But what is even more interesting is the prediction model behind this strange compound: it opens the doors to discover more strange compounds, by monitoring pressurized reactions as opposed to normal-pressure reactions, and that means we have a whole new class of materials to discover. Among them, I bet there will be a few more gems such as Na3Cl that come up!
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Sodium and Chlorine Combined Into Compound Textbooks Say Can't Happen
Researchers at Stony Brook University have come up with a way to combine Sodium and Chlorine, the constituents of common table salt, in ways that your old chemistry textbook says cannot be. Remember that Sodium has only one electron in the outermost shell, and Chlorine has 7 electrons in its outermost shell.
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. ⓐ
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. ⓑ
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.” ⓐ
His discovery may have application in the planetary sciences, where high-pressure phenomena abound. It may explain results of other experiments, where researchers compressed materials and got puzzling results. His computational methodology and structure-prediction algorithms will help researchers predict material combinations and structures that exhibit desired properties and levels of stability. ⓐ
“We have learned an important lesson—that even in well-defined systems, like sodium chloride, you can find totally new chemistry, and totally new and very exciting materials,” Oganov says. “It’s like discovering a new continent; now we need to map the land. Current rules cannot cope with this new chemistry. We need to invent something that will." ⓐ
Unexpected Stable Stoichiometries of Sodium Chlorides
Reformulating Table Salt Under Pressure
(Image: Stony Brook University)