We measured the neutron powder diffraction of 0.013 M KOD-doped D2O ice to investigate the formation process of ice XI, a hydrogen-ordered phase of ice Ih. The doped ice Ih transformed to ice XI after annealing at 57 and subsequently at 68 K. The mass fraction of ice XI to that of the doped ice (f) was estimated using Rietveld analysis for each sample. The f value of the doped ice, which had once experienced being ice XI (f = 0.23), was larger than that of the doped ice, which had never experienced being ice XI (f = 0.14). Results indicate that small hydrogen-ordered domains remained in the ice Ih, which had once transformed to ice XI, and accelerated the phase transition from ice Ih to ice XI. Results further suggest that large amounts of ice on icy bodies in our solar system can transform to ice XI, which might be detectable using infrared telescopes or planetary exploration in the near future.
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