How do crystals grow? In a two-step process, powered by their kinks

New light has been shed on how molecules leave a solution to become part of a growing crystal. Because the properties of crystals depend on their size and shape, and these are influenced by their growth rate, the research opens the door to tweaking crystal formation to improve usability.

Crystals are much more common and more important than just the pretty stones that most people first associate with the word. They can be formed from essential proteins in our bodies, power our lives in the form of solar cells and provide the semiconductors that allow you to read this article. Yet crucial aspects of the crystal formation process have remained mysterious. A new study reveals how molecules attach to sites on the crystal surface so the crystal can grow.

Whatever the word means to you, in materials science, buckles are abnormalities caused by a dislocation defect. They ensure that crystals can deform under shear forces. Furthermore, scientists from the University of Houston note in their new paper: “The growth rate of crystals is largely determined by the chemical reaction between solutes and kinks.” In other words, when molecules in solution attach to crystals so that they grow, they do so by binding to the kink.

As with many things in science, working out the details has proven difficult, but the Houston team has investigated the reactions of four solvents at kink sites in exceptional detail.

“We show that the binding of a solute molecule to a kink divides into two elementary reactions,” they report. “First, the incoming solute molecule sheds a fraction of its solvent shell and attaches to molecules from the kink through bonds that differ from those in the fully incorporated state. In the second step, the solute breaks these initial bonds and moves to the kink.”

A preliminary bond is formed between the solute molecule and the kink in the first stage, but this must be broken before final incorporation can be achieved, something the authors argue is counterintuitive.

“For decades, crystal growth researchers have dreamed of elucidating the chemical reaction between incoming molecules and the unique spots on a crystal surface that accept them, the kinks,” senior author Professor Peter Vekilov said in a statement. “The mechanism of that reaction, i.e. the characteristic time scale and length scale, possible intermediates and their stability, has been elusive and subject to speculation for more than 60 years.”

The authors discovered that molecules are absorbed into the crystal much more slowly than the solvent supplies them. This, they write, “announces the presence of an activation barrier.” The same is not seen when crystals grow from gas, indicating that the barrier is a product of shells in the solvate. The bonds between the solute and the solvent must be broken for the crystal to grow.

This means, the authors conclude, that the intermediate state before molecules move toward the kink determines the rate of crystal growth.

“The notions of an intermediate state and its decisive role in crystal growth refute and replace the dominant idea in this field,” Vekilov said.

Ironically, that dominant idea – that the activation barrier is determined by interactions in the solution prior to encounters with the crystal – was proposed decades ago by Vekilov’s PhD advisor AA Chernov. Perhaps the authors, in a peace offer to Vekilov’s former advisor (who previously served as president of the International Organization of Crystal Growth), acknowledge that some crystals can grow in a one-step process, but they suggest that this is “probably limited to high symmetry solute that is comparable in size to the solvent.”

To unravel the growth process, the authors used not only X-ray diffraction – our main tool for understanding crystal structures for a century – and absorption spectroscopy, but also more advanced techniques, including time-resolved in situ atomic force microscopy. The latter is now capable of resolving objects on an almost molecular scale.

The study was published in Proceedings of the National Academy of Sciences.

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