Element of Split
Personality
Theoretical and Experimental Studies
Find New Superhard Phase of Boron
Prof. Jiuhua Chen at
Brookhaven National Lab
Photo BNL
Research team sheds light on one of the
most complicated element in the Periodic Table
The AA E-Zine contacted Prof. Jiuhua Chen,
former SBU research professor and Asst. Dean of Admissions
for China, now at Florida International University and
Brookhaven National Lab. Dr. Chen started the project when
he was at Stony Brook. Dr. Chen said "the success of the
project is really the collaboration between experimentalists
and theoreticians. We not only produced the phase at high
pressure and temperature, we also characterized the
materials using infrared spectroscopy at the NSLS (National
Synchrotron Light Source) of BNL. This
is a true international, interdisciplinary collaboration.
Scientists involved in the project come from the US,
Switzerland, France, Italy and China."
STONY BROOK, N.Y.,
January 28, 2009 — 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.
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 been 16 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 and B2 dumbbells. 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.
The Department of Geosciences and New York Center for
Computational Science are major research divisions of Stony
Brook University. For more information go to
www.geosciences.stonybrook.edu and
www.newyorkccs.org.
