Grain boundary segregation is a critical issue in materials science because it determines the properties of individual grain boundaries and thus governs the macroscopic properties of materials. Recent progress in electron microscopy has greatly improved our understanding of grain boundary segregation phenomena down to atomistic dimensions, but solute segregation is still extremely challenging to experimentally identify at the atomic scale. Here, we report direct observations of atomic-scale yttrium solute segregation behaviours in an yttria-stabilized-zirconia grain boundary using atomic-resolution energy-dispersive X-ray spectroscopy analysis. We found that yttrium solute atoms preferentially segregate to specific atomic sites at the core of the grain boundary, forming a unique chemically-ordered structure across the grain boundary.
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-A 3 N 4 (where A is a Group IVA element), except c-Hf 3 N 4 , are all semiconductors with band gaps ranging from an indirect gap of 0.07 eV in c-Ti 3 N 4 to a direct gap of 3.45 eV in c-Si 3 N 4 . For double nitrides of the form c-AB 2 N 4 (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-CSi 2 N 4 (which has an exceptionally strong covalent bonding and large bulk modulus), c-SiGe 2 N 4 (which has an energetically favorable direct band gap of 1.85 eV), and c-SiTi 2 N 4 (which is metallic).
First-principles molecular orbital calculations using model clusters are made in order to reproduce and interpret experimental electron-energy-loss near-edge structure and near-edge x-ray absorption fine structure of MgO at Mg K, L 2,3 and O K edges. Ground-state calculations using a model cluster composed of 125 atoms and by a band-structure method are in good agreement, but they do not reproduce the experimental spectra satisfactory. They are well reproduced only by the cluster calculations for the Slater transition state, where a half-electron is removed from a core orbital and placed into the lowest unoccupied molecular orbital. The core-hole effect is therefore essential for theoretical reproduction of the spectral shapes. A large supercell is required to reproduce the experimental spectra when one uses a band-structure method. The origin of peaks appearing in the experimental spectra is interpreted in terms of orbital interactions using overlap-population diagrams. Some features of the spectra at different edges are pointed out to have common origins. Experimental spectra are aligned accordingly. The transition energies and qualitative features of experimental spectra are found to be reproduced even using a smaller cluster composed of 27 atoms, although some of fine structure is missing.
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 AB 2 N 4 are found to be stable when the counterparts BA 2 N 4 are metastable except for TiZr 2 N 4 . The four single nitrides C 3 N 4 , Si 3 N 4 , Ge 3 N 4 , and Sn 3 N 4 have direct band gaps at the ⌫ point ranging from 1.14 to 3.45 eV while Zr 3 N 4 and Ti 3 N 4 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, CSi 2 N 4 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.
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.
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