Stability of numerous novel potassium chlorides at high pressure

K-Cl is a simple system displaying all four main types of bonding, as it contains (i) metallic potassium, (ii) elemental chlorine made of covalently bonded Cl2 molecules held together by van der Waals forces, and (iii) an archetypal ionic compound KCl. Well-known octet rule (and its special case, the charge balance rule, assigning classical charges of +1 to K and -1 to Cl) predicts that no compounds other than KCl are possible. However, our quantum-mechanical variable-composition evolutionary simulations predict new stable compounds. The phase diagram of the K-Cl system turned out to be extremely complicated, featuring new thermodynamically compounds K3Cl, K2Cl, K3Cl2, K4Cl3, K5Cl4, K3Cl5, KCl3, KCl7. Of particular interest are 2D-metallic homologs Kn+1Cln, the presence of positively charged Cl atoms in KCl7, and the predicted stability of KCl3 already at the atmospheric pressure.