Scientists Discover New Surprising Details About Table Salt

April Flowers for – Your Universe Online
An international team of scientists has discovered a surprise hidden in the first chemical compound that children learn about: table salt.
Under certain high pressure conditions, table salt, scientifically known as sodium chloride, can take on some surprising forms that violate standard chemistry predictions. The findings, published in Science, may hold the key to answering lingering questions about planet formation.
The team used new computational methods, combined with structure-prediction algorithms and high-pressure experiments, to study the range of changes that simple sodium chloride undergoes when under pressure.
The team, led by Alexander Goncharov, of the Carnegie Institution for Science, and Prof. Artem Oganov, of Stony Brook University, predict some unanticipated reaction results under high pressure that could help geochemistry scientists reconcile ongoing mysteries involving minerals found in planetary cores.
Sodium chloride (NaCl) crystallizes in a cubic unit cell and is very simple. The simplicity continues with the chemical composition, as well — one sodium atom (Na) and one chlorine atom (Cl). Or at least, that’s the chemical composition under ambient conditions. Classic rules of chemistry forbid other compounds of the two elements. For example, the octet rule states that all chemical elements strive to fill their outermost shell with eight electrons, which is the most stable configuration. An example of this configuration would be found in noble gases. Sodium has one extra electron, while chlorine is missing one. This leaves both atoms with an outer shell containing eight electrons and forming a strong ionic bond.
The researchers used advanced algorithms to predict an array of possible stable structural outcomes that would result from compressing rock salt. Using a diamond anvil at DESY’s X-ray source PETRA III, they put the salt under high pressure of 200,000 atmospheres. They added an extra “dash” of either sodium or chlorine, creating new “forbidden” compounds like Na3Cl and NaCl3.
“Following the theoretical prediction, we heated the samples under pressure with lasers for a while,” explains Dr. Zuzana Konôpková of DESY, who supported the experiments at DESY’s Extreme Conditions Beamline P02 (ECB). “We found other stable compounds of Na and Cl which came as a surprise.”
Such compounds require a completely different form of chemical bonding with higher energy. Because nature always favors the lowest state of energy, such compounds should not happen.
Oganov’s team wasn’t surprised, however, as they had calculated that exotic compounds might form under extreme conditions and remain stable under these conditions.
“We have predicted and made crazy compounds that violate textbook rules: NaCl3, NaCl7, Na3Cl2, Na2Cl, and Na3Cl,” says Dr. Weiwei Zhang, a visiting scholar at Oganov’s lab at Stony Brook. The predictions were tested at PETRA III and Carnegie in what the researchers call the “cook and look” experiments, which targeted Na3Cl and NaCl3, the two compounds that were predicted to be more easily made than others, and indeed found them.
“These compounds are thermodynamically stable and once made, remain so indefinitely,” says Zhang. “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.”
The results of these experiments help to explore a broader view of chemistry. “I think this work is the beginning of a revolution in chemistry,” Oganov says. “We found, at low pressures achievable in the lab, perfectly stable compounds that contradict the classical rules of chemistry. If you apply rather modest pressure, 200,000 atmospheres – for comparison purposes, the pressure at the centre of the Earth is 3.6 million atmospheres – much of what we know from chemistry textbooks falls apart.”
One reason for this surprising discovery is that textbook chemistry usually applies to what we call ambient conditions.
“Here on the surface of the earth, these conditions might be default, but they are rather special if you look at the universe as a whole,” Konôpková explains. What may be “forbidden” under ambient conditions on earth, can become possible under more extreme conditions.
“‘Impossible’ really means that the energy is going to be high,” Oganov says. “The rules of chemistry are not like mathematical theorems, which cannot be broken. The rules of chemistry can be broken, because impossible means softly impossible. You just need to find the conditions where the energy balance shifts and the rules hold no more.”
This discovery could lead to new, practical applications, say the researchers.
“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,” Oganov 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 interest.”
The research team hopes that the table salt experiments will only be the beginning of the discovery of completely new compounds. “If this simple system is capable of turning into such a diverse array of compounds under high-pressure conditions, then others likely are, too,” Goncharov explains. “This could help answer outstanding questions about early planetary cores, as well as to create new materials with practical uses.”