Tight-binding molecular dynamics and density-functional simulations reveal detailed diffusion mechanisms of the compact silicon tri-interstitials Ib3 . The diffusion pathway of Ib3 s can be visualized as a five defect-atom object both translating and rotating in a screw-like motion along ⟨111⟩ directions. Density functional theory yields a diffusion constant of ∼ 10-5 exp(-0.49 eV/kBT) cm2/s. The diffusion path of Ib3 s suggests a similar collective diffusion for the ground state di-interstitial Ia2 s. While Ia2 s perform a translation/rotation step with a 0.3 eV barrier to diffuse along ⟨111⟩ bond directions, an additional reorientation step with a 90 meV barrier allows Ia2 s to achieve isotropic diffusion through the crystal. The resulting diffusion constant of Ia2 is ∼ 10-4 exp(-0.3 eV/kBT) cm2/s. The 0.3 eV diffusion barrier of Ia2 s is consistent with the experimental value of 0.6 +/- 0.2 eV. The low-diffusion barriers of Ia2 and Ib3 may be important in the growth of ion-implantation-induced extended interstitial defects.; I also calculate a low-lying transition path connecting the three lowest-energy silicon tri-interstitials Ib3 , Ic3 and ground state Ia3 . An examination of transition rates reveals that at an annealing temperature of 815°C, the relative populations of the three silicon tri-interstitials reach thermal equilibrium within ∼ 1 mus. In particular, I estimate the transition rate from Ib3 to Ia3 to be 7.8 THz exp(-1.4 eV/kBT). I find that the I3-chain structure rapidly decays to Ia3 by a strongly exothermic reaction with an activation of only 0.1 eV. The I4-chain, while not the lowest energy I4 structure, is a deep local minimum of the total energy with escape barriers of 0.6 eV. I find that it can easily form by an exothermic reaction of Ia3 plus a single interstitial. Because {lcub}311{rcub} planar defects are comprised of parallel interstitial-chain structures, this reaction may be an important step of the growth process by which {lcub}311{rcub} defects form.
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