Emergence of novel hydrogen chlorides under high pressure

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.