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A new phase of boron

In Chemistry on February 25, 2009 at 12:51 pm

This may be the first appearance of a chemical discovery on the Conan O’Brien show. A team of researchers led by Artem Oganov of Stony Brook University has discovered a new form of boron — a crystal structure that includes, for the first time ever seen, ionic bonds within a single element. Boron is a “schizophrenic element,” says Oganov — it was already known to have three forms, one dark red, one coal-like, and one hideously complicated structure called T-192.

All previous forms of boron have had B_12 clusters of 13 boron atoms — what makes this structure remarkable is that it also has B_2 pairs, which produce a faint but perceptible ionic character as charge is transferred from the structure to the pairs.

How was the new form discovered? Not by a lab accident but by an algorithm. The researchers used a computational technique known as evolutionary structure prediction that encodes parameters of the crystal structure; starting with a number of trial configurations for the atoms, it discards the high-energy (unstable) patterns, and tries again, eventually converging on a stable configuration. The algorithm may come up with stable configurations not yet observed in nature, which leads chemists to go looking for temperature and pressure conditions that can create them.

The article is here. More on evolutionary crystal structure prediction here — it actually gives a pretty intuitive sense of the algorithm. It’s modeled on biological evolution: different molecular configurations are produced by “heredity,” “mutation,” and “permutation” (that is, two crystal lattices may be averaged to get a new one, one lattice may be warped a random amount, or two atoms may be exchanged within a lattice.) Then, the individuals are selected to survive or die based on “fitness” — in this case, fitness means low chemical potential energy. The surviving configurations then have “descendants” which are further pruned. Eventually the algorithm finds local minima for potential energy: the stable configurations.


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