Credit: © 2009 NPG

Although boron has been known since 1808, a reasonably pure form was only isolated in 1909, and its full phase diagram is still not well understood and its stable phase at normal conditions is yet to be established experimentally. Now, Artem Oganov, from ETH Zurich and Moscow State University, with colleagues from across the world, has discovered a new form of boron at high pressure1.

All currently known structures of boron contain icosahedral clusters of 12 boron atoms and this is no exception. What makes the new form particularly interesting is that it features pairs of boron atoms as well. The B12 clusters and B2 pairs are arranged in a rock-salt-type lattice, with the B12 clusters taking the place of the cubic-close-packed anions and the pairs fitting in the octahedral spaces. This results in a more dense structure than those with just clusters, hence why it is formed at high pressures.

Charge transfer between the clusters and the pairs results in a small but not insignificant ionic character to the compound. This ionicity affects its dielectric, electronic and optical properties and 'boron boride' represents the first example of ionic bonding observed in pure elements, as well as helping to fill in the phase diagram for such a well-known element.