We report an ab initio study of the mechanical stability of platinum nitride ͑PtN͒, in four different crystal structures, the rock salt ͑rs-PtN͒, zinc-blende ͑zb-PtN͒, cooperite, and a face-centered orthorhombic phase. Of these phases only the rs-PtN phase is found to be stable and has the highest bulk modulus B = 284 GPa. Its electronic density of states shows no band gap making it metallic. The zb-PtN phase does not stabilize or harden by the nitrogen vacancies investigated in this study. Therefore, the experimental observation of super hardness in PtN remains a puzzle.

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We study the interface energetics of Ag nanocrystals on a H-passivated Si(111) surface by a transmission electron microscopy experiment and molecular dynamics simulations. The annealed nanocrystals are oriented with Ag(111)||Si(111). Azimuthally, epitaxy is preferred for nanocrystals with an interface larger than a coincident-site-lattice (CSL) cell. The equilibrium orientation, or interface energy minimum, depends on the interface size and shape. For interfaces approaching a CSL cell in size ( approximately 2 nm nanocrystals), fluctuations of a single atom at an interface can lead to large variations in nanocrystal orientations.

The superconducting properties of YBa 2 Cu 3 O 7-␦ ͑YBCO͒ are strongly dependent on atomic structure and composition. While the average structures of YBCO are well documented, the local oxygen concentration and ordering have received much less attention owing to the difficulty of experiments and analysis. We examined several samples of YBCO by quantitative high-resolution transmission electron microscopy and geometric phase analysis to determine their local atomic structure and symmetry. Our results reveal that adjacent domains separated by the ͑110͒ interfaces, commonly considered to be twin domains, can have different a o / b o ratios. The domains, therefore, do not retain the ideal twinned configuration commonly expected for this system. Assuming the a o / b o ratio is proportional to oxygen concentration, we suggest that these domains result from inhomogeneous oxygen distribution. Multiple measurements in both bulk and thin-film samples indicate that YBCO can consist of different phases, which may yield locally variable superconducting properties.