  Sodium chloride (NaCl), or rocksalt, is well characterized at ambient
pressure. Due to the large electronegativity difference between Na and Cl
atoms, it has highly ionic chemical bonding, with stoichiometry 1:1 dictated by
charge balance, and B1-type crystal structure. Here, by combining theoretical
predictions and diamond anvil cell experiments we show that new materials with
different stoichiometries emerge at pressure as low as 20 GPa. Compounds such
us Na3Cl, Na2Cl, Na3Cl2, NaCl3 and NaCl7 are theoretically stable and have
unusual bonding and electronic properties. To test this prediction, at 55-80
GPa we synthesized cubic and orthorhombic NaCl3 at 55-70 GPa and 2D-metallic
tetragonal Na3Cl. This proves that novel compounds, violating chemical
intuition, can be thermodynamically stable even in simplest systems at
non-ambient conditions.
