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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.
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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.”


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