Forming new compounds from sodium chloride challenges the rules of chemistry

Researchers have discovered that by compressing sodium chloride (rock salt) under certain high-pressure conditions, they can trigger a process which was found to form new compounds - despite chemistry textbooks saying this is impossible. Their study has been published in the journal Science.

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 and chlorine’s charge is -1; sodium will give away an electron, chlorine wants to take an electron. Thus, the only possible combination of these atoms in a compound is 1:1 - rock salt, or NaCl.

Yet Professor Artem R Oganov of Stony Brook University said his team found, at pressures that were achievable in the lab, “compounds that contradict the classical rules of chemistry”. Lead author Weiwei Zhang said these “crazy compounds” - including NaCl₃, NaCl₇, Na₃Cl₂, Na₂Cl, and Na₃Cl - “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 fulfil 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.”

Electron localisation function in the cubic NaCl₃ structure. (Credit: Oganov, Artem, Stony Brook University.)

Oganov’s theories predicted 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 further theorised that if you take NaCl, mix it with pure chlorine, compress and heat, you will get chlorine-rich compounds such as NaCl₃. These predictions were proved correct in experiments carried out by Alexander F Goncharov and his team at the Carnegie Institution for Science.

“When you change the theoretical underpinnings of chemistry, that’s a big deal,” Goncharov said. “But what it also means is that we can make new materials with exotic properties.”

Such materials created by Oganov and his team include Na₃Cl - a two-dimensional metal where electricity is conducted along the layers of the structure. This comprises “layers of NaCl and layers of pure sodium”, he said, where “the NaCl layers act as insulators; the pure sodium layers conduct electricity”.

Furthermore, the research could be applied to several other areas of science, such as the planetary sciences, where high-pressure phenomena abound; explaining the results of other experiments, where researchers compressed materials and got puzzling results; and predicting 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 said.

“Current rules cannot cope with this new chemistry. We need to invent something that will.”

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