  HCl, a 'textbook' example of a polar covalent molecule, is a well-known
compound of hydrogen and chlorine. Inspired by the discovery of unexpected
stable stoichiometries of sodium chlorides, we performed systematic searches
for all stable compounds in the H-Cl system from ambient pressure to higher
pressures up to 500 GPa using variable-composition ab initio evolutionary
algorithm USPEX. We found several compounds that are stable under pressure,
i.e. HCl, H$_2$Cl, H$_3$Cl, H$_5$Cl and H$_4$Cl$_7$, which display a rich
variety of chemical bonding types. At ambient pressure, H$_2$, Cl$_2$ and HCl
molecular crystals are formed by weak intermolecular van der Waals interactions
and adjacent HCl molecules connect with each other to form asymmetric zigzag
chains, which become symmetric under high pressure. In hydrogen-rich chlorides,
H$_2$ and HCl react to form the thermodynamically stable H$_3$Cl crystalline
compound in which molecular cyclic H$_3^+$ cations are stabilised by the Cl$^-$
sublattice. Increasing the amount of hydrogen leads to stable solid-state
H$_5$Cl, in which H$_2$ formally combines with H$_3^+$ to form H$_5^+$ cations.
Additionally, chlorine-based Kagom\'e layers are formed with intercalated
zigzag HCl chains in chlorine-rich hydrides. These discoveries help to
understand how varied bonding features can co-exist and evolve in one compound
under extreme conditions.

Materials Science