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

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• 作者：Stefan T. Bromley ; Bernhard Bandow ; Bernd Hartke
• 刊名：Journal of Physical Chemistry C
• 出版年：2008
• 出版时间：November 27, 2008
• 年：2008
• 卷：112
• 期：47
• 页码：18417-18425
• 全文大小：362K
• 年卷期：v.112,no.47(November 27, 2008)
• ISSN：1932-7455

文摘

Nanoclusters of two well-known tetrahedrally coordinated systems, (SiO₂)_N silica and (H₂O)_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 (H₂O)_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 (SiO₂)_N clusters having the same T-atom topology as these prismatic water clusters are also low in energy (per SiO₂ unit with respect to the corresponding lowest energy (SiO₂)_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 (H₂O)_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, (SiO₂)₂₄ clusters with the same T-atom topology as the amorphous (H₂O)₂₄ clusters are found to be high in energy with respect to both the (SiO₂)₂₄ ground-state and (SiO₂)₂₄ prismatic isomers. We discuss possible reasons for the existence and eventual breakdown of the topological analogy between (H₂O)_N and (SiO₂)_N nanoclusters with relation to the different bonding characteristics in each system.