ScienceShot: Salt Under Pressure

Artem R. Oganov & Weiwei Zhang

ScienceShot: Salt Under Pressure

Pressure changes people. It changes chemistry, too, according to a report online today in Science that shows the structure of table salt—sodium chloride—morphs under extreme pressure. Table salt normally consists of a crystalline lattice made up of alternating sodium and chloride ions. Under “normal” conditions, sodium and chlorine atoms, like all of their kin, strive to fill their outer electron shells with eight electrons, which is the most stable arrangement and a condition known as the octet rule. Sodium has one extra electron and chlorine is missing one. So sodium donates one electron to chlorine, giving them a positive and negative charge, respectively. The opposite charged ions attract one another, causing the two to form a tight “ionic” bond. But an international team of researchers has found that those rules of chemical bonding change under high pressure. It used an advanced computer algorithm to predict what would happen under a pressure of 200,000 atmospheres and up to 2000 kelvins. The program concluded that the compression should force atoms to merge into exotic solids, such as Na3Cl and NaCl3. When the scientists then compressed sodium chloride in a diamond anvil cell, they not only discovered the exotic salts, but also showed that they remained stable when much of the pressure was removed. The new work, the authors say, may force geochemists to reconsider the types of compounds that may form in the high-pressure interiors of stars and planets.

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