Tikalon Blog by Dev Gualtieri

Strange Salt

January 15, 2014

Sodium chloride (table salt, Na Cl ) is a chemical compound that appears in the very early chapters of nearly every chemistry textbook . Sodium , which has atomic number 11, and chlorine , which has atomic number 17, appear very early in the Periodic Table , in the third period . As a group 1 metal , sodium has a single positive charge when it's ionized by losing an electron (Na⁺). As a group 17 halogen , chlorine accepts an electron to become an ion with a single negative charge (Cl^-).

When these ions get together, it's easy to see how the stable compound, NaCl, will form. The crystal form of NaCl is called halite , and that's how the halogens ("salt makers") get their name. The crystal structure of halite is the aptly named rock salt structure , which is a face-centered cubic crystal (see figure).

Crystal structure of rock salt (NaCl, halite)

(Image by H. Hoffmeister, via Wikimedia Commons .)

Quantum mechanics has illuminated the atomic nature of the chemical elements , so we now know that the electron configuration of sodium is [Ne] 3s¹, and the electron configuration of chlorine is [Ne] 3s² 3p⁵. In forming NaCl, sodium loses its electron to become more like the stable noble gas, neon , while chlorine accepts an electron to be more like the stable noble gas, argon . What could be simpler, and that's the stuff of which textbook examples are made.

The reason most things happen is because energy is minimized . In the case of halite, the sodium and chlorine atoms form this crystal structure because it's an arrangement with low energy . That might be true for normal temperature and pressure conditions , but sometimes putting on the pressure will cause a transformation from one crystal structure to another.

One example of this is ice . At the very low temperature of 100 K and atmospheric pressure (100 kPa ), ice exists in its I_C crystalline form. Ramping up the pressure causes a transformation of crystal structures through the famous ice-nine (IX), II , XV , VIII , X and finally XI . Each transformation happens because the change lowers the crystal energy.

Increased pressure often has a beneficial affect on useful physical properties, such as superconductivity . When it's found that pressure increases the superconducting critical temperature of a particular superconductor, the material is doped with large atoms to simulate the pressure effect at atmospheric pressure. The transition temperatures of many high temperature superconductors are routinely increased using this technique.[1]

A large international group of scientists from China , the United States , Russia , and Germany have now demonstrated that pressure allows formation of unexpected compounds of sodium and chloride that violate the presumed charge-pairing rule for ionic compounds .[2-6] The team was led by Artem Oganov of Stony Brook University and Alexander Goncharov of the Carnegie Institution .[5-6] Compounds with the stoichiometries , Na₃Cl and NaCl₃, were proven to exist at high pressure by X-ray measurements .[5-6]

The "high pressures" used weren't all that high. The compounds were demonstrated at about 200,000 atmospheres , which is far less than the 3.6 million atmospheres at Earth's center .[5-6] Pressure studies of NaCl itself have demonstrated a transition from rock salt to the B2 phase, a cubic, eight-fold coordinated phase, at about 30 GPa .[3] Complete metallization of NaCl is predicted to occur above about 300 GPa, but no sodium-chlorine compounds other than NaCl were known before these experiments.[3]

As happens more often with today's better computation tools , theory led experiment . Crystal structure prediction algorithms predicted the existence of Na₃Cl, Na₂Cl, Na₃Cl₂, NaCl₃, and NaCl₇.[2] Cubic and orthorhombic NaCl₃, and two-dimensional metallic tetragonal Na₃Cl, were formed under pressure to test the predictions. Although these compounds violated the traditional rules of chemistry, they were found to be thermodynamically stable under the high pressure condition.[2]

Crystal structure of NaCl₃.

(Illustration by Artem Oganov, from Carnegie Institution press release .)

The lead author of the paper, Weiwei Zhang , a visiting scholar at Stony Brook, summarized how these results deviate from classical chemistry.

"These compounds are thermodynamically stable and once made, remain so indefinitely... 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."[5-6]

Alexander Goncharov of the Carnegie Institution sees the possibility for future work.

"If this simple system is capable of turning into such a diverse array of compounds under high-pressure conditions, then others likely are, too... This could help answer outstanding questions about early planet cores , as well as to create new materials with practical uses."[4]

This work was supported in the US by the National Science Foundation , the Army Research Office and DARPA , among other institutions.[4]

Strange Salt

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