We present a data analysis strategy to extract information about one-dimensional stacking disorder and anisotropic particle sizes from high-intensity medium-resolution neutron powder diffraction performed in situ at D20 at ILL and to obtain a full profile refinement of the observed diffraction patterns.

Ice Ic, so-called "cubic ice", can be obtained e.g. by warming recovered high-pressure forms of ice. It is usually obtained in the form of very small crystallites leading to particle size broadening of the diffraction pattern. This pattern also contains features incompatible with a well-crystallised cubic structure, the details of which depend on the parent phase and the prevailing temperature. We have corroborated an earlier suggestion that an important number of deformation stacking faults exist in cubic ice and present a model for a quantitative description of stacking faults and anisotropic particle size broadening in ice Ic suitable for profile refinements of its complex diffraction patterns (see figure).

Two samples of deuterated ice Ic from ice IX and ice V have been studied in situ as a function of time at temperatures between 145 and 240K. Small changes of stacking fault probability occur hours after formation and continue gradually upon heating towards a higher proportion of hexagonal at the expense of cubic stacking sequences. At 190K the intensities of the Bragg reflections change considerably and the peaks become sharper. The pattern matches exactly the one of ice Ih only above 240K. We show quantitatively the time evolution of stacking disorder and crystallite size at different temperatures for ice Ic.

The kinetics of gas hydrate decomposition, below the melting point of water, is a complicated multi-phase process, due to an ice cover is produced by the initial gas hydrate decomposition at the grain surface and hindering out-diffusion of gas formed. Ice formed below 240K is stacking-faulty and shows a complicated T-dependent annealing which in turn changes the hindrance to gas diffusion leading to a “self-preservation” phenomenon above 240K where hydrates sustain in a semi-stable state for long time scales. For the data analysis of our in situ diffraction data we extend the presented stacking-fault model to describe the ice formed during CO₂-hydrate decomposition and its subsequent annealing behaviour.

Finally, we attempt to apply the method to a completely different system of one-dimensional stacking-disordered materials: orthorhombic copper(II)-hydroxo-oxoruthenate(VI) CuRuO₂(OH)₄.

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