CRYSURFSIM · Crystal surface simulations

Crystal surface simulations

Crystals exhibit various external morphologies ranging from the dendritic forms of snow crystals, the needle shape of Epsom salt or the cubic form of rock salt. Understanding, simulating and influencing the crystal morphology is a major concern in modern crystal morphology engineering. A wide spread problem in industry is the appearance of undesired crystal morphologies giving rise to problems in handling and product quality, or to such issues concerning the blinding of filters and tubes. On the other hand, the design of crystal morphologies has gained an increasing interest, within, for example, the pharmaceutical industry or the industrial application of nanotechnology, because influencing the crystal morphology enables the crystal engineer to prepare materials with tailored physical and chemical properties. This opportunity to design materials having specific attributes is a driving factor of industrial crystallisation, where a large quantity of minute crystals with well defined size and form are required to satisfy the needs in pharmaceuticals, chemical seasonings, photographic emulsions, pigments of colour or even proteins. Also, different techniques have been developed that will produce bulk single crystals and thin films with high perfection and homogeneity by strict control of the growth parameters. Various crystalline materials with desired properties have been synthesised and this has driven the utilisation of single crystals in the production of semiconductor, piezoelectric and pyroelectric materials. The shapes of crystals results from the way in which crystals grow. The mechanism of growth is recorded in various forms in each individual crystal, regardless of size. The same crystal species may show macroscopic different crystal habits (polyhedral, skeletal and dendriticl), depending on growth conditions. At a microscopic level, spiral growth step patterns, which record the growth processes at the nanometre scale, have been observed on crystal surfaces. Further, in single crystals, fluctuations in growth rates during the growth process are recorded and can be observed and interpreted as variations in perfection and homogeneity during the growth of the mineral specimen. These fluctuations are observed not only in crystals formed by inorganic processes, but also in those formed in living organs like bones, teeth or shells, or in calculus formed in various organs through the excretion of unnecessary components. By application of the BVD-model (bond valence deficiency model) developed by Mutter (2007), it is possible to combine different crystal structure properties (reticular density, lattice spacing, crystal symmetry and crystal structure type), with the number of unsatisfied bonds per unit area of a given crystal surface. Thus, it is possible to simulate the ideal crystal form of a given mineral. Moreover, it is possible to relate this rather abstract form, via the interaction of the bond-valence deficiency of the surface and the bond-valence distribution of an adjacent fluid, to the growth morphology of a crystal. The CRYSURFSIM project, financed by the Marie Curie FP7-People – 2010 Intra-European Fellowship (IEF) program explore a new approach to combine classical crystal growth theories with modern quantum mechanical simulations, by application of the BVD-model. In a first approach the model was tested by simulating the surface free energy of fcc, bcc and hcp metals. The results obtained agreed well with results available in literature and the results of lattice simulations, and the crystal morphologies calculated were in accordance with the most common forms of the minerals observed in nature. It proved that the major advantage of this model is its speed, as crystal morphologies and crystal surface energies can be determined with only minor computational efforts. The application of the model to simulate the surface free energy of minerals AX and AX2 structure types has proven to be more challenging as models calculatd by BVD and quantum mechanical methods differ to a certain extent. While the BVD-model can calculate the surface free energy for charged and charge-neutral surfaces alike, lattice simulation methods need to convert the surface termination of charged surfaces to generate a new charge-neutral surface in ways that cannot be directly converted to the BVD-model. Thus results can only be compared to each other with great care and are still under debate. However, when the surface free energies obtained by application of the BVD-model for various halides, sulphides and oxides are taken as a basis to deduce their crystal morphological ranking, the results of the model agreed well with the most common forms of these minerals observed in nature. Thus despite the variance in the absolute values of the surface free energies obtained by either application of the BVD-model or quantum mechanical simulations, the BVD-model again has proven to be a fast and valuable tool to determine the morphology of crystals with only minor computational efforts.

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