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 C_2v 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 C_2v 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 Au⁰_s or Ag⁰_s. The equilibrium structures were found to correspond to a displacement of 0.20 Å for Ni^-, and 0.15 Å for Pt^-_s. For Au⁰_s and Ag⁰_s, 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 Au⁰_s. In the case of Au⁰_s and Pt⁰_s, 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 Pt_s^-, the TM ion moved back closer to its T_d site, to stabilise to within 0.01 Å of the lattice site, in evident disagreement with the predictions of the dihedral model.