A high resolution neutron powder diffraction study of D2O ice XI

The lattice parameters and diffraction peak profiles are presented for KOD-doped D₂O ice both in its low-temperature ordered state (ice XI) and as a function of temperature through the transition to ice Ih at 76 K. They are compared with parallel measurements on pure D₂O ice. We demonstrate that ice XI is orthorhombic, with lattice parameters a=4.465(3) Å, b=7.859(4) Å, and c=7.292(2) Å at 5 K, as compared with a=4.4974(1) Å and c=7.3236(1) Å for pure ice. The detransformed phase measured at 5 K has a different c-lattice parameter from that of pure ice, with a=4.4971(1) Å and c=7.3182(1) Å. The transformed sample is an intimate mixture of the orthorhombic transformed phase and hexagonal untransformed ice, giving rise to elastic strain effects such that the c-lattice parameter of the untransformed phase is shifted, giving a=4.499(1) Å and c=7.311(1) Å. These strain effects also produce broadening of the diffraction peaks for the transformed phase relative to those of the detransformed and pure phases. We demonstrate that the phase transition from ice XI to ice Ih is first-order. There is a shift in c-lattice parameter of the detransformed KOD ice relative to pure ice above the transition, which decreases with temperature, becoming negligible at 200 K. We suggest that this is due to local ordering in the doped ice.