Simulating intergrowth formation in zeolite crystals: impact on habit and functionality.

A kinetic Monte-Carlo methodology is presented for simulating crystal growth in materials which contain stacking faults. By simulating a large number of potential growth and dissolution events, a representation of the crystal is generated at various stages throughout the crystallisation, allowing the effects of disorder on the evolution of crystal habit and nanoscale surface topography to be explored. As examples, simulations were performed on two intergrown zeolite materials - zeolite T and zeolite beta. In both zeolite T and zeolite beta, simulations demonstrate how an intergrown structure leads to a characteristic roughening of certain crystal facets. In zeolite beta, this is accompanied by the development of internal defects which shows a non-homogeneous distribution. Results of simulations are validated by direct comparison to experimental scanning electron microscopy, atomic force microscopy and X-ray diffraction data. All simulations are performed using the CrystalGrower software package with modifications to account for disorder and should be generally applicable to all classes of crystals.

CrystalGrower: a generic computer program for Monte Carlo modelling of crystal growth.

A Monte Carlo crystal growth simulation tool, CrystalGrower, is described which is able to simultaneously model both the crystal habit and nanoscopic surface topography of any crystal structure under conditions of variable supersaturation or at equilibrium. This tool has been developed in order to permit the rapid simulation of crystal surface maps generated by scanning probe microscopies in combination with overall crystal habit. As the simulation is based upon a coarse graining at the nanoscopic level features such as crystal rounding at low supersaturation or undersaturation conditions are also faithfully reproduced. CrystalGrower permits the incorporation of screw dislocations with arbitrary Burgers vectors and also the investigation of internal point defects in crystals. The effect of growth modifiers can be addressed by selective poisoning of specific growth sites. The tool is designed for those interested in understanding and controlling the outcome of crystal growth through a deeper comprehension of the key controlling experimental parameters.

Predicting crystal growth via a unified kinetic three-dimensional partition model.

Understanding and predicting crystal growth is fundamental to the control of functionality in modern materials. Despite investigations for more than one hundred years, it is only recently that the molecular intricacies of these processes have been revealed by scanning probe microscopy. To organize and understand this large amount of new information, new rules for crystal growth need to be developed and tested. However, because of the complexity and variety of different crystal systems, attempts to understand crystal growth in detail have so far relied on developing models that are usually applicable to only one system. Such models cannot be used to achieve the wide scope of understanding that is required to create a unified model across crystal types and crystal structures. Here we describe a general approach to understanding and, in theory, predicting the growth of a wide range of crystal types, including the incorporation of defect structures, by simultaneous molecular-scale simulation of crystal habit and surface topology using a unified kinetic three-dimensional partition model. This entails dividing the structure into 'natural tiles' or Voronoi polyhedra that are metastable and, consequently, temporally persistent. As such, these units are then suitable for re-construction of the crystal via a Monte Carlo algorithm. We demonstrate our approach by predicting the crystal growth of a diverse set of crystal types, including zeolites, metal-organic frameworks, calcite, urea and l-cystine.

Anatomy of screw dislocations in nanoporous SAPO-18 as revealed by atomic force microscopy.

Defects in solids are often the source of functional activity, the trigger for crystal growth and the seat of instability. Screw dislocations are notoriously difficult to study by electron microscopy. Here we decipher the complex anatomy of one such defect in the industrially important nanoporous catalyst SAPO-18 by atomic force microscopy.

Combinatorial Approach to the Hydrothermal Synthesis of Zeolites.

One hundred or more solid-state syntheses can be conducted in parallel and employed for the combinatorial hydrothermal syntheses of zeolites by using a novel multiautoclave design. The operation of the multiautoclave was ascertained by the reinvestigation of the complete Na2 O-Al2 O3 -SiO2 ternary system in a single experiment. In the picture on the right, the shaded areas on the left show the crystallization fields of the different phases obtained.

Towards a Rational Synthesis of Large-Pore Zeolite-Type Materials?

A range of novel structures, such as the metalloaluminosilicate UCSB-8 (depicted schematically), are accessible by synthetic strategies that could provide the basis for the rational design of other large-pore zeolites and zeolite-type materials. The combination of experimental approaches with computational methods, for example host-guest shape analysis, may provide further breakthroughs in this field.