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)
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.