Structure, Bonding and Stability of Transition Metal Silicides: A Real-Space Perspective by Tight Binding Potentials
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Di~jp (F)= Re[Y-iC'i,(nk)Cjp(nk) Siajp
8(e-&n)]
can be achieved also in the case of orthogonal tight binding by substitution of the overlap element Siojp with the corresponding hopping element Haxj, in agreement to the HUckel guess Siqo = 2 Hi,,_jt/K(e•,,+'jt). This allows for a qualitative analysis of bonding and antibonding features in the density of states (DOS) and for the evaluation of the bond energy contributions to Uk U iEJP(sF)
= J Di.j(s) ds.
309 Mat. Res. Soc. Symp. Proc. Vol. 491 ©1998 Materials Research Society
Obviously, this is not a novelty in the standard LCAO analysis of the stability trends in solids, as nicely outlined in the last fifteen years by distinguished chemists like Roald Hoffman [2], or physicists like David Pettifor [3]. What makes still vital the subject is the need to import it into the standard analysis of molecular dynamics simulations, which is still mainly grounded on structural informations, such as the ones provided by pair correlation distributions. The repulsive potential, on the other hand, also convey some additional information, if we consider that (within a simple pair potential scheme) it contains the orbital overlap correction and the crystal field contribution. In fact, competing structures may display a different partition in Ub. and Urep, depending on the neighbours configurations, in terms of covalent bonding and Pauli repulsion, respectively. Clearly, in case of binary compounds, the need to extend interactions up to secon neighbours (as motivated by the achivement of a satisfactory DOS) may introduce some arbitrariness in the determination of the pair repulsion parameters. Still, a wise fitting on the equilibrium and stability conditions for a few structures always gives rise to a phenomenological
link between the atomic size and the repulsive character. In the last few years we have applied the orthogonal TB method to total energy calculations and molecular dynamics simulations of Ni-, Co- and Fe- silicides [4-12]. The silicon-rich compounds are particularly interesting from this point of view, since they display both a high sensitivity of the electronic features to the bond directions, as provided by the fairly covalent psidrm bonding, and a relevant polymorphic attitude, which is typical of metallic materials. The latter feature is emphasized in the case of thin epitaxial films, which frequently display pseudomorphic (i.e. substrate-induced) crystal phases, not present in the bulk phase diagram. One example is FeSi 2, which is presently the subject of a vivacious interst, due to the prosimising performances provided by its stable, orthorombic phase as a light emitting diode integrated at a silicon junction [13]. ONE EXAMPLE: FeSi 2 At variance with respect to the related compounds NiSi 2 and CoSi 2, the metallic fluorite phase y-FeSi 2 is not bulk-stable, and evolves towards the the 3 form, which exhibits a semiconductive gap as large as 0.8-0.9 eV [14]. The latter includes 24 atoms in a primitive, base-centered orthorhombic cell, and it is generated by
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