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Salt under pressure is not NaCl

Dec. 19, 2013 — In the very beginning of the school chemistry course, we are told of NaCl as an archetypal ionic compound. Being less electronegative, sodium loses its electron to chlorine, which, following the "octet rule," thus acquires the 8-electron electronic configuration of a noble gas. All the rules predict NaCl to be the only possible compound formed by chlorine and sodium. The research team led by Artem R. Oganov, Professor of Crystallography at Stony Brook University, has discovered new sodium chlorides that call for revision of textbook chemistry. In their article "Unexpected stable stoichiometries of sodium chlorides" published in the latest issue of "Science," they predicted stability of compounds that are impossible from the point of view of classical chemistry -- NaCl3, NaCl7, Na3Cl, Na2Cl3, or Na2Cl. The lead author of the paper, sharing equal credit with Oganov, is Weiwei Zhang, a visitor in Oganov's laboratory.

"We know from school chemistry that for many classes of compounds a simple charge balance rule applies to all stable compounds. In ionic compounds Cl atoms have formal charge of -1, and sodium has +1, hence the only possible compound is NaCl. This rules out, for example, NaCl2. And NaCl5 and Na2Cl3 should be impossible, too. Yet, one can perform a calculation and estimate how unfavourable they are. It turns out that a large number of "weird" compounds may be successfully stabilized by just increasing pressure. At that point traditional rules break down" -- says Oganov.

These structures radically altering our understanding of chemistry were calculated using the crystal structure prediction technique invented by Oganov and used currently by more than 1500 scientists around the world. Oganov has called his code USPEX (Universal Structure Predictor: Evolutionary Xrystallography) making this Russian word, standing for "success," popular around the crystallographers and material scientists.

Exotic compounds not only expand our understanding of chemistry but may find new practical applications in future. For example, NaCl7, NaCl3, Na3Cl2, and Na2Cl are metals (that explains the apparent violation of electroneutrality since charge balance rules are inapplicable to metals), and only one semiconducting phase of NaCl3 is stable in the pressure range between 250 and 480 thousand atmospheres. Metallic Na3Cl has a unique structure. It consists of alternating layers of NaCl and of pure sodium layers. Sodium chloride layers are dielectric, sodium layers are conducting, hence the crystal as a whole is a unique quasi-two-dimensional conductor. Such substances were recently found to show some interesting physical effects.

"I think that such materials should find practical applications. The only trouble is that these substances are stable only at high pressures. However, other extreme conditions may be used to produce such materials. For example, on the surfaces of other crystals. Surface is an extreme state, too, where nearly half of the bonds are broken, and the chemical composition is known to be completely different from that inside the crystal," -- says Oganov.

New sodium chlorides are not mere fantasy or an exotic numerical result. Alexander Goncharov, experimental physicist from the Carnegie Institute in Washington, has reproduced the high pressures required to stabilize the non-standard sodium chlorides in his laboratory and confirmed directly the existence of the compounds predicted by USPEX.

Oganov considers the new substances not a mere amusing exception but the first signs of a new chapter of chemistry. Such unusual compounds may exist in planetary interiors, where pressures are high.

"I think that sodium chloride could not be an exception. Most likely, we have encountered a new class of compounds that will show up throughout a wide range of chemical systems. We have confirmed this for KCl where the phase diagram is even richer. For a planet-forming system magnesium-oxygen, we predicted two new compounds, MgO2 and Mg3O2. We assume that magnesium silicate is always MgSiO3 or Mg2SiO4 in the interior of the Earth. We have always thought this way but now we ask ourselves -- couldn't it be otherwise? Even if these are the only compositions in the Earth's core, the rocky core of Jupiter can contain its own unexpected silicates," -- supposes Oganov.

Though the existence of these new compounds was confirmed experimentally, the nature of their stability is not all cases easy to understand.

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