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さまざまな不純物を添加した氷XIの生成と水素秩序化のメモリー効果

Growth of ferroelectric ice XI doped with alkali metal hydroxide observed using neutron diffraction

荒川 雅*; 小松 一生*; 飯塚 理子*; 鍵 裕之; Fernandez-Baca, J. A.*; Chakoumakos, B.*; 山内 宏樹; 深澤 裕

Arakawa, Masashi*; Komatsu, Kazuki*; Iizuka, Riko*; Kagi, Hiroyuki; Fernandez-Baca, J. A.*; Chakoumakos, B.*; Yamauchi, Hiroki; Fukazawa, Hiroshi

0.1から0.0001mol/lの不純物濃度の氷をさまざまな温度で静置した後、中性子回折の測定を行った。得られた回折パターンのリートベルト解析を行い、生成した氷XIの割合を決定し、どのような条件で氷XIが生成するのかを詳しく調べた。すると、氷XIの生成に要する時間や生成量は不純物の種類によらず、不純物濃度と温度履歴のみに依存することがわかった。一連の実験結果から、不純物由来のOH-が相転移の触媒としてはたらき、カチオンに由来する結晶歪みやクーロン力の影響はないと結論された。以上のことは、宇宙における氷XIの存在を裏付ける。さらに、相転移に適した不純物濃度及び温度履歴を明らかにした。

We prepared ice XI from different kinds of alkali metal hydroxide solutions with different concentrations, and measured neutron powder diffraction at JRR-3 (Japan Atomic Energy Agency, Japan) and HFIR (Oak Ridge National Laboratory, TN, USA). Rietveld analysis was carried out for the obtained diffraction patterns using a two-phase model, which includes ices Ih and XI. The hydrogen-ordered structure in the lithium-doped ice was the same as that in the sodium-doped and potassium-doped ices. We also confirmed the optimum formation condition of ice XI for the temperature history and concentration of solution. Furthermore, our results indicate that the phase transition from ice Ih to XI occurs in the doped ice that had once been ice XI to a notably larger degree than in the doped ice that had never transformed to ice XI. We named this phenomenon memory effect of hydrogen ordering.

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