Evaluation of correlated studies using liquid cell and cryo-transmission electron microscopy: Hydration of calcium sulphate and the phase transformation pathways of bassanite to gypsum.

Insight into the nucleation, growth and phase transformations of calcium sulphate could improve the performance of construction materials, reduce scaling in industrial processes and aid understanding of its formation in the natural environment. Recent studies have suggested that the calcium sulphate pseudo polymorph, gypsum (CaSO4 ·2H2 O) can form in aqueous solution via a bassanite (CaSO4 ·0.5H2 O) intermediate. Some in situ experimental work has also suggested that the transformation of bassanite to gypsum can occur through an oriented assembly mechanism. In this work, we have exploited liquid cell transmission electron microscopy (LCTEM) to study the transformation of bassanite to gypsum in an undersaturated aqueous solution of calcium sulphate. This was benchmarked against cryogenic TEM (cryo-TEM) studies to validate internally the data obtained from the two microscopy techniques. When coupled with Raman spectroscopy, the real-time data generated by LCTEM, and structural data obtained from cryo-TEM show that bassanite can transform to gypsum via more than one pathway, the predominant one being dissolution/reprecipitation. Comparisons between LCTEM and cryo-TEM also show that the transformation is slower within the confined region of the liquid cell as compared to a bulk solution. This work highlights the important role of a correlated microscopy approach for the study of dynamic processes such as crystallisation from solution if we are to extract true mechanistic understanding.

Energetics of Ce and Pu incorporation into zirconolite waste-forms.

The General Utility Lattice Program (GULP) has been used to model the zirconolite lattice, calculate the energies of substituting Ce(3+), Ce(4+), Pu(3+), Pu(4+) and Fe(3+) into the lattice both as single and multi-defect systems and model the formation of Ce(3+), Ce(4+), Pu(3+) and Pu(4+) doped zirconolite lattices. These results have been compared against experimental observations, with particular emphasis on those Ce containing solid solutions that exhibit Ce(3+)/Ce(4+) mixed valence characteristics. It is found that the Ce(3+)/Ce(4+) mixed valence is as a result of reduction within the lattice, with the Ce(3+) being stabilised on the Ca site, and that this behaviour would not be expected for the corresponding Pu solid solutions.

Sampling the structure of calcium carbonate nanoparticles with metadynamics.

Metadynamics is employed to sample the configurations available to calcium carbonate nanoparticles in water, and to map an approximate free energy as a function of crystalline order. These data are used to investigate the validity of bulk and ideal surface energies in predicting structure at the nanoscale. Results indicate that such predictions can determine the structure and morphology of particles as small as 3-4 nm in diameter. Comparisons are made to earlier results on 2 nm particles under constant volume conditions which support nanoconfinement as a mechanism for enhancing the stability of amorphous calcium carbonate. Our results indicate that crystalline calcitelike structure is thermodynamically preferred for nanoparticles as small as 2 nm in the absence of nanoconfinement.

Metadynamics simulations of calcite crystallization on self-assembled monolayers.

We show that recent developments in the application of metadynamics methods to direct simulations of crystallization make it possible to predict the orientation of crystals grown on self-assembled monolayers. In contrast to previous studies, the method allows for dynamic treatment of the organic component and the inclusion of explicit surface water without the need for computationally intensive interfacial energy calculations or prior knowledge of the interfacial structure. The method is applied to calcite crystallization on carboxylate terminated alkanethiols arrayed on Au (111). We demonstrate that a dynamic treatment of the monolayer is sufficient to reproduce the experimental results without the need to impose epitaxial constraints on the system. We also observe an odd-even effect in the variation of selectivity with organic chain length, reproducing experimentally observed orientations in both cases. Analysis of the ordering process in our simulations suggests a cycle of mutual control in which both the organic and mineral components induce complementary local order across the interface, leading to the formation of a critical crystalline region. The influence of pH, together with some factors that might affect the range of applicability of our method, is discussed.

Nucleation and growth on defect sites: experiment-theory comparison for Pd/MgO(001).

It is well established that nucleation of metal clusters on oxide and halide surfaces is typically dominated by defect sites. Rate equation models of defect nucleation have been developed and applied to these systems. By comparing the models with nucleation density experiments, energies for defect trapping, adsorption, surface diffusion and pair binding have been deduced in favourable cases, notably for Pd deposited on Ar-cleaved MgO(001). However, the defects responsible remain largely unknown. More recently, several types of ab initio calculation have been presented of these energies for Pd and related metals on MgO(001) containing several types of surface defect; these calculated values are surveyed, and some are widely divergent. New rate equation nucleation density predictions are presented using the calculated values. Some calculations, for some defect types, are much closer to experiment than others; the singly charged F(s)(+) centre and the neutral divacancy emerge as candidate defects. In these two cases, the Pd/MgO(001) nucleation density predictions agree well with experiment, and the corresponding surface defects deserve to be taken seriously. Energy and entropy values are discussed in the light of differences in calculated charge redistribution between the metal atoms, clusters and (charged) surface defects, and (assumed or calculated) cluster geometries.

Surface diffusion and surface growth in nanofilms of mixed rocksalt oxides.

We have modelled the surface diffusion and growth of BaO and SrO both in the homoepitaxial and heteroepitaxial (BaO on SrO and SrO on BaO) cases. The diffusion proceeds most favourably by an exchange mechanism involving the surface layer. When impurities are adsorbed on the surface this can lead to intermixing between the layers. This strongly suggests that ionic materials may not be grown on a substrate with a similar structure without significant intermixing. Island growth begins with the formation of individual clusters which grow and merge together.

Selective nucleation and controlled growth: quantum dots on metal, insulator and semiconductor surfaces.

Nucleation and growth models are well developed for nucleation on homogeneous substrates, and they can typically be described in terms of three energy parameters. Nucleation on substrates containing point-defect traps has been investigated, at the cost of introducing more energy parameters. This paper outlines the quantitative description of such growth models, using rate and rate-diffusion equations, in terms of energies for individual surface processes, with examples taken from metal-metal, metal-insulator and semiconductor growth. The challenge to modelling is to describe the large range of length and time-scales in thin-film fabrication and degradation, without relying on too many (unknown) material parameters, which often occur in combination. Separating them into elementary processes often proves to be a challenge. One typically requires selective nucleation using patterned substrates, in combination with controlled, self-organized, growth for reliable nanotechnology. Reconstructed semiconductor surfaces offer both a further challenge to modelling and an opportunity for future technology; these paradoxes are discussed briefly.