Benchmarking acid and base dopants with respect to enabling the ice V to XIII and ice VI to XV hydrogen-ordering phase transitions.

Abstract

Doping the hydrogen-disordered phases of ice V, VI, and XII with hydrochloric acid (HCl) has led to the discovery of their hydrogen-ordered counterpart ices XIII, XV, and XIV. Yet, the mechanistic details of the hydrogen-ordering phase transitions are still not fully understood. This includes, in particular, the role of the acid dopant and the defect dynamics that it creates within the ices. Here we investigate the effects of a wide range of acid and base dopants on the hydrogen ordering of ices V and VI with calorimetry and X-ray diffraction. Surprisingly, lithium-hydroxide doping achieves a performance comparable to hydrofluoric-acid doping in ice V, but it is ineffective in the case of ice VI. Ice V is therefore the first phase of ice that can be hydrogen-ordered with both acid and base doping. Hydrobromic-acid doping facilitates hydrogen ordering of ice VI, but it is ineffective in the case of ice V. HCl is reaffirmed to be the most effective for both phases which is attributed to a favorable combination of high solubility and strong acid properties. Sodium-hydroxide, potassium-hydroxide (as previously shown), and perchloric-acid doping are ineffective for both phases. These findings highlight the need for future computational studies but also raise the question why lithium hydroxide is the best-performing alkali hydroxide for hydrogen-ordering ice V whereas potassium-hydroxide doping is most effective for the "ordinary" ice Ih.