Emergence of novel hydrogen chlorides under high pressure

TL;DR

This study systematically explores the stability and bonding of novel hydrogen chlorides under high pressure, revealing diverse compounds and bonding types that emerge at pressures up to 500 GPa.

Contribution

The paper reports the discovery of new stable hydrogen chloride compounds under high pressure, expanding understanding of their structural and bonding diversity.

Findings

Stable H$_2$, H$_3$, H$_4$, H$_5$, and H$_7$Cl compounds identified at high pressures.

Formation of cyclic H$_3^+$ and H$_5^+$ cations within crystalline structures.

Complex bonding features including van der Waals, covalent, and layered Kagomé structures.

Abstract

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.