Energetics and structures of the initial stages of nucleation of (SiO2)N species:: possible routes to highly symmetrical tetrahedral clustersw

A detailed survey of the low energy isomer spectrum of (SiO2)(N), N = 6-10, 13, 16 has been performed using interatomic potential based global optimisations refined via high-level density functional calculations. Within these spectra, including many isomers reported for the first time, structurally and energetically viable pathways for the initial stages of silica cluster growth through SiO2 nucleation are identified. The role of the exceptionally stable (SiO2)(8) ground state magic'' cluster is highlighted in the possible formation of highly symmetric fully tetrahedral clusters of size (SiO2)(10) and (SiO2)(16). These clusters are found to form a part of a natural (SiO2)(N) = 7, 10, 13, 16 sequence together with the C-3v ground states for (SiO2)(7) and (SiO2)(13). The fully tetrahedral clusters are argued to be likely relatively long-lived metastable species in the process of gas phase SiO2 nucleation due to the manner of their termination. It is speculated that larger tetrahedral (SiO2)(40) clusters may exhibit porous structures.