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  • 1
    Electronic Resource
    Electronic Resource
    Electronic Structure and Bonding of All Crystalline Phases in the Silica–Yttria–Silicon Nitride Phase Equilibrium Diagram (2004)
    Oxford, UK : Blackwell Publishing Ltd
    Journal of the American Ceramic Society 87 (2004), S. 0 
    Details
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: This paper reviews the structures and properties of 10 binary, ternary, and quaternary crystals within the equilibrium phase diagram of the SiO2–Y2O3–Si3N4 system. They are binary compounds SiO2, Y2O3, Si3N4; ternary compounds Si2N2O, Y2Si2O7, and YSi2O5; and quaternary crystals Y2Si3N4O3 (M-melilite), Y4Si2O7N2, (N-YAM), YSiO2N (wallastonite), and Y10(SiO4)6N2 (N-apatite, N-APT). Although the binary compounds are well-known and extensively studied, the ternary and the quaternary crystals are not. Most of the ternary and the quaternary crystals simply have been referenced as secondary phases in the processing of nitrogen ceramics. Their crystal structures are complex and not precisely determined. In the quaternary crystals, there exists O/N disorder in that the exact atomic positions of the anions cannot be uniquely determined. It is envisioned that a variety of cation–anion bonding configurations exist in these complex crystals. The electronic structure and bonding in these crystals are, therefore, of great interest and are indispensable for a fundamental understanding of structural ceramics. We have used ab initio methods to study the structure and bonding properties of these 10 crystals. For crystals with unknown or incomplete structural information, we use an accurate total energy relaxation scheme to obtain the most likely atomic positions. Based on the theoretically modeled structures, the electronic structure and bonding in these crystals are investigated and related to various local cation–anion bonding configurations. These results are presented in the form of atom-resolved partial density of states, Mulliken effective charges, and bond order values. It is shown that Y–O and Y–N bonding are not negligible and should be a part of the discussion of the overall bonding schemes in these crystals. Spectroscopic properties in the form of complex, frequency-dependent dielectric functions, X-ray absorption near-edge structure (XANES), and the electron energy-loss near-edge structure (ELNES) spectra in these crystals also are calculated and compared. These results are discussed in the context of specific bonding configurations between cations (silicon and yttrium) and anions (oxygen and nitrogen) and their implications on intergranular thin films in polycrystalline Si3N4 containing rare-earth elements.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2004.tb06352.x
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  • 2
    Electronic Resource
    Electronic Resource
    Ab-initio Calculation of Yttrium Substitutional Impurities in alpha-Al2O3 (1997)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 80 (1997), S. 0 
    Details
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Yttrium has a very low solubility in bulk α-Al2 O3 and a great propensity for segregation to the grain boundaries in polycrystalline Al2O3. The segregation of yttrium influ-ences many properties of the material. To understand the nature of the chemical bonding of yttrium within Al2O3, we have, as a first step, conducted an ab-initio self-consistent calculation for a yttrium impurity atom replacing an aluminum atom in Al2O3. We have used the first-principles orthogonalized linear combination of atomic orbitals (OLCAO) method and a supercell with 120 atoms in the hexagonal lattice. The relaxation of the nearby atoms, be-cause of the presence of yttrium, is studied via total energy calculation within the local density approximation. The nearest-neighbor oxygen atoms of yttrium move outward by 8% and the next-nearest-neighbor aluminum atoms move inward by 5% of the respective separations in the undistorted lattice. A substitutional energy of 4.79 eV is obtained. The yttrium impurity introduces three defect states in the gap near the conduction band edge, thus natu-rally explaining the “donor effect.” Two of the defect states are degenerate, and all three are derived mainly from the Y4d orbitals with very different local symmetries. The effec-tive-charge and bond-order calculations show a substantial covalent bonding character between yttrium and the re-laxed oxygen and aluminum atoms in the host.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.1997.tb03250.x
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  • 3
    Electronic Resource
    Electronic Resource
    Electronic Structure and Bonding in Crystalline Y10[SiO4]6N2 (2003)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 86 (2003), S. 0 
    Details
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The electronic structure and bonding of the complex ceramic crystal Y10[SiO4]6N2 is studied by a first-principles method. It is shown that this crystal is an insulator with a direct band gap of 1.3 eV. It has some unique properties related to the one-dimensional chain structure in the c-direction and the planar N-Y bonding in the x-yplane.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2003.tb03487.x
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  • 4
    Electronic Resource
    Electronic Resource
    Structure and Properties of Advanced Nitrides (2002)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 85 (2002), S. 0 
    Details
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2002.tb00028.x
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  • 5
    Electronic Resource
    Electronic Resource
    Calculation of XANES/ELNES Spectra of All Edges in Si3N4 and Si2N2O (2002)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 85 (2002), S. 0 
    Details
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: Using a recently developed first-principles supercell method that includes the electron and core-hole interaction, the XANES/ELNES spectra of Si-L2,3, Si-K, and N-K edges in α-Si3N4, β-Si3N4, spinel c-Si3N4, and Si2N2O were calculated and compared. The difference in total energies between the initial ground state and the final core-hole state provides the transition energy. The calculated spectra are found to be in good agreement with the experimental measurements on β-Si3N4 and c-Si3N4. The differences in the XANES/ELNES spectra for the same element in different crystals are explained in terms of differences in local bonding. The use of orbital-decomposed local density of states to explain the measured spectra is shown to be inadequate. These results reaffirm the importance of including the core-hole effect in any XANES/ELNES spectral calculation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2002.tb00030.x
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  • 6
    Electronic Resource
    Electronic Resource
    Theoretical Prediction of the Structure and Properties of Cubic Spinel Nitrides (2002)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 85 (2002), S. 0 
    Details
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: 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).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2002.tb00042.x
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  • 7
    Electronic Resource
    Electronic Resource
    “Electronic Structure and Bonding of All Crystalline Phases in the Silica–Yttria–Silicon Nitride Phase Equilibrium Diagram” (2005)
    Oxford, UK : Blackwell Science Inc
    Journal of the American Ceramic Society 88 (2005), S. 0 
    Details
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1551-2916.2005.00605.x
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  • 8
    Electronic Resource
    Electronic Resource
    Geometry Optimization and Ground-State Properties of Complex Ceramic Oxides (2001)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 84 (2001), S. 0 
    Details
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The crystal structures of the several complex oxides including MgAl2O4, α-Al2O3, θ-Al2O3, r-Al2O3, Y2O3, Y3Al5O12, and YAlO3 are fully optimized by means of first-principles total energy calculations with relaxation of all internal parameters. This is achieved by the implementation of a simple scheme based on the finite difference method for the energy gradient. In most cases, the predicted lattice constants are within 0.5% of the experimental values. The bulk moduli and the pressure coefficients for these crystals are calculated from fitting the energy vs volume data to the equation of state. Because all of the internal parameters of the crystal are optimized at different volumes, the calculated ground-state properties are found to be in excellent agreement with measured data. For the polymorphs of the Al2O3 system, comparison to other existing calculations are made and discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2001.tb00744.x
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  • 9
    Electronic Resource
    Electronic Resource
    Electronic Structure and Bonding of β-SiAlON (2000)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 83 (2000), S. 0 
    Details
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The α- and β-SiAlONs are ceramic solid solutions with charge-neutral substitutions in α- and β-Si3N4. They have high potential for applications as structural materials. We have calculated the electronic structure and bonding of β-Si6—zAlzOzN8—z for z= 0, 1, 2, 3, 4 using a first-principles method. Total energy calculations show that the bulk modulus of β-Si6—zAlzOzN8—z decreases as z increases, in general agreement with experimental trends. Simultaneous substitution of the (Si,N) pair by (Al,O) results in impurity-like states in the upper portion of the bandgap of β-Si3N4. As z increases, more and more states are introduced into the gap, forming a new conduction band (CB) edge for SiAlON. At z= 4, the calculated bandgap is ∼1.3 eV. Density of states (DOS) calculations show the top of the valence band remains steep for all z, and the bottom of the CB is formed predominately by Si—O antibonding states. Orbitally resolved partial DOS calculations in the CB region are used to predict the trends of the electron-energy-loss near-edge spectra (ELNES) of Si-L2,3, Al-L2,3, Si-K, Al-K, O-K, and N-K edges in β-SiAlON. The impurity-like states near the CB edge result in pre-edge structures in all ELNES spectra. Effective charge and bond order calculations show that the overall bond strength in β-SiAlON decreases only slightly as z increases. Although the stronger Si—N bonds are replaced by weaker Al—O bonds, the remaining Si—N and Al—O bonds actually strengthen as z increases because of the effective charge redistribution after substitution. This is a very interesting finding that may partly explain the superior mechanical properties of the SiAlON system that render them suitable for structural applications.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.2000.tb01274.x
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  • 10
    Electronic Resource
    Electronic Resource
    Ab-Initio Total Energy Calculation of α- and β-Silicon Nitride and the Derivation of Effective Pair Potentials with Application to Lattice Dynamics (1998)
    Westerville, Ohio : American Ceramics Society
    Journal of the American Ceramic Society 81 (1998), S. 0 
    Details
    ISSN: 1551-2916
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Mechanical Engineering, Materials Science, Production Engineering, Mining and Metallurgy, Traffic Engineering, Precision Mechanics , Physics
    Notes: The ground state total energies of α- and β-Si3N4 crystals are calculated by an ab-initio method based on local density functional theory. The calculated bulk modulus and pressure coefficients for both crystals are in good agreement with recent experimental measurements. Interatomic pair potentials of the Buckingham form with an additional repulsive term are derived using ab-initio effective charges and total energy data. The effective pair potentials give excellent results on equilibrium lattice parameters, elastic constants, phonon spectra, and lattice specific heat for both crystals. The zone-center optical phonon modes are in good agreement with measured infrared and Raman spectra. The pair potentials for α- and β-Al2O3 are similar but not the same, despite their similarities in the local short-range order. Applications of the pair potentials to the simulation and modeling of surfaces and interfaces in Si3N4 ceramics are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1151-2916.1998.tb02755.x
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