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ABSTRACT: In order to examine x-ray absorption near-edge structures (XANES) of disordered solid solutions of metal oxides, a combined approach of a first principles supercell method and a cluster expansion method is developed. Zn-L_3 edge XANES are measured on a series of Mg_1−xZn_xO with a rocksalt structure in the range of x=0.025–0.3 using synchrotron source. A first principles orthogonalized linear combinations of an atomic orbital method is employed to obtain a theoretical spectrum of a given model. A Zn-2p core hole is included in the calculation, and a set of 128 atom supercells is used. Theoretical XANES of disordered solid solutions are obtained as a weighted sum of theoretical spectra for four ordered structures, with the weighting factors determined by the cluster expansion method. The dependence of the spectral shape on the solute concentration is reproduced only when the averaged environment of solute atoms as determined by the solute concentration and the effect of the disordering is taken into account. The formation of the disordered Mg_1−xZn_xO solid solution is confirmed by the Monte Carlo calculations.
Physical Review B 12/2007; 76(19). · 3.69 Impact Factor
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ABSTRACT: The structure and properties of cubic spinel nitrides were investigated based on first-principles theoretical calculations. The lattice constants, bulk moduli, band structures, electronic bonding, and lattice stability of thirty-nine single and double nitrides were studied. The single spinel nitrides of the form c-A3N4 (where A is a Group IVA element), except c-Hf3N4, are all semiconductors with band gaps ranging from an indirect gap of 0.07 eV in c-Ti3N4 to a direct gap of 3.45 eV in c-Si3N4. For double nitrides of the form c-AB2N4 (where A and B are Group IVA (Ti, Zr, Hf) or IVB (C, Si, Ge, Sn) elements), both metallic and insulating band structures are possible. The stability of the double spinel nitrides, relative to single nitrides, is dependent on the optimal cation radii and polyhedral volumes at the tetrahedral A sites and the octahedral B sites. Of the thirty-two double nitrides, only nine are predicted to be energetically favorable. Among the potentially stable phases, the most interesting ones are c-CSi2N4 (which has an exceptionally strong covalent bonding and large bulk modulus), c-SiGe2N4 (which has an energetically favorable direct band gap of 1.85 eV), and c-SiTi2N4 (which is metallic).
Journal of the American Ceramic Society 12/2004; 85(1):75 - 80. · 2.27 Impact Factor
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ABSTRACT: The first principles calculations of ELNES/XANES of AlN polytypes were carried out by the first-principles OLCAO method using large supercells composed of more than 100 atoms. It can quantitatively reproduce the experimental spectra from wurtzite AlN using a 108-atoms supercell. ELNES from rock-salt and zinc-blend AlN were predicted by using 128 atoms supercells. The spectral features of rock-salt phase are different from other phases, whereas that of zinc-blend phase have numerous similarities with that from wurtzite AlN. Characteristic differences between the wurtzite and zinc-blend phases are predicted to appear at the first peak of Al L(2,3) and K edges. The first peak of zinc-blend AlN is broader than that of wurtzite AlN. The same tendency was found in the case of SiC. In order to elucidate the cause of the broadness at the first peak, partial density of states and chemical bondings were investigated. The theoretical analysis revealed that the broadness of the first peak is related to the covalency of the compounds. This result suggests that the spectral features at the first peak of L(2,3) and K edges contain information about the covalency at the illuminated area.
Micron 02/2003; 34(3-5):249-54. · 1.53 Impact Factor
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ABSTRACT: Electron-energy-loss near edge structures (ELNES) at the Zn–L2,3 edge and the O–K edge have been measured for 10 mol%ZnO-doped MgO, and were compared with spectra from reference materials. In order to interpret the spectra, first principles molecular orbital calculations were made using model clusters composed of 125 and 153 atoms. Photoabsorption cross sections (PACS) were computed at the Slater's transition state in which a half-filled core hole was included in the self-consistent calculations. The difference in the coordination numbers of Zn was found well distinguishable by the Zn–L2,3-edge ELNES. The experimental spectra in the first 25 eV were well reproduced by the theoretical PACS. In this energy region, the Zn–L2,3-edge ELNES from four-fold coordinated Zn showed four sets of peaks, whereas the six-fold coordinated Zn exhibits three sets of peaks. The origin of these peaks can be explained by the point symmetry within the first coordination unit. A small shift toward the lower energy side was observed in the O–K edge ELNES of the ZnO-doped MgO as compared with pure MgO. This can be ascribed to the lower energy of the Zn-4s orbital as compared with the Mg-3s orbital, which is the common mechanism to the difference in the band gap between MgO and ZnO.
Ultramicroscopy 03/2001; · 2.47 Impact Factor
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ABSTRACT: Lattice constants, bulk moduli, band gaps, electronic bonding, and the stability of 20 new nitrides with spinel structure are studied by first-principles calculations. Double nitrides AB2N4 are found to be stable when the counterparts BA2N4 are metastable except for TiZr2N4. The four single nitrides C3N4, Si3N4, Ge3N4, and Sn3N4 have direct band gaps at the Γ point ranging from 1.14 to 3.45 eV while Zr3N4 and Ti3N4 have small indirect gaps. For double nitrides, both metallic and insulating band structures are possible. The total bond orders of the stable double nitrides are larger than those of constituent single nitrides. Among them, CSi2N4 shows exceptionally strong covalent bonding and a large bulk modulus. A simple scaling law based on bond lengths can describe the bulk moduli of these spinel nitrides quite well.
Phys. Rev. B. 01/2001; 63(6).
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ABSTRACT: Effects of a core hole in the unoccupied density of states are systematically studied using model clusters of h-BN, c-BN, and w-BN, in order to reproduce and interpret experimental electron-energy-loss near-edge structure /near-edge x-ray absorption fine structure at both B- and N-K edges. Wave functions are found to localize significantly near the core hole, thereby changing their energies as well as spatial distribution. They are very different from the Bloch states assumed in a band-structure calculation on the basis of a structural unit cell. When the presence of the core hole is ignored, in other words at the ground state, small clusters such as h-(B7N12) exhibit better agreement with the experiment as compared with large clusters such as h-(B91N108) because wave functions are made to localize in the small clusters. Features appeared in the experimental spectra are interpreted in terms of chemical bondings among atomic orbitals using overlap population diagrams. Absolute transition energies by Slater’s transition state method agree with experimental values within an error of 1%.
Phys. Rev. B. 08/1999; 60(7).
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ABSTRACT: First principles molecular orbital calculations of three titanium oxides are made in order to quantify the absolute transition energies of ELNES/NEXAFS at the O K and Ti edges and to clarify the origin of their chemical shifts. The absolute transition energies as well as their chemical shifts at two edges are satisfactorily reproduced using clusters composed of 24 to 63 atoms when Slater's transition state method is employed allowing temporary spin-polarization. The O K edge shows a positive shift with the increase of the formal number of d electrons per Ti ion. The shift can be mainly ascribed to the variation of the energy of the -like band, although the energy of the O 1s core-orbital varies slightly. On the other hand, the Ti edge shows negative shift, which is found to be explained by the balance of energies of the Ti 2p and the -like band. The magnitude of the chemical shifts is not significantly altered by the manner of the octahedral linkage.
Journal of Physics Condensed Matter 12/1998; 11(16):3217. · 2.55 Impact Factor
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ABSTRACT: Maximum solubility of Si in MgO has been reported to be 9.0 at.%. The local environment of the Si solute is of great interest. In order to foresee the difference of the O K-edge electron-energy-loss near-edge structure (ELNES) or x-ray absorption near-edge structure (XANES) due to the presence of the Si solute, first-principles molecular orbital calculations are performed using model clusters composed of 27 and 125 atoms that include a core hole. Extra peaks are found in the theoretical O K-edge photoabsorption cross section (PACS) of the Si-doped cluster. Their positions are found to depend strongly on the Si-O bond length. A realistic model cluster for the Si-doped MgO is constructed by total energy minimization using interatomic potentials. An Mg vacancy is included to keep the charge neutrality. The extra feature in the O K-edge PACS due to the presence of Si is then expected to be located mainly at the higher-energy shoulder of the main peak of the undoped MgO.
Journal of Physics Condensed Matter 12/1998; 11(29):5661. · 2.55 Impact Factor
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ABSTRACT: A first-principles molecular orbital calculation of NiO is made using a model cluster that includes a half-filled core hole. Major features of O K-edge electron energy loss near-edge structure up to 20 eV above the edge are satisfactorily reproduced simply by taking the O 2p partial density of states as a theoretical spectrum. Four peaks appeared in the experimental spectrum are well interpreted from the viewpoint of interactions among atomic orbital. Contrary to the previous multiple-scattering calculations, we found the major features in this energy range are determined within a smaller range, i.e., two Ni shells.
Phys. Rev. B. 10/1998; 58(15).
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ABSTRACT: High-resolution transmission electron microscopy (HRTEM) and analytical electron microscopy (AEM) have been carried out on Si-doped sintered α-Al2O3. HRTEM shows that there is no amorphous phase at grain boundaries. The Si-segregated boundary is found to be much more sensitive to irradiation damage than undoped Al2O3 grain boundaries. AEM with energy dispersive x-ray spectroscopy (EDS) shows the significant segregation of Si at grain boundaries, and AEM with electron energy-loss spectroscopy (EELS) reveals the existence of six-fold coordinated Si at the grain boundaries. The theoretical calculations obtained by the molecular orbital method support the data obtained by EELS. © 1998 American Institute of Physics.
Applied Physics Letters 01/1998; 72(2):191-193. · 3.84 Impact Factor
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ABSTRACT: In order to examine x-ray absorption near-edge structures (XANES) of disordered solid solutions of metal oxides, a combined approach of a first principles supercell method and a cluster expansion method is developed. Zn-L3 edge XANES are measured on a series of Mg1−xZnxO with a rocksalt structure in the range of x=0.025–0.3 using synchrotron source. A first principles orthogonalized linear combinations of an atomic orbital method is employed to obtain a theoretical spectrum of a given model. A Zn-2p core hole is included in the calculation, and a set of 128 atom supercells is used. Theoretical XANES of disordered solid solutions are obtained as a weighted sum of theoretical spectra for four ordered structures, with the weighting factors determined by the cluster expansion method. The dependence of the spectral shape on the solute concentration is reproduced only when the averaged environment of solute atoms as determined by the solute concentration and the effect of the disordering is taken into account. The formation of the disordered Mg1−xZnxO solid solution is confirmed by the Monte Carlo calculations.
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ABSTRACT: In order to examine x-ray absorption near-edge structures (XANES) of disordered solid solutions of metal oxides, a combined approach of a first principles supercell method and a cluster expansion method is developed. Zn-L3 edge XANES are measured on a series of Mg1−xZnxO with a rocksalt structure in the range of x=0.025–0.3 using synchrotron source. A first principles orthogonalized linear combinations of an atomic orbital method is employed to obtain a theoretical spectrum of a given model. A Zn-2p core hole is included in the calculation, and a set of 128 atom supercells is used. Theoretical XANES of disordered solid solutions are obtained as a weighted sum of theoretical spectra for four ordered structures, with the weighting factors determined by the cluster expansion method. The dependence of the spectral shape on the solute concentration is reproduced only when the averaged environment of solute atoms as determined by the solute concentration and the effect of the disordering is taken into account. The formation of the disordered Mg1−xZnxO solid solution is confirmed by the Monte Carlo calculations.
Phys. Rev. B. 76(19).
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ABSTRACT: Our recent works to reproduce and interpret experimental electron energy‐loss near‐edge structures (ELNES) are reviewed. Wave functions are significantly localized at the final state of the excitation to make the ELNES. Inclusion of core‐hole effects is, therefore, mandatory to calculate ELNES. The molecular orbital approach using small clusters can reproduce the spectral features qualitatively well, since wave functions are forced to be localized by the cluster method. The linear combination of atomic orbitals (LCAO)‐based cluster method has the advantage of interpreting the origin of the spectral features in a simple manner, especially when combined with overlap population diagrams. Quantitative reproduction of ELNES at edges of normal sp‐elements can be made by LCAO‐based band‐structure calculations when sufficiently large supercells are chosen and the dipole matrix elements are computed. Contrary to these ‘normal’ edges, the L 2,3 edge of the 3d‐transition metal elements cannot be reproduced in these ways, because the interactions between 2p‐hole and 3d electrons as well as the spin‐orbit coupling are not negligible. The need for relativistic many‐electrons calculations is, therefore, emphasized.
