The equilibrium phase boundary between single-bonded, threefold-coordinated polymeric forms of nitrogen, and the observed, triple-bonded diatomic phases, is predicted to occur at relatively low (50 + 15 GPa) pressure. This conclusion is based on extensive local-density-functional total-energy calculations for polymeric structures (including that of black phosphorus, and another with all gauche dihedral angles) and diatomic structures (including that of the observed high-pressure e-N2 phase). We believe the diatomic phase of nitrogen, observed up to 180 Gpa and room temperature, to be metastable at these conditions, and that such hysteresis enhances the prospects for the existence of a metastable polymeric form of nitrogen at ambient conditions. In this regard, we show that the black-phosphorus and cubic gauche polymeric forms of nitrogen would encounter signi6cant barriers along high-symmetry paths to dimerization at atmospheric pressure.

A series of molecular-dynamics simulations using a many-body interatomic potential has been performed to investigate the behavior under load of several ͗001͘ and ͗011͘ symmetrical tilt grain boundaries ͑GB's͒ in diamond. Cohesive energies, the work for fracture, maximum stresses and strains, and toughness as a function of GB type are evaluated. Results indicate that special short-period GB's possess higher strengths and greater resistance to crack propagation than GB's in nearby misorientation angles. Based on dynamic simulations, it was found that the mechanism of interface failure for GB's without preexisting flaws is not that implied by Orovan's criterion, but rather GB strength is defined by GB type instead of cleavage energy. In simulations of crack propagation within GB's on the other hand, it was found that critical stresses for crack propagation from atomistic simulation and from the Griffith criterion are consistent, indicating that GB cleavage energy is an important characteristic of GB toughness. Crack propagation in polycrystalline diamond samples under an applied load was also simulated and found to be predominantly transgranular rather than intergranular.

We present an overview of recent work on quantum-based atomistic simulation of materials properties in transition metals performed in the Metals and Alloys Group at Lawrence Livermore National Laboratory. Central to much of this effort has been the development, from fundamental quantum mechanics, of robust many-body interatomic potentials for bcc transition metals via model generalized pseudopotential theory (MGPT), providing close linkage between ab initio electronic-structure calculations and large-scale static and dynamic atomistic simulations. In the case of tantalum (Ta), accurate MGPT potentials have been so obtained that are applicable to structural, thermodynamic, defect, and mechanical properties over wide ranges of pressure and temperature. Successful application areas discussed include structural phase stability, equation of state, melting, rapid resolidification, high-pressure elastic moduli, ideal shear strength, vacancy and self-interstitial formation and migration, grain-boundary atomic structure, and dislocation core structure and mobility. A number of the simulated properties allow detailed validation of the Ta potentials through comparisons with experiment and/or parallel electronic-structure calculations. Elastic and dislocation properties provide direct input into higher-length-scale multiscale simulations of plasticity and strength. Corresponding effort has also been initiated on the multiscale materials modelling of fracture and failure. Here large-scale atomistic simulations and novel real-time characterization techniques are being used to study void nucleation, growth, interaction, and coalescence in series-end fcc transition metals. We have so investigated the microscopic mechanisms of void nucleation in polycrystalline copper (Cu), and void growth in single-crystal and polycrystalline Cu, undergoing triaxial expansion at a large, constant strain rate - a process central to the initial phase of dynamic fracture. The influence of pre-existing microstructure on the void growth has been characterized both for nucleation and for growth, and these processes are found to be in agreement with the general features of void distributions observed in experiment. We have also examined some of the microscopic mechanisms of plasticity associated with void growth.

A class of spintronic materials, the zinc-blende ͑ZB͒ half metals, has recently been synthesized in thin-film form. We apply all-electron and pseudopotential ab initio methods to investigate the electronic and structural properties of ZB Mn and Cr pnictides and carbides, and find six compounds to be half metallic at or near their respective equilibrium lattice constants, making them excellent candidates for growth at low strain. Based on these findings, we further propose substrates on which the growth may be accomplished with minimum strain. Our findings are supported by the recent successful synthesis of ZB CrAs on GaAs and ZB CrSb on GaSb, where our predicted equilibrium lattice constants are within 0.5% of the lattice constants of the substrates on which the growth was accomplished. We confirm previous theoretical results for ZB MnAs, but find ZB MnSb to be half metallic at its equilibrium lattice constant, whereas previous work has found it to be only nearly so. We report here two low-strain half metallic ZB compounds, CrP and MnC, and suggest appropriate substrates for each. Unlike the other five compounds, we predict ZB MnC to become/remain half metallic with compression rather than expansion, and to exhibit metallicity in the minority-rather than majority-spin channel. These fundamentally different properties of MnC can be connected to substantially greater p-d hybridization and d-d overlap, and correspondingly larger bonding-antibonding splitting and smaller exchange splitting. We examine the relative stability of each of the six ZB compounds against NiAs and MnP structures, and find stabilities for the compounds not yet grown comparable to those already grown.

Water is known to exhibit fascinating physical properties at high pressure and temperature. Its remarkable structural and phase complexities suggest the possibility of exotic chemical reactivity under extreme conditions, although this remains largely unstudied. Detonations of high explosives containing oxygen and hydrogen produce water at thousands of kelvin and tens of gigapascals, similar to conditions in the interiors of giant planets. These systems thus provide a unique means of elucidating the chemistry of 'extreme water'. Here, we show that water has an unexpected role in catalysing complex explosive reactions--contrary to the current view that it is simply a stable detonation product. Using first-principles atomistic simulations of the detonation of the high explosive pentaerythritol tetranitrate, we discovered that H(2)O (source), H (reducer) and OH (oxidizer) act as a dynamic team that transports oxygen between reaction centres. Our finding suggests that water may catalyse reactions in other explosives and in planetary interiors.

We propose and investigate the properties of a digital ferromagnetic heterostructure consisting of a delta-doped layer of Mn in Si, using ab initio electronic-structure methods. We find that (i) ferromagnetic order of the Mn layer is energetically favorable relative to antiferromagnetic, and (ii) the heterostructure is a two-dimensional half-metallic system. The metallic behavior is contributed by three majority-spin bands originating from hybridized Mn-d and nearest-neighbor Si-p states, and the corresponding carriers are responsible for the ferromagnetic order in the Mn layer. The minority-spin channel has a calculated semiconducting gap of 0.25 eV. The band lineup is found to be favorable for retaining the half-metal character to near the Curie temperature. This kind of heterostructure may be of special interest for integration into mature Si technologies for spintronic applications.

The structural and electronic properties of a single pentacene molecule and a pentacene molecular crystal, an organic semiconductor, are examined by a first-principles method based on the generalized gradient approximation of density functional theory. Calculations were carried out for a triclinic unit cell containing two pentacene molecules. The bandwidths of the valence and conduction bands which determine the charge migration mechanism are found to depend strongly on the crystallographic direction. Along the triclinic reciprocal lattice vectors A and B which are orientated approximately perpendicular to the molecular axes the maximal valence (conduction) band width amounts to only 75 (59) meV, even smaller values are obtained for the C direction parallel to molecular axes even less. Along the stacking directions A+B and A-B, however, the maximal valence (conduction) band width is found to reach 145 (260) meV. The value for the conduction band width is larger than estimates for the polaron binding energy but significantly smaller than recent results obtained by semiempirical methods. The single molecule has a HOMO-LUMO gap of about 1.1 eV as deduced from the Kohn-Sham eigenvalue differences. When using the self-consistent field method, which is expected to yield more reliable results, a value of 1.64 eV is obtained. The theoretical value for the band gap in the molecular solid amounts to 1.0 eV at the Γ-point.

Monovacancies for seven bcc d-transition metals V, Cr, Fe, Nb, Mo, Ta, and W have been studied in detail from first-principles calculations. A full-potential, linear muffin-tin-orbital ͑FP-LMTO͒ method has been used in conjunction with both the local-density approximation ͑LDA͒ and the generalized-gradient approximation ͑GGA͒ to calculate volume-relaxed vacancy formation energies in all seven metals. A complementary ab initio pseudopotential ͑PP͒ method has been used to calculate both volume-and structure-relaxed LDA formation energies and formation volumes in V, Nb, Mo, Ta, and W. Fully relaxed PP geometries have also been applied to FP-LMTO LDA and GGA calculations. From these results, the following clear trends and conclusions emerge: ͑i͒ for the same fully relaxed geometry, FP-LMTO-LDA and PP-LDA formation energies are nearly identical; ͑ii͒ the lowest calculated formation energies are within or close to experimental error bars for all bcc metals except Cr, and the overall agreement with experiment is better for the 4d and 5d metals than the 3d metals; ͑iii͒ GGA and LDA formation energies are very similar for the 4d and 5d metals but for the 3d metals, and especially Fe, GGA performs better; ͑iv͒ volume-and structural-relaxation contributions lower the calculated formation energy by 0.1-0.5 eV, and improve agreement with experiment; ͑v͒ fully relaxed LDA formation volumes are in the narrow range (0.45-0.62)⍀ 0 , where ⍀ 0 is the equilibrium atomic volume; and ͑vi͒ the dominant structural effects are an approximate 5% inward relaxation of the first near-neighbor shell for group-V metals and a corresponding 1% inward relaxation for group-VI metals, with the exception of Mo, for which the second-shell atoms also relax inward by about 1%.