Thermodynamics of high-pressure ice phases explored with atomistic simulations

Most experimentally known high-pressure ice phases have a body-centred cubic (bcc) oxygen lattice. Our large-scale molecular-dynamics simulations with a machine-learning potential indicate 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, the transformation between ice X and the Pbcm phase has 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 ices and reveals peculiar solid-solid transition mechanisms not known in other systems. Many experimentally known high-pressure ice phase are structurally very similar. Here authors elucidate the phase behaviour of the high-pressure insulating ices and reveal solid-solid transition mechanisms not known in other systems.