A New Twist On Table Salt

The conventional wisdom about sodium chloride—plain old table salt—assumes that it has a one-to-one ratio of sodium and chlorine. But at high pressures, that conventional wisdom breaks down. A new study by Artem R. Oganov of Stony Brook University, SUNY; Weiwei Zhang of China Agricultural University, in Beijing; and coworkers reports that at high pressure sodium and chlorine can combine with different stoichiometries (Science 2013, DOI: 10.1126/science.1244989).

“The most intriguing aspect of this work is that it represents the fall of a textbook idol,” writes Jordi Ibáñez Insa of the Spanish National Research Council, Barcelona, in an accompanying commentary. “Under high pressure, the familiar rules of chemistry are modified and the simplicity of highly ionic compounds such as NaCl is totally lost.”

The researchers used a quantum mechanical algorithm called USPEX to predict structures of various combinations of sodium and chlorine at pressures ranging from 20 to 200 gigapascals. The technique predicted five new stoichiometries: Na₃Cl, Na₂Cl, Na₃Cl₂, NaCl₃, and NaCl₇. The resulting compounds depend on the pressure and composition of the mixture. NaCl is stable over the entire pressure range, so it doesn’t spontaneously decompose to form any of them. Appropriate conditions can exist within Earth, but such compounds are more likely found elsewhere in the universe.

To test the predictions, the researchers synthesized Na₃Cl and two forms of NaCl₃ by placing NaCl in a diamond-anvil cell with excess sodium or chlorine at high pressure and temperature. They measured the resulting compounds using X-ray diffraction and Raman spectroscopy. “The experiments fully confirmed everything we predicted,” Oganov says.

The compounds have interesting structural features. With the exception of one form of NaCl₃, which is a semiconductor, all the compounds are metals. The semiconducting form of NaCl₃ contains linear trichloride units. The crystal structure of NaCl₇ is similar to that of metallic NaCl₃, except that chlorine and sodium are in central positions, respectively. The structure of Na₃Cl consists of sodium layers that alternate with insulating NaCl layers; this unusual material is a two-dimensional metal, Oganov notes.