Chemical bonding in zeolites

A review is given with 28 refs. on the current status of chem. bonding of zeolites and AlPO's. Short range covalent bonding dominates, the tetrahedra have to be considered relatively rigid and the Si-O-Si bond angle flexible. Differences in energy of the SiO2 or AlPO4 polymorphs are small. The relative stability depends on a proper accounting of the small changes in electrostatic energy. The deprotonation energy is also mainly detd. by short range covalent interactions. These are only properly accounted for when full lattice relaxation is included in the calcns. Isomorphous substitution effects are also dominated by changes in covalent interaction energ

Accurate thermal conductivities from optimally short molecular dynamics simulations

The evaluation of transport coefficients in extended systems, such as thermal conductivity or shear viscosity, is known to require impractically long simulations, thus calling for a paradigm shift that would allow to deploy state-of-the-art quantum simulation methods. We introduce a new method to compute these coefficients from optimally short molecular dynamics simulations, based on the Green-Kubo theory of linear response and the cepstral analysis of time series. Information from the full sample power spectrum of the relevant current for a single and relatively short trajectory is leveraged to evaluate and optimally reduce the noise affecting its zero-frequency value, whose expectation is proportional to the corresponding conductivity. Our method is unbiased and consistent, in that both the resulting bias and statistical error can be made arbitrarily small in the long-time limit. A simple data-analysis protocol is proposed and validated with the calculation of thermal conductivities in the paradigmatic cases of elemental and molecular fluids (liquid Ar and H2O) and of crystalline and glassy solids (MgO and a-SiO2). We find that simulation times of one to a few hundred picoseconds are sufficient in these systems to achieve an accuracy of the order of 10% on the estimated thermal conductivities

Electron-beam-assisted superplastic shaping of nanoscale amorphous silica

At room temperature, glasses are known to be brittle and fracture upon deformation. Zheng et al. show that, by exposing amorphous silica nanostructures to a low-intensity electron beam, it is possible to achieve dramatic shape changes, including a superplastic elongation of 200% for nanowires

Accelerated discovery of two crystal structure types in a complex inorganic phase field

The discovery of new materials is hampered by the lack of efficient approaches to the exploration of both the large number of possible elemental compositions for such materials, and of the candidate structures at each composition1. For example, the discovery of inorganic extended solid structures has relied on knowledge of crystal chemistry coupled with time-consuming materials synthesis with systematically varied elemental ratios2,3. Computational methods have been developed to guide synthesis by predicting structures at specific compositions4,5,6 and predicting compositions for known crystal structures7,8, with notable successes9,10. However, the challenge of finding qualitatively new, experimentally realizable compounds, with crystal structures where the unit cell and the atom positions within it differ from known structures, remains for compositionally complex systems. Many valuable properties arise from substitution into known crystal structures, but materials discovery using this approach alone risks both missing best-in-class performance and attempting design with incomplete knowledge8,11. Here we report the experimental discovery of two structure types by computational identification of the region of a complex inorganic phase field that contains them. This is achieved by computing probe structures that capture the chemical and structural diversity of the system and whose energies can be ranked against combinations of currently known materials. Subsequent experimental exploration of the lowest-energy regions of the computed phase diagram affords two materials with previously unreported crystal structures featuring unusual structural motifs. This approach will accelerate the systematic discovery of new materials in complex compositional spaces by efficiently guiding synthesis and enhancing the predictive power of the computational tools through expansion of the knowledge base underpinning them

Computational studies of zeolite framework stability

For the purpose of detg. the relative stabilities of topol. different Al-free tetrahedral networks, Hartree-Fock-level ab-initio calcns. were done of the relative stability of 3-, 4-, 5-, and 6-unit SiO(OH)2 rings. Very small differences per T unit are found for the 4-, 5-, and 6-rings; however, the energy per T unit is unfavorable for the 3-ring. Rigid ion lattice minimization calcns. were performed on Al-free as well as high-Al-content zeolite systems. The results are discussed for ZSM-5, mordenite and faujasite structures. Very small energy differences, of the order of .apprx.20 kJ/mol, are again found for the Al-free networks. Open structures have less favorable energy than dense structures due to decreased Madelung energy. Large changes in relative energy are found on variation of the Al/Si ratio. Medium- and small-pore zeolites are much more sensitive to an increase in Al content than the wide-pore material. This should be ascribed to stacking of the cations in the channels of the zeolite. The implications of these results for zeolites synthesis are discusse

Relation between crystal symmetry and ionicity in silica polymorphs

THE structure and stability of an inorganic solid is determined in general both by short-range covalent and by long-range electrostatic forces. Here we describe the use of interatomic force fields developed recently from first-principles quantum- chemical cluster calculations 1, 2 in the study of the structures of SiO2 tetrahedral networks. We find that the symmetry of these structures depends sensitively on the balance between ionic and covalent forces: high-symmetry structures are stabilized for relatively large ion partial charges, and low-symmetry structures are stabilized when the ionicity is small. For some SiO2 polymorphs, the low-symmetry structures found in our simulations correspond to the low-temperature phases of these polymorphs found experimentally. A reinterpretation of structural data on quartz provides evidence for temperature dependence of the ionicity, which can explain the change of symmetry observed when temperature is increased. Our preliminary calculations on aluminophosphates suggest that this symmetry-breaking mechanism may also provide insight into the structural changes observed for complex molecular sieve