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

Abstract

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. A tight-binding based bond-order potential ͑BOP͒ 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.

Bond-order potential for simulations of extended defects in tungsten

“…4 results for the hcp structures because they almost coincide with those for the fcc structure and are difficult to discern.͒ Figures 4͑a͒ and 4͑b͒ show that not only is the bcc structure most stable in the whole range of investigated volumes but that both BOP parametrizations reproduce well the DFT results for the fcc, hcp, and sc structures even though the only fitted quantities were the cohesive energy and elastic moduli of the bcc structure at its equilibrium volume. Similarly as in the case of Mo 58 and Ir, 59 the transferability of BOP's to the A15 structure is less satisfactory and the energy of this structure is overestimated when compared with ab initio results. In order to gain an insight into the origin of this discrepancy, we separated the structural energy differences of the three lowest-energy structures, calculated at bcc equilibrium volume per atom, into two parts: the bond contribution E bond , shown in Fig.…”

“…V͑R ij ͒ is a pair potential and, similarly as in previous studies, [58][59][60]81 it is taken as a sum of cubic splines:…”

“…They are then fitted to reproduce accurately the values of the screened bond integrals that are obtained numerically from calculations employing the first-principles tight-binding linearized-muffintin-orbital ͑TB-LMTO͒ method. 56 The BOP's have now been constructed for three elemental transition metals, titanium, 57 molybdenum, 58 and iridium, 59 as well as for Ti-Al alloys. 60 These potentials were employed extensively in studies of dislocations and led to a number of important findings.…”

Tight-binding potential for atomistic simulations of carbon interacting with transition metals: Application to the Ni-C system

“…52 The fcc lattice is then completely unstable, and actually the bcc structure is found to be more stable for nearly filled d bands, when performing total energy calculations. 53,54,55 Similarly the cohesive energies are much too low.…”

Dislocation Driven Problems in Atomistic Modelling of Materials

“…14) We have therefore showed that the environmentally dependent bond-order potentials describing correctly not only bonding character but also elastic Cauchy relation in fcc-Ir, have predicted very well planar defect behavior. The latter is crucial for investigating mechanical properties in metals and in particular the origin of brittleness in iridium.…”