. Applying our results, the lattice thermal conductivity for an unknown half-Heusler compound can be immediately predicted. In the future, low-cost and short-time development of new functional materials can be realized, leading to breakthroughs in the search of novel functional materials.
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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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    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.