The new structure can be viewed as a NaCl-type structure, with anionic and
cationic positions occupied by two different clusters of boron atoms (B12 and
B2). The difference of the electronic properties of these clusters brings about
charge transfer, making this material a partially ionic boron boride (B2)d+(B12)d-.
Results have been published in today's "Nature" online magazine.
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 the research have now come a big step closer to unders-tanding boron.
A separate publication by the authors in the "Journal of Superhard Materials"
demonstrated that the new phase is superhard.
Independently synthesized
The new superhard material was independently synthesized by two researchers
who eventually joined forces with crystallographer Artem Oganov's theoretical
team. Initially, Jiuhua Chen, a material scientist at Florida International
University, and Vladimir Solozhenko, a physical chemist at the Centre National
de la Recherche Scientifique (CNRS) in France, conducted experiments on extremely
pure boron material, containing at most one foreign atom to one million boron
atoms. 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 both teams of experimentalists
found a new polymorph of boron, but could not solve its structure.
New method leads to breakthrough
Artem Oganov, working at ETH
Zurich's Department of Material Science, has now developed a computational
method for predicting the stable crystal structures of materials. His calculations
reveal that in the new phase, boron atoms form two different kinds of nanoclusters:
an icosahedron B12 consisting of twelve atoms and dumbbell B2 consisting of
just two boron atoms.
These nanoclusters are arranged in the new phase of boron just as are sodium
and chlorine ions in the rock salt (table salt) structure (see diagram). The
new phase is predicted to remain stable to 89 GPa. 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.
Unusual properties identified
The unexpected structure of the new phase, which the authors called ?-B, contains
atoms which are ionized, meaning that the electrons are distributed between
the atoms unevenly. According to classical textbooks, ionic bonds are possible
only between two different elements, such as sodium and chlorine in table salt.
But 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
for an element phenomena in dielectric properties, lattice dynamics, and anomalous
electronic properties. Additional experiments carried out by the researchers
also show that the new phase is superhard.
Oganov and his colleagues expect that forms of other elements, such as carbon
heterofullerites, might display charge transfer and partial ionicity. Now a
professor at State University of New York at Stony Brook (USA), Oganov anticipates
that sooner or later applications will be developed which are based on ionic
elements. These applications could be based on switching on or off the anomalous
properties (for example, strong infrared absorption) possessed by ionic elements
– such properties will display dramatic changes as a result of pressure-
or temperature-induced phase transitions. In addition, interesting effects related
to superconductivity may appear as well.
Posted January 29th, 2009
