Scientists have found the first case of an ionic crystal consisting of just
one chemical element - boron. This is the densest and hardest known phase of
this element. The new phase turned out to be a key to understanding the phase
diagram of boron - the only element for which the phase diagram was unknown
since its discovery 200 years ago. This work is published in the January 28,
2009 issue of Nature. Results on the hardness of the new phase are published
in a separate paper in the Journal of Superhard Materials.
 | | Dr. Artem Oganov shows the crystal structure of the new phase of boron. |
The team of authors is made of sub-teams led by Artem R. Oganov (theoretical
crystallographer from Stony
Brook University), Jiuhua Chen (materials scientist from Florida International
University), Carlo Gatti (theoretical chemist from the University of Milano,
Italy), and Vladimir Solozhenko (physical chemist from Centre national de la
recherche scientifique/CNRS, France). Such a large effort was necessary to crack
down on what is likely the most complicated element in the Periodic Table.
Boron has long been known as a graveyard of great scientific reputations. Its
bizarre tale started in 1808, when two great teams – J.-L. Gay-Lussac
and L.-J. Thénard in Paris and Sir Humphrey Davy in London – independently
announced the discovery of a new element, boron. Later it was proven that in
both cases the “element” was a compound containing not more than
60-70% boron. The most definitive proof of this was made by another great chemist,
H. Moissan, but his material was later also shown to be a compound with less
than 90% boron. In 1858, F. Wöhler wrote in his classical book that boron
has two polymorphs – a graphite-like one, and a diamond-like one. Now
we know that both forms are actually compounds, AlB12 and B48C2Al, respectively.
The first time 99% boron was synthesized was in 1909, but this was not the end
of story. Even 1% of impurities, or even less, can change the structure and
properties of boron in an amazingly strong way and compounds such as PuB100
are known.
“Such sensitivity to impurities is unprecedented among the elements and
makes studies of this element nothing short of a nightmare,” says Artem
R. Oganov, Associate Professor at the Department of Geosciences and New York
Center for Computational Science at Stony Brook.
There have been16 polymorphic modifications of boron reported to date, but
most of these are likely to be impurity-stabilized forms. This is the only element
for which the ground state is not experimentally known even at ambient conditions.
Among the many abnormalities found for boron is also the recent suggestion that
it violated the third law of thermodynamics (which states that stable phases
at zero Kelvin must be perfectly ordered) at atmospheric pressure. High-pressure
behavior remained even more mysterious.
The stage for present research was set in 2004, when Chen and Solozhenko independently
synthesized a new form of boron at high temperatures and pressures above 100,000
atmospheres. The structure could not be solved from experimental data alone,
and required a new theoretical method that was developed by Dr. Oganov at the
time.
“The method is a purely theoretical, requires no experimental information,
and is based on ideas of natural evolution applied to the search for the most
stable crystal structure,” said Dr. Oganov. “The computer generates
dozens of trial crystal structures, whose energies are evaluated using quantum-mechanical
calculations, and the most favorable of the sampled structures mate and mutate
to produce child structures until the most stable structure is found.”
Using this method Dr. Oganov was able to find the structure and determined
that it is a true ground state of boron and there is significant charge transfer
between boron atoms within the structure.
Dr. Gatti’s sophisticated analysis confirmed bond ionicity, and Dr. Solozhenko’s
further experiments revealed that the new phase is superhard with a Vickers
hardness of about 50 GPa. Quantum-mechanical calculations suggest a very large
stability field for this new phase, extending up to 900,000 atmospheres in pressure.
How can an element be ionic? Classical chemistry textbooks indicate that charge
transfer occurs when atoms have different electronegativities and this automatically
disqualifies pure elements as possible ionic phases. Boron finds a surprising
solution to this problem – its new structure contains two very different
types of nanoclusters, B12 icosahedra (blue in the figure above) and B2 dumbbells
(orange in the figure above). The electronic structures of these two clusters
are very different – in fact, the dependence of electronic properties
on the size of the cluster is well known and is the main idea of nanotechnology.
Electronegativities of the B12 icosahedra and B2 pairs are different, and this
causes charge redistribution and the emergence of partial ionicity in this elemental
structure.
“What’s also striking,” said Dr. Oganov, is that the centers
of mass of clusters in this new structure occupy the same positions as atoms
in the structure of NaCl, an archetypal ionic compound.
As a result of these findings, Oganov and colleagues anticipate other ionic
forms of the elements, and propose several stable or metastable possibilities.
Furthermore, elemental liquids are likely to have some degree of instantaneous
charge transfer between the atoms. Apart from being a curiosity, ionic elements
have interesting and potentially important properties. The properties most affected
by ionicity include the dielectric constants, vibrational spectra, and electronic
band gap. Among these anomalous properties, the predicted infrared absorption
spectrum (which is entirely due to charge redistribution between the atoms)
has already been fully confirmed in experiments of Chen’s team. Inducing
transitions from non-ionic to ionic structure will result in such changes in
the properties that otherwise would be hard to achieve.
Posted January 28th, 2009
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