A team of researchers led by Prof Artem Oganov of Stony Brook University has shown that, under certain conditions, ordinary rock salt can take on some surprising forms that violate textbook rules of chemistry.
The scientists first used new computational methods and structure-prediction algorithms to identify an array of possible stable structural outcomes from compressing rock salt (sodium chloride, NaCl).
They then attempted to verify these predictions, using a diamond anvil to put salt mixed with molecular chlorine or metallic sodium under high pressure.
“We discovered that the standard chemistry textbook rules broke down,” said Dr Alexander Goncharov from Carnegie Institution of Washington, who is a co-author of the study 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, chlorine’s charge is -1; sodium will give away an electron, chlorine wants to take an electron.
According to chemistry texts and common sense, the only possible combination of these atoms in a compound is 1:1 – rock salt, or NaCl.
“We found crazy compounds that violate textbook rules – NaCl3, NaCl7, Na3Cl2, Na2Cl, and Na3Cl,” said lead author Dr Weiwei Zhang of Stony Brook University.
“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. Well, here that rule is not satisfied.”
NaCl turned into stable compounds of Na3Cl, Na2Cl, Na3Cl2 and NaCl7, all of which have highly unusual chemical bonding and electronic properties.
Their structures were calculated using the crystal structure prediction technique invented by Prof Oganov and called USPEX (Universal Structure Predictor: Evolutionary Xrystallography) making this Russian word, standing for ‘success’, popular around the crystallographers and material scientists.
These compounds not only expand our understanding of chemistry but may find new practical applications in future.
For example, NaCl7, NaCl3, Na3Cl2, and Na2Cl are metals (that explains the apparent violation of electroneutrality since charge balance rules are inapplicable to metals), and only one semiconducting phase of NaCl3 is stable in the pressure range between 250 and 480 thousand atmospheres.
“Na3Cl has a fascinating structure. 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 of interest,” Prof Oganov said.
“If this simple system is capable of turning into such a diverse array of compounds under high-pressure conditions, then others likely are, too,” Dr Goncharov said.
“This could help answer outstanding questions about early planet cores, as well as to create new materials with practical uses.”
Weiwei Zhang et al. 2013. Unexpected Stable Stoichiometries of Sodium Chlorides. Science, vol. 342, no. 6165, pp. 1502-1505; doi: 10.1126/science.1244989