We develop an empirical potential for silicon which represents a considerable improvement over existing models in describing local bonding for bulk defects and disordered phases. The model consists of two-and three-body interactions with theoretically motivated functional forms that capture chemical and physical trends as explained in a companion paper. The numerical parameters in the functional form are obtained by fitting to a set of ab initio results from quantum mechanical calculations based on density functional theory in the local density approximation, which include various bulk phases and defect structures. We test the potential by applying it to the relaxation of point defects, core properties of partial dislocations and the structure of disordered phases, none of which are included in the fitting procedure. For dislocations, our model makes predictions in excellent agreement with ab initio and tight-binding calculations. It is the only potential known to describe both the 30 • -and 90 • -partial dislocations in the glide set {111}. The structural and thermodynamic properties of the liquid and amorphous phases are also in good agreement with experimental and ab initio results. Our potential is the first capable of simulating a quench directly from the liquid to the amorphous phase, and the resulting amorphous structure is more realistic than with existing empirical preparation methods. These advances in transferability come with no extra computational cost, since force evaluation with our model is faster than with the popular potential of Stillinger-Weber, thus allowing reliable atomistic simulations of very large atomic systems.

We use recent theoretical advances to develop a new functional form for interatomic forces in bulk silicon. The theoretical results underlying the model include a novel analysis of elastic properties for the diamond and graphitic structures and inversions of ab initio cohesive energy curves. The interaction model includes two-body and three-body terms which depend on the local atomic environment through an effective coordination number. This formulation is able to capture successfully: (i) the energetics and elastic properties of the ground state diamond lattice; (ii) the covalent re-hybridization of undercoordinated atoms; (iii) and a smooth transition to metallic bonding for overcoordinated atoms. Because the essential features of chemical bonding in the bulk are built into the functional form, this model promises to be useful for describing interatomic forces in silicon bulk phases and defects. Although this functional form is remarkably realistic by usual standards, it contains a small number of fitting parameters and requires computational effort comparable to the most efficient existing models. In a companion paper, a complete parameterization of the model is given, and excellent performance for condensed phases and bulk defects is demonstrated.

We performed a first principles investigation on the structural and electronic properties of group-IV (C, SiC, Si, Ge, and Sn) graphene-like sheets in flat and buckled configurations and the respective hydrogenated or fluorinated graphane-like ones. The analysis on the energetics, associated with the formation of those structures, showed that fluorinated graphane-like sheets are very stable, and should be easily synthesized in laboratory. We also studied the changes on the properties of the graphene-like sheets, as result of hydrogenation or fluorination. The interatomic distances in those graphane-like sheets are consistent with the respective crystalline ones, a property that may facilitate integration of those sheets within three-dimensional nanodevices.

The thermoelastic properties of ferropericlase Mg1؊xFexO (x ‫؍‬ 0.1875) throughout the iron high-to-low spin cross-over have been investigated by first principles at Earth's lower mantle conditions. This cross-over has important consequences for elasticity such as an anomalous bulk modulus (K S) reduction. At room temperature the anomaly is somewhat sharp in pressure but broadens with increasing temperature. Along a typical geotherm it occurs across most of the lower mantle with a more significant K S reduction at Ϸ1,400 -1,600 km depth. This anomaly might also cause a reduction in the effective activation energy for diffusion creep and lead to a viscosity minimum in the mid-lower mantle, in apparent agreement with results from inversion of data related with mantle convection and postglacial rebound.Earth's lower mantle ͉ viscosity ͉ thermodynamics ͉ thermal expansivity U nderstanding of the Earth's lower mantle relies on indirect lines of evidence. Comparison of elastic properties extracted from seismic models with computed or measured elastic properties of candidate minerals at mantle conditions is a fruitful line of enquiry. For instance, it has shed light on the lower mantle composition (1-3) and on the nature of the DЉ layer (4, 5). Such comparisons support the notion that the lower mantle consists primarily of ferrosilicate perovskite, Mg 1Ϫy Fe y SiO 3 , and ferropericlase, Mg 1Ϫx Fe x O (hereafter, Pv and Fp, respectively). In contrast, evidence based on solar and chondritic abundances suggests a deep lower mantle chemical transition into a pure Pv composition at Ϸ1,000 km depth (6). A chemical transition with wide topography, gentle, and diffuse changes in elasticity and density is also supported by geodynamic modeling (7). The discovery of the spin cross-over in Fp and Pv at lower mantle pressures (8, 9) introduces a new dramatic ingredient that demands a careful reexamination of these phases' elastic properties at appropriate conditions, the consequences for mantle elasticity, and reanalysis of lower mantle properties. This may, after all, support lower mantle models containing a chemical transition. Here, we show the effect of the spin cross-over on the bulk modulus and bulk velocity of Fp at high temperatures. We also show the effect it should have on the bulk modulus of a homogeneous lower mantle with pyrolite composition and confirm and justify the origin of anomalies in the elasticity of Fp recently demonstrated at room temperature (10). We point out that such an elastic anomaly might alter the activation energy for diffusion creep (11,12) in Fp, which might affect mantle viscosity. Results and DiscussionsThe high-spin (HS) to low-spin (LS) cross-over (13) in ferrous iron in Fp has been detected by several techniques at room temperature (8,10,(14)(15)(16)(17)(18) and recently up to 2,000 K (19). For typical mantle compositions the cross-over may start as low as Ϸ35 GPa (18) and end as high as 75 GPa (8) at room temperature. The observed variations in the pressure range of the transition seem to b...

The discovery of a pressure induced iron-related spin crossover in Mg((1-x))Fe(x)O ferropericlase (Fp) and Mg-silicate perovskite, the major phases of Earth's lower mantle, has raised new questions about mantle properties which are of central importance to seismology. Despite extensive experimental work on the anomalous elasticity of Fp throughout the crossover, inconsistencies reported in the literature are still unexplained. Here we introduce a formulation for thermoelasticity of spin crossover systems, apply it to Fp by combining it with predictive first principles density-functional theory with on-site repulsion parameter U calculations, and contrast results with available data on samples with various iron concentrations. We explain why the shear modulus of Fp should not soften along the crossover, as observed in some experiments, predict its velocities at lower mantle conditions, and show the importance of constraining the elastic properties of minerals without extrapolations for analyses of the thermochemical state of this region.

The thermodynamics properties of ferropericlase ͑Mg 1−x Fe x O where x = 0.1875͒ ͑Fp͒ throughout its spin crossover were investigated by first principles. Fp was treated as an ideal solid solution of pure high-spin and low-spin states. The Gibbs free energies of the pure states were addressed using the LDA+ U method. A vibrational virtual-crystal model was developed to address the vibrational properties of the pure spin cases and used in conjunction with quasiharmonic theory to compute their vibrational free energies. The thermodynamics properties of Fp display significant anomalies that should be typical of spin crossover systems in general. In Fp, in particular, they are fundamental for understanding the state of earth's interior, where the pressure and temperature conditions of the crossover are realized.

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