First-principles calculations of structural and electronic properties of Ta-doped Si clusters, wires, and bulk systems

Recent experiments have shown that Ta@Si-16(+) is a very stable cation from which it should be possible to create Si-based cluster assembled materials. In this paper we have studied, by means of first-principles spin-dependent generalized gradient approximation calculations, the structural and electronic properties of the following systems: (i) Ta@Si-n(+) clusters in the range n = 14-18; (ii) (Ta@Si16F) m aggregates with sizes m = 1-8 formed by Ta@Si-16(F) molecules; (iii) infinite wires formed by stacking triangular (Ta@Si-16(F)) 3 aggregates twisted 60 degrees. to each other along the vertical axis; and (iv) the fcc phase of bulk Ta@Si-16(F). Theminimum-energy Ta@Si-16(+) cluster shows C-3v symmetry, having 40 meV smaller total energy than a fullerenelike D-4d isomer. However, the molecule Ta@Si-16(F) formed with that D-4d isomer is 40 meV more stable than that formed with the C-3v one. We have optimized several [ Ta@Si-16(F)] n aggregates (n = 1-8) which contain the Ta@Si-16 unit with D-4d symmetry. The more bound (Ta@Si-16(F)) 6 aggregate is formed by stacking vertically two triangular (Ta@Si-16(F)) 3 aggregates which are twisted 60 degrees to each other. The infinite wire formed with that (Ta@Si-16(F)) 6 aggregate as the unit cell has a cohesive energy 1.88 eV and a small highest occupied molecular orbital-lowest occupied molecular orbital gap. We have optimized also a metastable fcc bulk phase having the Ta@Si-16(F) supermolecule as the unit cell. A Birch-Murnaghan fit to that phase produces a cohesive energy 0.84 eV at lattice constant 12.27 angstrom, with bulk modulus 7.55 GPa and a phase stability to isotropic compression smaller than 0.75 GPa. That phase is nonmagnetic and shows a band gap of 0.20 eV. Using the values of hardness of Ta@Si-16(F) molecules, we estimated a correction enhancement factor similar to 3 to that small band gap. For that metastable solid we performed a 13.5-ps run of first-principles molecular dynamics annealing at 300 K and constant volume, and we found that the Ta@Si-16(F) supermolecule in the fcc cell becomes severely distorted after the first 5 ps.