Reformulating Table Salt Under Pressure

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Science  20 Dec 2013:
Vol. 342, Issue 6165, pp. 1459-1460
DOI: 10.1126/science.1247699

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Sodium chloride (NaCl), the main component of table salt, is the archetypical ionic compound of chemistry textbooks. The large electronegativity difference between the participating elements drives salt formation. Metallic sodium transfers electrons to chlorine, and the resulting positively and negatively charged ions are held together by electrostatic attraction—ionic bonds. At ambient conditions, NaCl crystallizes in the so-called rocksalt structure, a cubic array of Na and Cl atoms in equal proportions (1:1 stoichiometry) and with six-fold coordination. With increasing pressure, a structural phase transition to the cubic, eight-fold coordinated NaCl-B2 phase is observed at ∼30 GPa [(1) and references therein]. Theory suggested that complete metallization of NaCl should occur above ∼300 GPa, and no Na-Cl compounds other than NaCl were known to exist. However, on page 1502 of this issue, Zhang et al. (2) show that stable Na-Cl phases with stoichiometries different from 1:1 and intriguing properties can be synthesized in the lab with high-pressure techniques.