Hydrogen bonding exists in a range of simple molecular systems, such as water and ammonia. The hydrogen-halides also exhibit hydrogen bonding and consequently have extensive polymorphism. On tuning the hydrogen bonding interaction via compression or cooling, the hydrogen-halides progress through a series of solid phases depending on the ordering of the hydrogen atoms, denoted as phase I through to phase III[1]. Only by means of compression and with hydrogen chloride (HCl) is it possible to achieve a further phase, phase IV, whereby the former hydrogen bond distance and the intramolecular distance equalize, reported at ~50 GPa[1], known as hydrogen-bond symmerisation.  A transition absent in the heavier hydrogen halides, due to their preference to dissociate (2HX→H₂+X₂, X=Br, I)[2].

Beyond symmeterisation, hydrogen bonding is poorly understood, including in the archetypal system, water ice, where it is believed to transform to a lower symmetry orthorhombic ice XI[3] with further transitions, including metallisation, to occur in the terapascal regime[4]. Due to the stability of the HCl system in comparison to the heavier hydrogen-halides, it can provide further insight into the weakly constrained hydrogen-bonded systems in a regime beyond symmeterisation. For example, currently, at compression beyond the observation of symmeterisation, unusual modified stoichiometries are suggested from numerical simulations with proposed metallic properties[5].

In this presentation, I will discuss our recent observations, including X-ray diffraction, Raman spectroscopy and DFT calculations on the HCl system into the multi-Mbar regime. Further, I will discuss recent measurements on the pure chlorine system to pressures in excess of 300 GPa[6], which allow us to constrain the observed behaviour of dense HCl. These results give us new insight into hydrogen-bonding at extreme conditions and therefore could be applicable in a wide range of hydrogen-bearing systems[3].

[1] E. Katoh et al., Phys. Rev. B. 61, 119 (2000).

[2] J. Binns et al., Phys. Rev. B. 96, 144105 (2017).

[3] M. Benoit et al., Phys. Rev. Lett. 89, 205503 (2002).

[4] Y. Wang et al., Nat. Comms. 2, 563 (2011).

[5] Q. Zheng et al., Phys. Chem. Chem. Phys. 19, 8236 (2017).

[6] Dalladay-Simpson et al., Nat. Comms. 10, 1134 (2019).