Employing a polyatomic version of the basin hopping global optimization algorithm, together with interatomic potentials specifically tailored to accurately describe the structures and energetics of nanoscale silica, a large number of energetically low-lying nanoclusters for (SiO 2 ) N (N ) 6-12) was generated. Substantial subsets of particularly low-energy candidate structures for each cluster size were subsequently further evaluated using density functional theory (DFT) energy minimization calculations. We report the resulting lowest energy nanoclusters, together with the energetically nearest lying nanoclusters, for each cluster class, (SiO 2 ) N (N ) 6-12). The majority of the clusters obtained display no structural motifs typical of bulk crystalline silica. Of all the clusters studied, the lowest energy (SiO 2 ) 8 cluster found is shown to be especially thermodynamically favored compared to other similarly sized cluster isomers and with respect to addition or removal of SiO 2 units. The clusters are discussed with respect to their structure, their reactivity, and their suitability as building blocks for new materials.

Bromley et al. Reply: In our Letter [1] we explicitly demonstrate that defect-terminated SiO 2 chain clusters may be stabilized as fully coordinated rings, and speculate that ''. . .we envisage. . . fully connected clusters, such as our molecular rings, as materials building blocks. '' In their Comment [2], Sun et al. propose SiO 2 clusters possessing terminating defects but lower energies as building blocks. While never claiming that SiO 2 rings have particularly low energies, we hold that the suitability of a cluster/molecule to act as a material building block is not primarily determined by its energy, but by other criteria: (i) structural stability, (ii) thermodynamic accessibility, (iii) formation selectivity, and (iv) resistance to coalescence. We stress that there is no requirement for a cluster's structural stability to be dependent on its energy. Si 12 O 24 rings, as tested by molecular dynamics (T 1000 K), for example, are particularly resistant to collapse or rupture. The energy of a cluster/molecule is also often a poor guide to the ease with which it may be formed. In recent plasma experiments, for example, ground state clusters are not necessarily, or even usually, formed, but, rather, metastable clusters [3]. For thermodynamic accessibility, (ii), it is thus not sufficient that a potential building block has relatively low energy. For our SiO 2 N rings, formation could be envisaged by deformations of long two-ring chains, or linking smaller chains, both thermodynamically downhill processes [1]. As it has long been established that two-ring chains of arbitrary length can be synthesized in the formation of silica-W [4], and that two-ring chain clusters are formed in (Si,O)-plasma reactions [5], the manipulation of such sources indicates viable routes to ring formation. We note that the proposed low energy SiO 2 building blocks of Sun et al. are also not ground state clusters [see Figs. 1(a) and 1(b)], nor is any route to their formation obvious to us. Building blocks can be utilized only if made in large quantities, thus, ideally, one would like them to be selectively identifiable. Our rings have IR bands at 886, 902, and 933 cm ÿ1 . Jena et al. pointed out that their Si 12 O 24 cluster also has similar bands, seemingly making identification difficult [2]. We note, however, that their structure also has numerous other significant bands (e.g., 827, 951 cm ÿ1 ), which are not present in the spectra of the rings [6]. Furthermore, both the rings and the fully coordinated cage [see Fig. 1(b)] have particularly high ionization potentials facilitating their separation as neutral species from an ionizing environment. Finally, criterion (iv) provides, we feel, the defining property of a cluster-based, or molecular material, i.e., being an assembly of interacting but discrete structurally stable units. High relative reactive stability is thus of great importance when bringing clusters together to form a material. Terminated SiO 2 clusters are particularly reactive to each other and are thus prone to coalesce ...

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Extensive large-scale global optimizations refined by ab initio calculations are used to propose SiO 2 N N 14-27 ground states. For N < 23 clusters are columnar and show N-odd-N-even stability, energetically and electronically. At N 23 a columnar-to-disk structural transition occurs reminiscent of that observed for Si N . These transitions differ in nature but have the same basis, linking the nanostructural behavior of an element (Si) and its oxide (SiO 2 ). Considering the impact of devices based on the nanoscale manipulation of Si=SiO 2 the result is of potential technological importance. DOI: 10.1103/PhysRevLett.95.185505 PACS numbers: 61.46.+w, 36.40.Ei, 36.40.Mr, 68.65.2k The technological importance of nanoscale silica (SiO 2 ) can hardly be overstated considering its established utilization in microelectronics, catalysis, and composite materials and its promise in the emerging field of photonics. Although much experimental and theoretical work has focused upon the stabilities and structures of the many silica bulk polymorphs and their surfaces, the low energy structures and potential energy surface (PES) of nanoscale SiO 2 clusters is relatively unknown. In this Letter we provide the beginnings of an energetic baseline of the complex PES of nanoscale SiO 2 by finding the lowest energy forms of silica for SiO 2 N N 14-27 providing new limits on the stability of this centrally important material in the physical, chemical, and geophysical sciences. The clusters we study all have at least one dimension between 1 and 2.5 nm and are all substantially lower in energy than any previously reported. Our results strongly indicate that one-dimensional columnar structures are energetically favored up to a length scale of at least 2 nm where upon a transition to two-dimensional trigonal disklike structures occurs at N 23. The transition is characterized by sharp changes in measures of the structural form, electronic structure, and bonding topology of the cluster series, but appears smooth with respect to cluster energies. The structural transition in SiO 2 N clusters is compared to that known for clusters of its parent element Si N [1]. In both cases for N 23-25 a structural transition from elongated to more compact forms appears to occur in the thermodynamically preferred clusters [2]. Usually oxides and their parent elements display different but complementary physical and chemical properties and typically bear little structural relation to one another. Our finding that, at the nanoscale, the size-dependent structural behavior of SiO 2 and its parent element follow a similar general pattern is at once relatively unexpected and fundamentally suggests an inherent link between the two materials. Because of the immense dependence on the combined structural and physical properties of both nanoscale Si and SiO 2 in electronic and optical devices, our demonstration of such a connection is of potential significant technological importance.Although relatively small, M 20 homogeneous clusters of low atomic weight atoms are...

In order to investigate the technical feasibility of crystalline porous silicates as hydrogen storage materials, the self-diffusion of molecular hydrogen in all-silica sodalite is modeled using large-scale classical molecular-dynamics simulations employing full lattice flexibility. In the temperature range of 700-1200 K, the diffusion coefficient is found to range from 1.6•10 Ϫ10 to 1.8•10 Ϫ9 m 2 /s. The energy barrier for hydrogen diffusion is determined from the simulations allowing the application of transition state theory, which, together with the finding that the pre-exponential factor in the Arrhenius-type equation for the hopping rate is temperature-independent, enables extrapolation of our results to lower temperatures. Estimates based on mass penetration theory calculations indicate a promising hydrogen uptake rate at 573 K.