This is actually
a breakthrough in what can be acceptable in chemistry and completely alters our
expectations on dealing with high pressures.
That is to the well as we have skirting this issue for a long time.
I expect that we
will learn in time that conventional chemistry is merely the most likely choice
available and that the alternatives do occur as we change the environment. I have seen such evidence and I would be
unsurprised that every chemist shared similar thoughts.
Chemistry needs
this sort of a boost and it is time to properly explore high pressure and high
temperature chemistry.
Scientists turn
table salt into forbidden compounds that violate textbook rules
January 20, 2014
A sample of NaCl3, which was considered 'forbidden'
in classical chemistry (Photo: Alexander Goncharov)
In the field of exotic new materials, we've examined
one of the strongest
stones and another declared to be impossible;
scientists now report creating "forbidden" materials out of ordinary
table salt that violate classical rules of chemistry. Not only does the
development challenge the theoretical foundation of chemistry, but it is also
expected to lead to the discovery of new exotic chemical compounds with
practical uses and shed light on the deep interiors of planets.
The international team of researchers led by Artem
R. Oganov, a Professor of Crystallography at Stony Brook University, predicted
that taking table salt and subjecting it to high pressure in the presence of an
excess of one of its constituents (either chlorine or sodium) would lead to the
formation of totally unexpected compounds. In spite of salt being one of
the most thoroughly studied chemical compounds out there, the researchers
predicted the formation of compounds forbidden by classical chemistry, such as
Na3Cl and NaCl3. Their predictions were proven by subsequent experiments.
"Sodium has one electron in its outermost
shell, and chlorine has seven," Weiwei Zhang, the lead author, a Professor
of Physics at China Agricultural University and visiting scholar at Oganov's
lab, tells Gizmag. "When sodium meets chlorine, sodium would like to give
away an electron and chlorine wants to take one according to the Octet rule.
Since one Na can supply only one electron to one Cl, the only possible
combination of these atoms in a compound is 1:1, rocksalt NaCl. Take NaCl3 as
an example, when you try to satisfy three Cl by one Na, there is no way to
distribute electrons according to this rule. So NaCl3 is forbidden in the
classical frame of chemistry."
###
In addition to NaCl3, the team has predicted other
new "crazy" compounds, such as NaCl7, Na3Cl2, Na2Cl, and Na3Cl, based
on a sophisticated algorithm developed by Oganov and his students involving
quantum-mechanical calculations. The experiment verified their prediction that
these exotic materials would be thermodynamically stable at high pressures.
Their research opens up the way to successfully create and stabilize a huge
number of compounds previously considered to be forbidden.
"They are 'forbidden' at ambient pressure,
but the situation changes at high pressures," explains Alexander
Goncharov, a Senior Staff Scientist at the Carnegie Institution of Washington
and a key team member, responsible for the experimental confirmation of Zhang's
and Oganov's predictions. "Pressure stabilizes new compounds and changes
rules of chemistry we are used to at ambient pressure. These materials violate
textbook chemistry at ambient pressure, but they do not violate any laws of
physics, or more general chemistry rules at high pressures. The only trouble is
that these more general rules still remain to be discovered."
Most of what we know about textbook chemistry only
applies to ambient conditions, or the default conditions that one normally
finds on the surface of the Earth, says the team. So it's entirely possible
that what's forbidden on the Earth's surface under ambient conditions, becomes
possible when things become more extreme, or in the presence of high energy
that causes energy balances to shift, they claim. More than just broadening
existing views, the researchers anticipate that it will start a
revolution, or at least a new chapter in chemistry.
"We have found not just several sodium
chlorides, but a whole new class of compounds, which are formed under pressure
and to which standard chemical rules do not apply,"
Oganov tells us. "Now we need to formulate rules that apply to such
compounds, explore their properties and possible applications."
Given how many weird compounds they were able to
create with ordinary table salt, the team expects that it'll be possible to
discover and create more new ones by subjecting other materials to high
pressures, too. When created, these new materials could have many unusual
properties with practical uses. For instance, Na3Cl is a two dimensional metal,
made of layers of pure sodium and salt that can conduct electricity. The salt
layers act as insulators while the sodium layers act as conductors. They've
predicted other exotic materials that can be made in large quantities and
patented their possible use.
All the materials they've created so far are stable
only at high pressures and transform back into their initial states when the
pressure is lowered. The next step, the team says, is to get them to be stable
at ambient pressure by manipulating factors like material parameters,
dimensionality, reaction kinetics and more. Further experiments, that start out
with compounds other than NaCl, are expected to yield insights into the kind of
compounds that may exist in planetary cores.
"For example, with Mg-O (Magnesium Oxide),
which comprises a large part of mantles of terrestrial planets, and probably
also of cores of gas giant planets, we found totally unexpected compounds
(including Mg3O2) that might exist within some planets in large
quantities," says Oganov.
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