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Substitutional Au0 and Pt-


For substitutional Pt- and Au0, there are three electrons in the one-electron t2 manifold, which makes this state Jahn-Teller sensitive. Experiment and theoretical studies have shown that the JT-stabilised structures have C2v symmetry, spin S=1/2 for a ground-state configuration $b_2^{\uparrow\downarrow}\,b_1^{\uparrow}\,a_1^0$, in agreement with the vacancy model [#!watkins-83!#]. According to this model, the electronic properties of substitutional TM impurities near the end of the 3d, 4d and 5d series can be described by considering a neutral TM atom with a saturated d10 shell occupying a negatively charged lattice vacancy. As a consequence, the structural and electronic properties of these impurities are dictated by the vacancy-like character of the sp3 Si hybrid orbitals and not by the many-electron character of the partially filled TM nd shell.

The distortions have been investigated by breaking the symmetry and relaxing the Si inner atoms including the impurity, in a specific electronic configuration under the appropriate symmetry constraint [#!jones-95!#]. The C2v structures with the impurity displaced off-site along the dihedral [100] direction have proven to be stable with small shifts of the neighbouring Si atoms. The magnitude of distortion is slightly larger for Ni-s and Pt-s than for Au0s or Ag0s. The equilibrium structures were found to correspond to a displacement of 0.20 Å for Ni-, and 0.15 Å for Pt-s. For Au0s and Ag0s, these were 0.03 and 0.01 Å, respectively. Our calculations find a small barrier for thermal re-orientation, consistent with experimental results for Pt-s and Au0s. In the case of Au0s and Pt0s, the thermal barrier was calculated to be 0.11 and 0.26 eV, respectively.

An alternative model has been introduced to explain apparent discrepancies between the observed g values for Pt- and those predicted by the vacancy model for the same centre. To test the predictions of the two-neighbouring-bonding or dihedral model [#!ammerlaan-89!#], the TM ion was displaced off-site by 0.8 Å along the [100] direction, closer to two neighbouring Si, and the opposite Si atoms to the vacancy were brought closer, to simulate the stronger tetragonal distortion. For Pts-, the TM ion moved back closer to its Td site, to stabilise to within 0.01 Å of the lattice site, in evident disagreement with the predictions of the dihedral model.


next up previous
Next: TM-Hn complexes Up: Methodology & results Previous: Methodology & results
Antonio Resende
1998-06-12