  At ambient pressure, sodium, chlorine, and their only known compound NaCl,
have well-understood crystal structures and chemical bonding. Sodium is a
nearly-free-electron metal with the bcc structure. Chlorine is a molecular
crystal, consisting of Cl2 molecules. Sodium chloride, due to the large
electronegativity difference between Na and Cl atoms, has highly ionic chemical
bonding, with stoichiometry 1:1 dictated by charge balance, and rocksalt
(B1-type) crystal structure in accordance with Pauling's rules. Up to now,
Na-Cl was thought to be an ultimately simple textbook system. Here, we show
that under pressure the stability of compounds in the Na-Cl system changes and
new materials with different stoichiometries emerge at pressure as low as 25
GPa. In addition to NaCl, our theoretical calculations predict the stability of
Na3Cl, Na2Cl, Na3Cl2, NaCl3 and NaCl7 compounds with unusual bonding and
electronic properties. The bandgap is closed for the majority of these
materials. Guided by these predictions, we have synthesized cubic NaCl3 at
55-60 GPa in the laser-heated diamond anvil cell at temperatures above 2000 K.
