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The electronic structure and spectroscopic properties of two ternary aluminum silicon carbide ceramics Al 4 SiC 4 and Al 4 Si 2 C 5 are studied by density functional theory calculations based on the orthogonalized linear combination of atomic orbitals method. Both crystals are shown to be small gap semiconductors with indirect band gaps of 1.05 and 1.02 eV, respectively. The calculated hole and electron effective mass and the interband optical properties, in the form of the complex dielectric function, show a high degree of anisotropy which can be traced to the unique structures of these two crystals. The calculated refractive indices are consistent with the values proposed in the literature. Mulliken effective charge and bond order calculations show that these crystals have a high degree of covalency with considerable charge transfer from Al and Si to the C atoms. The x-ray absorption near-edge-structure for all crystallographically nonequivalent sites ͑K and L edges͒ is calculated and compared with those of cubic SiC. It is shown that the site-averaged Si-K and Si-L 3 edges, and also the C-K edges are slightly different and broader than those of cubic SiC. Potential applications of these ternary ceramics are also discussed.

We present results of molecular dynamics ͑MD͒ simulations and density functional theory ͑DFT͒ calculations of the diffusion of Cu adatom and dimer on Ag͑111͒. We have used potentials generated by the embedded-atom method for the MD simulations and pseudopotentials derived from the projected-augmentedwave method for the DFT calculations. The MD simulations ͑at three different temperatures: 300, 500, and 700 K͒ show that the diffusivity has an Arrhenius behavior. The effective energy barriers obtained from the Arrhenius plots are in excellent agreement with those extracted from scanning tunneling microscopy experiments. While the diffusion barrier for Cu monomers on Ag͑111͒ is higher than that reported ͑both in experiment and theory͒ for Cu͑111͒, the reverse holds for dimers ͓which, for Cu͑111͒, has so far only been theoretically assessed͔. In comparing our MD result with those for Cu islets on Cu͑111͒, we conclude that the higher barriers for Cu monomers on Ag͑111͒ results from the comparatively large Ag-Ag bond length, whereas for Cu dimers on Ag͑111͒ the diffusivity is taken over and boosted by the competition in optimization of the Cu-Cu dimer bond and the five nearest-neighbor Cu-Ag bonds. Our DFT calculations confirm the relatively large barriers for the Cu monomer on Ag͑111͒-69 and 75 meV-compared to those on Cu͑111͒ and hint a rationale for them. In the case of the Cu dimer, the relatively long Ag-Ag bond length makes available a diffusion route whose highest relevant energy barrier is only 72 meV and which is not favorable on Cu͑111͒. This process, together with another involving an energy barrier of 83 meV, establishes the possibility of low-barrier intercell diffusion by purely zigzag mechanisms.

A fully transformed α″ martensite with stress-induced nanoscale {110}, {021}-type compound twin or a 90° rotation twin has been experimentally explored and unambiguously characterized in traditional β-type Ti alloys, which usually undergo a partial martensitic transformation [a β grain partially transformed into α″ with internal (111) twin] by quenching. This newly observed twinning, which matches with the predication based on the deformation twinning theory of Bilby and Crocker [Proc. R. Soc. 288, 241 (1965)], can help to explain the deformation mechanism and aid future development of advanced materials.

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