Tanaka, H., Yagasaki, T. & Matsumoto, M. On the role of intermolecular vibrational motions for ice polymorphs. III. Mode characteristics associated with negative thermal expansion. J. Chem. Phys. 155, 214502 (2021)

水を冷やすと4℃以下で膨張しはじめることは、水の変わった性質として有名ですが、0℃まで冷やすと氷になり、その先は冷やすほど体積は小さくなります。ところが、実は氷になってからも、温度を非常に低くすると、絶対温度60 K以下で再び膨張が始まります。この論文では、なぜ極低温の氷が膨張しはじめるのか、その機構を明らかにしています。氷と水はどちらも冷やすと膨張しはじめますが、そのメカニズムは全く異なります。

It is well known as an unusual property of liquid water that when it is cooled down, it begins to expand at a temperature below 4 degree celsius. When it is cooled down to 0 degree, it becomes ice, and after that, its volume becomes smaller as it is cooled down. However, even after it becomes ice, if the temperature is kept very low, it begins to expand again at an absolute temperature of 60 K or lower. This paper clarifies the mechanism of why ice begins to expand at very low temperatures. Both ice and water begin to expand when they are cooled, but their mechanisms are completely different.

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