Boron is the chemical element most sensitive to impurities. This enhanced sensitivity makes experimental studies of this element very difficult. However, with the discovery of a new, superhard phase of the element, the theorists and experimentalists involved in this research have now come a big step closer to understanding boron.
The international team carried out experiments on extremely pure boron material, containing at most one foreign atom to one million boron atoms. On different synchrotron sources, such as BNL, Hasylab-DESY and ID27 at the ESRF, they exposed this material to temperatures of over 1,500 degrees Celsius and to pressures in the range 12-30 GPa, similar to those found several hundreds of kilometers inside the Earth. Under these conditions they found a new polymorph of boron. The new knowledge obtained in this study allowed the researchers to propose a phase diagram for boron – the only light element whose phase diagram remained unknown until now.
According to classical textbooks, ionic bonds are possible only between two different elements, such as sodium and chlorine in table salt. In the new structure ionic bonds occur between atoms of the same element, though belonging to two kinds of nanoclusters. This ionicity leads to unusual phenomena in an element, such as dielectric properties, lattice dynamics, and anomalous electronic properties.
Above 19 Gigapascal boron atoms form nanoclusters that are arranged just
as are sodium and chlorine ions in the rock salt (table salt) structure. Credits:ETH
Reference: Oganov A.R., et al, Nature advance online publication 28 January 2009. doi:10.1038/nature07736.