Structural Correspondences between the Low-Energy Nanoclusters of Silica and Water

Nanoclusters of two well-known tetrahedrally coordinated systems, (SiO 2 ) N silica and (H 2 O) N water, are compared in the size range N = 8−26 with respect to energetic stability and T-atom topology (where T = Si and O, respectively). Because of the similar cage-forming tendencies of both systems in a number of bulk phases (e.g., clathrates), we focus on clusters with an even number of T-atoms for which cluster structural decomposition into a union of convex polyhedra (i.e., face-sharing cages with T-atom-vertices) is always possible. For (H 2 O) N, in the range N = 8−24, the even- N ground-state clusters are almost exclusively formed from face-sharing cubes and pentaprisms of T-atoms. We show that (SiO 2 ) N clusters having the same T-atom topology as these prismatic water clusters are also low in energy (per SiO 2 unit with respect to the corresponding lowest energy (SiO 2 ) N cluster), and have the same groundstate structure for N = 12. Along with similarities, an important difference is discussed, namely that, rather than a converging tendency for the two systems to have similar prismatic topologies with increasing size, for (H 2 O) N at N = 24, self-solvating amorphous clusters compete strongly with the prismatic ground states, becoming ground states themselves for N > 24. Contrary to this shift in water cluster structural preference, (SiO 2 ) 24 clusters with the same T-atom topology as the amorphous (H 2 O) 24 clusters are found to be high in energy with respect to both the (SiO 2 ) 24 ground-state and (SiO 2 ) 24 prismatic isomers. We discuss possible reasons for the existence and eventual breakdown of the topological analogy between (H 2 O) N and (SiO 2 ) N nanoclusters with relation to the different bonding characteristics in each system.