Neutron Diffraction Study of Ice Polymorphs under Helium Pressure

Neutron diffraction studies of several of the D₂O ice polymorphs were made with helium pressures to 3.5 kbar. Volume compressions $\mathrm{(\Delta V\hspace{0.17em}/\hspace{0.17em}V)}$ were obtained for ices Ih, Ic, and IX at 2.1 to 2.8 kbar. The equilibrium phase boundaries between ices I–II and I–III in the presence of helium are shifted toward higher pressure values. The crystal structures of ices Ih, Ic, II, and IX at approximately 2.5 kbar are the same as the structures found for these ice forms at atmospheric pressure. A tetragonal structure is indicated for ice III with lattice parameters at − 23°C and 2.7 kbar of $\text{a\hspace{0.17em}=\hspace{0.17em}6.674\hspace{0.17em}±\hspace{0.17em}0.015 Å}$ and $\text{c\hspace{0.17em}=\hspace{0.17em}6.97\hspace{0.17em}±\hspace{0.17em}0.03 Å}$ . The $\mathrm{c\hspace{0.17em}/\hspace{0.17em}a}$ ratio for ice IX has been found to be 1.000 ± 0.003 both at 1 bar and at 2.8 kbar.