Ultra-slow relaxation in ice and related substances
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概要
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Of prime interest in numerous studies on water, an important substance to all living systems, may be its physical, chemical, biological characteristics in our internal and external environments. One of the central problems underlying all these researches is a basic structural problem. An important question was why ordinary ice did not obey the third law of thermodynamics. The proton-disordered phase Ih remains down to the lowest temperature without exhibiting any indication of phase transition. We found that the glass transition is not a characteristic property of liquid but of wide occurrence in condensed systems that failed to maintain thermal equilibrium during continuous cooling. Crystalline substance that exhibited freezing-in process on cooling was designated as "glassy crystals". In fact, ice Ih exhibited a glass transition at around 110 K due to slowing down of reorientational motion of the water molecules. A particular kind of impurity was found to accelerate dramatically the motion, and to induce a long-awaited phase transition at 72 K. The transition removed a substantial fraction of the residual entropy of ice. The dopant acted as a kind of catalysis for releasing the immobilized non-equilibrium state to recover thermal equilibrium in the laboratory time. Structure and some properties of the ordered low temperature phase, designated as ice XI, are discussed. The same ordering processes observed in some clathrate hydrates possessing hydrogen-bonded networks similar to ice Ih are described. Interplay between phase transformation and glass transition in relation to important role played by particular dopant is discussed. (Communicated by Syûzô SEKI, M.J.A.)
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