Analytic Environment-Dependent Tight-Binding Bond Integrals: Application to \u3cstrong\u3eMoSi\u3csub\u3e2\u3c/sub\u3e\u3c/strong\u3e

We present the first derivation of explicit analytic expressions for the environmental dependence of the σ, π, and δ bond integrals within the orthogonal two-center tight-binding approximation by using the recently developed bond-order potential theory to invert the nonorthogonality matrix. We illustrate the power of this new formalism by showing that it not only captures the transferability of the bond integrals between elemental bcc Mo and Si and binary C11b MoSi2 but also predicts the absence of any discontinuity between first and second nearest neighbors for the ddσ bond integral even though large discontinuities exist for ppσ, ppπ, and ddπ

Bond-Order Potentials with Analytic Environment-Dependent Tight-Binding Integrals: Application to BCC Molybdenum

We present a new Screened Bond-Order Potential (SBOP) for molybdenum in which the environmental dependence of two-center tight-binding bond integrals has been implemented via a recently developed analytic expression. These bond integrals reproduce very well the numerical ab-intio values of screened LMTO bond integrals. In particular, they display the large discontinuity in ddpi between the first and second nearest neighbor of the bcc lattice whereas they do not show any discontinuity in ddsigma. This dependence can be traced directly to the angular character of the analytic screening function and is shown to be critical for the behavior of the second nearest neighbor force constants. The new BOP eliminates the problem of the very soft T2 phonon mode at the N point that is found in most two-center tight-binding models. Preliminary study of the core structure of 1/2\u3c111\u3e screw dislocations performed using SBOP indicates that the core is narrower and less asymmetric than structures found in previous studies, in agreement with recent ab-initio calculations

Bond-Order Potential for Molybdenum: Application to Dislocation Behavior

The bond-order potential (BOP) for transition metals is a real-space semiempirical description of interactions between the atoms, which is based on the tight-binding approximation and the d-band model. This scheme provides a direct bridge between the electronic level modeling and the atomistic modeling, where the electronic degrees of freedom have been coarse grained into many-body interatomic potentials. In this paper we construct BOP in which both the attractive and the repulsive contributions to the binding energy are environmentally dependent due to both the nonorthogonality of the orbitals and the breathing of the screening charges. The construction of the BOP is described and tested in detail. First, the energies of alternative crystal structures (A15, fcc, hcp, simple cubic) are calculated and compared with those evaluated ab initio. The transferability of the BOP to atomic configurations that deviate significantly from the bcc lattice is studied by computing the energies along tetragonal, trigonal, and hexagonal transformation paths. Next, the phonon spectra are evaluated for several symmetrical crystallographic directions and compared with available experiments. All these calculations highlight the importance of directional bonding and the investigation of phonons demonstrates that the environmental dependence of the bond integrals is crucial for the phonons of the N branch not to be unphysically soft. Finally, the constructed BOP was applied in the modeling of the core structure and glide of the 1/2⟨111⟩ screw dislocation. The calculated structure of the core agrees excellently with that found in the recent ab initio calculations and the observed glide behavior not only agrees with available ab initio data but is in agreement with many experimental observations and explains the primary reason for the breakdown of the Schmid law in bcc metals

A Combined \u3ci\u3eAb Initio\u3c/i\u3e and Bond-Order Potentials Study of Cohesion in Iridium

The extremely high melting point and excellent resistance to oxidation and corrosion offered by iridium suggest numerous applications of this transition metal in static components at high temperatures and in aggressive environments. However, the mechanical and physical properties of f.c.c. Ir exhibit numerous anomalies when compared to other metals that crystallize in the f.c.c. structure. Notable examples include a negative Cauchy pressure, 1/2 (C12 – C44), brittle transgranular cleavage after a period of plastic flow even in pure single crystals and anomalous [zeta zeta 0] branches in the phonon spectra. Atomistic studies of extended defects are needed to elucidate the origin of anomalous mechanical properties, such as brittleness. For this purpose we developed a Bond-Order Potential (BOP), an O(N) tight-binding formalism, employing physically transparent parameterizations that use experimental and ab initio data, generated in this study using the Full Potential Augmented Plane Wave plus Local Orbitals (APW+lo) method. The constructed BOP reproduces then both equilibrium as well as a variety of non-equilibrium properties of Ir and represents an excellent description of cohesion in f.c.c. Ir. This description of interatomic interactions is imminently suitable for studies of defects, such as dislocations and grain boundaries, that control plastic deformation and fracture

Construction, assessment, and application of a bond-order potential for iridium

A tight-binding based bond-order potential (BOP) has been constructed for the fcc transition metal iridium that includes explicitly only d orbitals in the evaluation of the total energy. We show that hybridization between the nearly free electron sp band and the unsaturated covalently bonded d orbitals is important in determining the relative stabilities of the close-packed structures and that this effect can be accurately captured through the use of a central force term. The BOP is found to provide an excellent description of the equilibrium properties of iridium, including its negative Cauchy pressure that is fitted using a many-body repulsive term. The transferability of the BOP is assessed by calculating energy differences between different crystal structures, the energetics of the tetragonal and trigonal deformation paths, the phonon spectra, stacking fault, and vacancy formation energies. Comparison of the results of these studies with either experiments or first principles calculations is found to be good. We also describe briefly the application of the constructed BOP to the atomistic simulation of the core structure of the screw dislocation that led to an explanation of the anomalous deformation and fracture behavior exhibited of iridium

Atomistic Studies of Dislocation Glide in gamma-TiAl

Computer simulation of the core structure and glide of ordinary 1/2\u3c110] dislocations and \u3c101] superdislocations in L10 TiAl has been performed using the recently constructed Bond-Order Potentials. This description of atomic interactions includes explicitly, within the tightbinding approximation, the most important aspects of the directional bonding, namely d-d, p-p and d-p bonds. The ordinary dislocation in the screw orientation was found to have a non-planar core and, therefore, high Peierls stress. The superdislocation was found to possess in the screw orientation either a planar (glissile) or a non-planar (sessile) core structure. However, the glissile core transforms into the sessile one for certain orientations of the applied stress. This implies a strong asymmetry of the yield stress and the break down of the Schmid law when the plastic flow is mediated by superdislocations. At the same time, this may explain the orientation dependence of the dislocation substructure observed in the single-phase gamma-TiAl by electron microscopy

A Bond-Order Potential Incorporating Analytic Screening Functions for the Molybdenum Silicides

The intermetallic compound MoSi2, which adopts the C11b crystal structure, and related alloys exhibit an excellent corrosion resistance at high temperatures but tend to be brittle at room and even relatively high temperatures. The limited ductility of MoSi2 in ambient conditions along with the anomalous temperature dependence of the critical resolved shear stress (CRSS) of the {110)\u3c111], {011)\u3c100] and {010)\u3c100] slip systems and departure from Schmid law behavior of the {013)\u3c331] slip system can all be attributed to complex dislocation core structures. We have therefore developed a Bond-Order Potential (BOP) for MoSi2 for use in the atomistic simulation of dislocations and other extended defects. BOPs are a real-space, O(N), two-center orthogonal tight-binding formalism that are naturally able to describe systems with mixed metallic and covalent bonding. In this development novel analytic screening functions have been adopted to properly describe the environmental dependence of bond integrals in the open, bcc-based C11b crystal structure. A many-body repulsive term is included in the model that allows us to fit the elastic constants and negative Cauchy pressures of MoSi2. Due to the internal degree of freedom in the position of the Si atoms in the C11b structure which is a function of volume, it was necessary to adopt a self-consistent procedure in the fitting of the BOP. The constructed BOP is found to be an excellent description of cohesion in C11bMoSi2 and we have carefully assessed its transferability to other crystal structures and stoichiometries, notably C40, C49 and C54 MoSi2, A15 and D03 Mo3Si and D8m Mo5Si3 by comparing with ab initio structural optimizations

Negative compressibility in platinum sulfide using density-functional theory

Copyright © 2010 The American Physical SocietyThe structural and dynamic properties of the mineral Cooperite (PtS) are investigated using density-functional theory. The results show that a competition with the less symmetric but more compact PdS structure leads to a phase transition when the pressure is increased. However, before the phase transition, PtS displays a rare anomalous elastic behavior by expanding along its long axis under hydrostatic pressure. We report the elastic constants of PtS and interpret this negative linear compressibility in the context of a displacive phase transition. We also show that the real structure of PtS is less symmetric than originally determined by experiment