http://www.sciencedaily.com/releases/2013/12/131219142138.htm
New Salt Compounds Challenge the Foundation of Chemistry
Dec. 19, 2013 — All good research breaks new ground, but rarely does the
research unearth truths that challenge the foundation of a science. That's
what Artem R. Oganov has done, and the professor of theoretical
crystallography in the Department of Geosciences will have his work
published in the Dec. 20, 2013 issue of the journal Science.
The paper, titled "Unexpected stable stoichiometries of sodium chlorides,"
documents his predictions about, and experiments in, compressing sodium
chloride -- rock salt -- to form new compounds. These compounds validate
his methodology for predicting the properties of objects -- a methodology
now used worldwide for computational material discovery -- and hold the
promise of novel materials and applications.
"I think this work is the beginning of a revolution in chemistry," Oganov
says. "We found, at low pressures achievable in the lab, perfectly stable
compounds that contradict the classical rules of chemistry. If you apply
the rather modest pressure of 200,000 atmospheres -- for comparison
purposes, the pressure at the center of the earth is 3.6 million
atmospheres -- everything we know from chemistry textbooks falls apart."
Standard chemistry textbooks say that sodium and chlorine have very
different electronegativities, and thus must form an ionic compound with a
well-defined composition. Sodium's charge is +1, chlorine's charge is -1;
sodium will give away an electron, chlorine wants to take an electron.
According to chemistry texts and common sense, the only possible
combination of these atoms in a compound is 1:1 -- rock salt, or NaCl.
"We found crazy compounds that violate textbook rules -- NaCl3, NaCl7,
Na3Cl2, Na2Cl, and Na3Cl," says Weiwei Zhang, the lead author and visiting
scholar at the Oganov lab and Stony Brook's Center for Materials by Design,
directed by Oganov. "These compounds are thermodynamically stable and, once
made, remain indefinitely; nothing will make them fall apart. Classical
chemistry forbids their very existence. Classical chemistry also says atoms
try to fulfill the octet rule -- elements gain or lose electrons to attain
an electron configuration of the nearest noble gas, with complete outer
electron shells that make them very stable. Well, here that rule is not
satisfied."
This opens all kinds of possibilities. Oganov posited that, if you mix NaCl
with metallic sodium, compress in a diamond anvil cell, and heat, you will
get sodium-rich compounds like Na3Cl. He likewise theorized that, if you
take NaCl, mix it with pure chlorine, and compress and heat, you will get
chlorine-rich compounds such as NaCl3. This is exactly what was seen in the
experiments, which were performed by the team of Alexander F. Goncharov of
Carnegie Institution of Washington, confirming Oganov's predictions. "When
you change the theoretical underpinnings of chemistry, that's a big deal,"
Goncharov says. "But what it also means is that we can make new materials
with exotic properties."
Among the compounds Oganov and his team created are two-dimensional metals,
where electricity is conducted along the layers of the structure. "One of
these materials -- Na3Cl -- has a fascinating structure," he says. "It is
comprised of layers of NaCl and layers of pure sodium. The NaCl layers act
as insulators; the pure sodium layers conduct electricity. Systems with
two-dimensional electrical conductivity have attracted a lot of interest."
Like much of science, Oganov's pursuit began with curiosity -- and
obstinacy.
"For a long time, this idea was haunting me -- when a chemistry textbook
says that a certain compound is impossible, what does it really mean,
impossible? Because I can, on the computer, place atoms in certain
positions and in certain proportions. Then I can compute the energy.
'Impossible' really means that the energy is going to be high. So how high
is it going to be? And is there any way to bring that energy down, and make
these compounds stable?"
To Oganov, impossible didn't mean something absolute. "The rules of
chemistry are not like mathematical theorems, which cannot be broken," he
says. "The rules of chemistry can be broken, because impossible only means
'softly' impossible! You just need to find conditions where these rules no
longer hold."
Oganov's team harnessed their own energy to bring the research to fruition.
"We have a fantastic team," he says. "The theoretical work was done here at
Stony Brook; the experimental work took place at the Geophysical Laboratory
in the Carnegie Institution of Washington."
Additionally, Oganov's team utilized the NSF-funded Extreme Science and
Engineering Discovery Environment (XSEDE) by running USPEX code -- the
world-leading code for crystal structure prediction -- on Stampede, a
supercomputer at the Texas Advanced Computing Center at the University of
Texas at Austin. USPEX was developed by Ogano
