Newly-discovered rock salts are rocking classical chemistry

"I think this work is the beginning of a revolution in chemistry," proclaims Artem R. Oganov, a professor of theoretical crystallography from the Department of Geosciences whose experiments with sodium chloride (rock NaCl) could reshape the foundations of classical chemistry.

 

Challenging chemistry conventions

 

Oganov and his team have managed to form new compounds with properties that challenge what conventional rules teach about sodium and chlorine. As the two elements are said to have very different electronegatives, it would only be possible to form an ionic compound from the two with a well-defined composition. As such, they could only exist in a 1:1 relationship as NaCl (commonly known as rock salt), where sodium’s +1 charge plays off against chlorine’s -1 charge.

 

However, according to study lead Weiwei Zhang, the team discovered “crazy compounds that violate textbook rules” in the form of sodium-rich compounds (such as Na3Cl2, Na2Cl, and Na3Cl) and chlorine-rich combinations (such as NaCl3 and NaCl7).

 

“These compounds 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 fulfill 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,” said Zhang, a visiting scholar at Stony Brook's Center for Materials by Design.

 

“Well, here that rule is not satisfied.”

 

Oganov provides further details about the study. “We found, at low pressures achievable in the lab, perfectly stable compounds that contradict the classical rules of chemistry. If you apply the rather modest pressure of 200,000 atmospheres – for comparison purposes, the pressure at the center of the Earth is 3.6 million atmospheres – everything we know from chemistry textbooks falls apart.”

 

"One of these materials – Na3Cl – has a fascinating structure," shares Oganov. The compound, which is a two-dimensional metal, conducts electricity among the layers of its structures.

 

“The NaCl layers act as insulators; the pure sodium layers conduct electricity. Systems with two-dimensional electrical conductivity have attracted a lot of interest,” adds Oganov.

 

Impossible is nothing

 

As it turns out, it was the word “impossible” that motivated Oganov and his team to take a closer look at possible new compound configurations.

 

“For a long time, this idea was haunting me -- when a chemistry textbook says that a certain compound is impossible, what does it really mean, impossible? Because I can, on the computer, place atoms in certain positions and in certain proportions. Then I can compute the energy. 'Impossible' really means that the energy is going to be high. So how high is it going to be? And is there any way to bring that energy down, and make these compounds stable?”

 

Their findings, which were published on December 20 in the journal Science, may contribute greatly to the scientific community in various ways, from facilitating a better understanding of planetary phenomena to interpreting complicated experiments with “puzzling results.”

 

Says Oganov, “The rules of chemistry can be broken, because impossible only means 'softly' impossible! You just need to find conditions where these rules no longer hold.” — TJD, GMA News