Same and interconvertible high-pressure ice phases

TL;DR

This study uses atomistic simulations to clarify the phase relationships and transition mechanisms among high-pressure ice phases, revealing some phases are interconvertible and identifying unique transition behaviors.

Contribution

It demonstrates that certain high-pressure ice phases are the same thermodynamic phase under different conditions and uncovers novel transition mechanisms at high pressures.

Findings

Ice VII, VII', and X are the same phase under different conditions.

Ice X transforms into Pbcm phase with a barrierless transition at 300 GPa.

Superionic ice VII'' has a first-order boundary with ice VII'.

Abstract

Most experimentally known high-pressure ice phases have a body-centred cubic (bcc) oxygen lattice. Our atomistic simulations show that, amongst these bcc ice phases, ices VII, VII' and X are the same thermodynamic phase under different conditions, whereas superionic ice VII'' has a first-order phase boundary with ice VII'. Moreover, at about 300 GPa, ice X transforms into the Pbcm phase with a sharp structural change but no apparent activation barrier, whilst at higher pressures the barrier gradually increases. Our study thus clarifies the phase behaviour of the high-pressure insulating ices and reveals peculiar solid-solid transition mechanisms not known in other systems.