Crystals under pressure: Questioning bonds
- Published: Jan 1, 2014
- Author: David Bradley
- Channels: X-ray Spectrometry
Theoretical crystallographer Artem Oganov is about to turn the world of ionic solids upside down. He and his colleagues have demonstrated that, under modest pressure, that seemingly simplest of salts, sodium chloride, does not follow the usual rules of bonding and can form materials prohibited by conventional theory, such as NaCl3, NaCl7, Na3Cl2, Na2Cl, and Na3Cl.
Science progresses as observations are formulated into explanations, explanations become testable theories. Once new observations fail to support the theory, a new explanation must be sought. Now, Artem Oganov, professor of theoretical crystallography in the Department of Geosciences at the State University of New York at Stony Brook (SUNYSB) describes findings that might disrupt a well-worn theory regarding the bonding and stoichiometry of crystalline solids, in particular, the rather familiar sodium chloride.
Oganov and colleagues have made theoretical predictions and thence experimental observations on the structures that exist when sodium chloride is compressed. While the data validate Oganov's theoretical structure-predicting framework they also suggest that it is time for a re-think, a re-formulation, if you will, of the framework on which we hang our understanding of chemical bonds in so-called ionic solids. As with any such disruptive research there will now be a period during which the data and predictions are debated, and during which independent researchers attempt to reproduce the findings. If they fail, there may be many more rounds of experiments to follow. If they succeed, not only will Oganov's work require a rewriting of the textbooks, but it will offer new promise of potentially useful materials and applications that were not even hinted at by the conventional bonding models.
Worth its salt
"I think this work is the beginning of a revolution in chemistry," Oganov suggests. "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 200000 atmospheres - for comparison purposes, the pressure at the center of the earth is 3.6 million atmospheres - much of what we know from chemistry textbooks falls apart."
In the simplistic high-school chemistry of the modern textbook, sodium and chlorine atoms are considered to have essentially diametrically opposed electronegativities. When they are combined the sodium readily loses an electron to become a positive ion. Its atomic orbitals in the atom "2,8,1", then become "2,8" and so with the loss of the outer electron it thus has a full outer shell, fulfilling the octet rule. Conversely, the chlorine atom has an electron configuration "2,8,7", if it grabs the electron from the sodium it can become a negatively charged chloride ion, "2,8,8". This much seems obvious, sodium and chloride ions will form inevitably, it seems, form a 1:1 ionic compound with a well-defined composition.
However, not all compounds behave so well, ionic bonds and their electron-sharing counterparts covalent bonds are often not quite so clear-cut in their distinctions in many exceptions to the rules where bonds are polar and hybridized with characteristics of both bonding types to different degrees. Sodium chloride was always the solid, the salt of the earth, when it comes to ionic compounds. But, not any more.
"We found crazy compounds that violate textbook rules," says team member Weiwei Zhang, a visiting scholar in Oganov's laboratory and his Center for Materials by Design. "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."
Oganov had theorized that if sodium chloride were to break the rules then mixing and heating it with metallic sodium in a diamond anvil cell, and applying the pressure would give rise to sodium-rich compounds, such as Na3Cl. Conversely, NaCl, mix and heated with pure chlorine and compressed would generate NaCl3. Now, the theoretical team's collaborator Alexander Goncharov of the Carnegie Institution of Washington has performed these very experiments and demonstrated that such compound are indeed formed under such conditions. "When you change the theoretical underpinnings of chemistry, that's a big deal," Goncharov suggests. "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," he explains. "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."
Oganov's pursuit of the seemingly impossible began with an obstinate curiosity. He too had read all the conventional wisdom in the chemistry textbooks that said these exotic materials were somehow forbidden because their existence would break the bonding rules laid down by previous generations of chemistry.
"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 just 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?"
The notion of impossible compounds is not an absolute concept. If it is simply about finding a way to change relative energies by changing conditions so that previously "impossible" compounds can rest easy. Indeed, while Oganov's work is breaking new ground, it is not really shattering any theory, just pushing the boundaries of what experimental chemists might try with real materials in the laboratory by using high computational power to show what might exist in the chemical space that has not been seen in the test tube before.
"The rules of chemistry are not like mathematical theorems, which cannot be broken," Oganov explains. "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." This is exactly what the researchers have done, their work might also now lead to the anticipation of new phenomena in planetary interiors where this exotic new chemistry may produce anomalies that could be observed spectroscopically and remotely.
"We have learned an important lesson - that even in well-studied and simple systems, like sodium chloride, you can find totally new chemistry, and totally new and very exciting materials," Oganov says. "It's like discovering a new continent; now we need to map the land. Current rules cannot cope with this new chemistry. We need to invent something that will." He told SpectroscopyNOW that, "The next phase will be to test other systems, gather better understanding of this new bizarre chemistry and to explore ways to technologically exploit them."
Science, 2013, 342, 1502-1505: "Unexpected Stable Stoichiometries of Sodium Chlorides"
Article by David Bradley
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.