A representation of the new M-Carbon structure. |
An international
team led by Artem R. Oganov, PhD, a professor of theoretical crystallography in
the Department of Geosciences at Stony
Brook University,
has established the structure of a new form of carbon. The results of their
work, “Understanding the Nature of Superhard Graphite,” were published in Scientific Reports.
Oganov and his
team used a novel computational method to demonstrate that the properties of
what had previously been thought to be only a hypothetical structure of a
superhard form of carbon called “M-carbon”—constructed by Oganov in 2006—matched
perfectly the experimental data on “superhard graphite.”
“Most of the
known forms of carbon have a colorful story of their discovery and a multitude
of real or potential revolutionary applications,” said Oganov. “Think of diamond,
a record-breaking material in more than one way. Think of graphene, destined to
become the material of electronics of the future. Or of fullerenes, the
discovery of which has started the field of nanoscience.”
The story of yet
another form of carbon started in 1963, when Aust and Drickamer compressed
graphite at room temperature. High-temperature compression of graphite is known
to produce diamond, but at room temperature an unknown form of carbon was
produced. This new form, like diamond, was transparent and superhard—but its
other properties were inconsistent with diamond or other known forms of carbon.
“The
experiment itself is simple and striking: you compress black ultrasoft
graphite, and then it suddenly turns into a colorless, transparent, superhard,
and mysterious new form of carbon—’superhard graphite,'” said Oganov. “The
experiment was repeated several times since, and the result was the same, but
no convincing structural model was produced, due to the low resolution of
experimental data.”
Using his
breakthrough crystal structure prediction methodology, Oganov in 2006
constructed a new low-energy superhard structure of “M-carbon.” That work
resulted in a stream of scientific papers that within two years proposed
different “alphabetic” structures, such as F-, O-, P-, R-, S-, T-, W-, X-, Y-,
Z-carbons. “The irony was that most of these also had properties compatible
with experimental observations on ‘superhard graphite.’ To discriminate between
these models, higher-resolution experimental data and additional theoretical insight
are required,” he said.
According to
Oganov, the reason why diamond is not formed on cold compression of graphite is
that the reconstruction needed to transform graphite into diamond is too large
and is associated with too great an energy barrier, which can be overcome only
at high temperatures, when atoms can jump far. At low temperatures, graphite
chooses instead a transformation associated with the lowest activation barrier.
One could
establish the structure of ‘superhard graphite’ by finding which structure has
the lowest barrier of formation from graphite. To do that, Oganov, his
postdoctoral associate Salah Eddine Boulfelfel, and their German colleague,
Professor Stefano Leoni, of Dresden University of Technology, used a powerful
simulation approach, recently adapted to solid materials, known as transition
path sampling. These simulations required some of the world’s most powerful
supercomputers, and finally proved that “superhard graphite” is
indeed identical to M-carbon, earlier predicted by Oganov.
“These
calculations are technically extremely challenging, and it took us many months
to perform and analyze them. Searching for the truth, you have to be prepared
for any outcome, and we were ready to accept if another of the many proposed
structures won the contest. But we got lucky, and our own proposal—M-carbon—won,”
said Oganov.
Another result
of this study is a set of detailed mechanisms of formation of several potential
carbon allotropes. These could be used to engineer ways of their synthesis for
potential technological applications.
“We don’t know yet which applications M-carbon will find, but most forms of
carbon did manage to find revolutionary applications, and this amazing material
might do so as well,” said Oganov.
Source: Stony Brook University
