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Using hybrid density functional theory combined with a semiempirical van der Waals dispersion correction, we have investigated the structural and electronic properties of vacancies and self-interstitials in defective few-layer phosphorene. We find that both a vacancy and a self-interstitial defect are more stable in the outer layer than in the inner layer. The formation energy and transition energy of both a vacancy and a self-interstitial P defect decrease with increasing film thickness, mainly due to the upward shift of the host valence band maximum in reference to the vacuum level. Consequently, both vacancies and self-interstitials could act as shallow acceptors, and this well explains the experimentally observed p-type conductivity in few-layer phosphorene. On the other hand, since these native point defects have moderate formation energies and are stable in negatively charged states, they could also serve as electron compensating centers in n-type few-layer phosphorene.Comment: 10 pages, 12 figure

The embrittling and strengthening effects of hydrogen, boron, and phosphorus on a ⌺5(210) ͓100͔ nickel grain boundary are investigated by means of the full-potential linearized augmented plane-wave method with the generalized-gradient approximation formula. Optimized geometries for both the free surface and grainboundary systems are obtained by atomic-force calculations. The results obtained show that hydrogen and phosphorus are embrittlers and that boron acts as a cohesion enhancer. An analysis of the atomic, electronic, and magnetic structures indicates that atomic size and the bonding behavior of the impurity with the surrounding nickel atoms play important roles in determining its relative embrittling or cohesion enhancing behavior.

Simple structures of MnX binary compounds, namely hexagonal NiAs and zincblende, are studied as a function of the anion (X = Sb, As, P) by means of the all-electron FLAPW method within local spin density and generalized gradient approximations. An accurate analysis of the structural, electronic and magnetic properties reveals that the cubic structure greatly favours the magnetic alignment in these compounds leading to high magnetic moments and nearly half-metallic behaviour for MnSb and MnAs. The effect of the anion chemical species is related to both its size and the possible hybridization with the Mn d states; both contributions are seen to hinder the magnitude of the magnetic moment for small and light anions. Our results are in very good agreement with experiment -where available -and show that the generalized gradient approximation is essential to correctly recover both the equilibrium volume and magnetic moment.PACS numbers 71.20.-b,75.30.-m,75.50.-y Typeset using REVT E X 1

Using density functional theory combined with a semi-empirical van der Waals dispersion correction, we have investigated the stability of lattice defects including boron vacancy, substitutional and interstitial X (X=H, C, B, N, O) and Σ5 tilt grain boundaries in borophene and their influence on the anisotropic mechanical properties of this two-dimensional system. The pristine borophene has significant in-plane Young's moduli and Poisson's ratio anisotropy due to its strong and highly coordinated B-B bonds.The concentration of B vacancy and Σ5 grain boundaries could be rather high given that their formation energies are as low as 0.10 eV and 0.06 eV/Å respectively. In addition, our results also suggest that borophene can react easily with H 2 , O 2 and N 2 when exposed to these molecules. We find that the mechanical strength of borophene are remarkably reduced by these defects. The anisotropy in Poisson's ratio, however, can be tuned by some of them. Furthermore, the adsorbed H or substitutional C may induce negative Poisson's ratio in borophene, and the substitutional C or N can significantly increase the Poisson's ratio by constrast.

The toughness and ductility of ultrahigh-strength alloys is often limited by intergranular embrittlement, particularly under conditions of unfavorable environmental interactions such as hydrogen embrittlement and stress corrosion cracking. Here we investigated the mechanism by which the segregated substitutional additions cause intergranular embrittlement. An electronic level phenomenological theory is proposed to predict unambiguously the effect of a substitutional alloying addition on grain boundary cohesion of metallic alloys, based on first-principles full-potential linearized augmented plane-wave method ͑FLAPW͒ calculations on the strengthening and embrittling effects of the metals Mo and Pd on the Fe grain boundary cohesion. With the bulk properties of substitutional alloying addition A and the matrix element M as inputs, the strengthening or embrittling effect of A at the grain boundary of M can be predicted without carrying out first-principles calculations once the atomic structure of the corresponding clean grain boundary is determined. Predictions of the embrittlement potency of a large number of metals, including the 3d, 4d, and 5d transition metals, are presented for the Fe ⌺3 ͑111͒ and the Ni ⌺5 ͑210͒ grain boundaries. Rigorous FLAPW calculations on the effect of Co, Ru, W, and Re on the Fe ⌺3 ͑111͒ grain boundary and Ca on the Ni ⌺5 ͑210͒ grain boundary cohesion confirm the predictions of our model. This model is expected to be applicable to other high-angle boundaries in general and instructive in the quantum design of ultrahigh-strength alloys with resistance to intergranular fracture.

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Knowing how the contact geometry influences the conductance of a molecular wire junction requires both a precise determination of the molecule/metallic-electrode interface structure and an evaluation of the conductance for different contact geometries with a fair accuracy. With a greatly improved method to solve the Lippmann-Schwinger equation, we are able to include at least one atomic layer of each electrode into the extended molecule. The artificial effect of the jellium model used for the electrodes is therefore significantly reduced. Our first-principles calculations on the transport properties of a single benzene dithiolate molecule sandwiched between Au(111) surfaces show that the transmission of the bridge site contact, which is the most stable adsorption configuration in equilibrium, displays different features from those of other configurations, and that the inclusion of the surface layers of Au electrodes into the extended molecule shifts and broadens the transmission peaks due to a stronger and more realistic S-Au bonding. We discuss the geometry dependence of the transport properties by analyzing the density of states of the molecular orbitals.