Recrystallisation of HDA ice under pressure by in-situ neutron diffraction to 3.9 GPa

We have studied by in-situ neutron diffraction the recrystallisation behaviour of HDA ice in the pressure range 0.3-3.9 GPa, i.e. the entire stability range of HDA. We report quantitative and detailed structural information on the various high pressure ice phases formed metastably at low temperatures. We find that between similar to0.4-0.7 GPa, HDA transforms at 175 K to mainly phases IV and V, and XII, and at 1-1.2 GPa to a mixture of ice VI and XH. On isothermal compression at 100 K, HDA recrystallises to an ice VII-like structure which is either partially ordered or mixed with ice VIII. Full structural data obtained by Rietveld refinements are reported for all these phases at low temperatures. Phases IV, V and XII are fully hydrogen disordered when obtained from HDA. The transformation behaviour in the 0.5-1.2 GPa range is in good agreement with the picture reported from quenched-recovered samples, although some differences persist in the recrystallisation to IV/XII mixtures. The recrystallisation behaviour of HDA over the entire pressure range appears to follow closely that of liquid water under pressure.