In situ neutron diffraction studies of high density amorphous ice under pressure

We review recent in situ neutron diffraction studies on the structural pressure dependence and the recrystallization of dense amorphous ices up to 2 GPa. Progress in high pressure techniques and data analysis methods allows the reliable determination of all three partial structure factors of amorphous ice under pressure. The strong pressure dependence of the g_OO(r) correlation function shows that the isothermal compression of high density amorphous ice (HDA) at 100 K is achieved by a contraction (∼20%) of the second-neighbour coordination shell leading to a strong increase in coordination. The g_DD(r) and g_OD(r) structure factors are, in contrast, only weakly sensitive to pressure. These data allow a comparison with structural features of the recently reported 'very high density amorphous ice' (VHDA) which indicates that VHDA at ambient pressure is very similar to compressed HDA, at least up to the second-neighbour shell. The recrystallization of HDA has been investigated in the range 0.3-2 GPa. It is shown that hydrogen-disordered phases are produced which normally grow only from the liquid, such as ice XII, and in particular ice IV: These findings are in good agreement with results on quench-recovered samples.