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 7 6 K. They are compared with parallel measurements on pure D2O ice. We demonstrate that ice XI is orthorhombic, with lattice parameters a=4.4 65(3) Angstrom, b=7.859(4) Angstrom, and c=7.292(2) Angstrom at 5 K, a s compared with a=4.4974(1) Angstrom and c=7.3236(1) Angstrom for pure ice. The detransformed phase measured at 5 K has a different c-lattic e parameter from that of pure ice, with a=4.4971(1) Angstrom and c=7.3 182(1) Angstrom. 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 t he untransformed phase is shifted, giving a=4.499(1) Angstrom and c=7. 311(1) Angstrom. These strain effects also produce broadening of the d iffraction peaks for the transformed phase relative to those of the de transformed 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 pa rameter of the detransformed KOD ice relative to pure ice above the tr ansition, which decreases with temperature, becoming negligible at 200 K. We suggest that this is due to local ordering in the doped ice. (C ) 1996 American Institute of Physics.