Ice is the most common solid in the universe. It is the foundation of star formation and the source of life. In nature, ice is a crystal with a hexagonal dense structure, known as "hexagonal ice", which is why snowflakes are always hexagonal flakes. So, in nature, can ice form cubes like diamonds? Researchers from the Chinese Academy of Sciences and Peking University used in situ transmission electron microscopy to deepen the experimental study of ice to the molecular level, giving a new solution to this problem: water crystallization can also directly form cubic ice, and the key factor affecting the formation of cubic ice may be the ubiquitous heterogeneous interface. This achievement was published online on the 29th in the international academic journal Nature. "For a long time, scientists have spent a great deal of effort on the research of cubic ice, and have also prepared cubic ice in the laboratory. However, for the physical process of water crystallization, it is always difficult to provide corresponding experimental data at its molecular level. Therefore, the controversy over whether cubic ice exists in nature has never stopped." One of the main authors of the paper Bai Xuedong, a researcher at the Institute of Physics of the Chinese Academy of Sciences/National Research Center for Condensed Matter Physics in Beijing, said: "From this perspective, the real-time microscopic imaging technology of water icing with high spatial resolution and low damage is of great significance, Successfully achieved the observation of ice growth and crystallization process at molecular level resolution, and in situ characterization of structural evolution. The researchers demonstrated the process of condensation of vapor phase water onto a low-temperature substrate at about - 170 ℃ to form ice crystals, and discovered the preferential nucleation and growth of cubic ice on this low-temperature substrate. Molecular level imaging has confirmed that water crystals can form various single crystal cubic ice with different morphologies. With the increase of time, the proportion of hexagonal ice in the overall ice crystal gradually increases. Researchers analyzed that this indicates that heterogeneous interfaces play an important role in the formation of cubic ice. Most common snowfall in nature is the condensation and growth of water molecules on surfaces such as dust and minerals, and this heterogeneous interface is ubiquitous. Further, the researchers characterized common defects within cubic ice. Experimental observation and molecular dynamics simulation results show that this defect rich structure is unstable, and under the perturbation of the electron beam, the defect layer undergoes a synergistic distortion of its structure and configuration, even a global climb. "This may be the reason why we can hardly see cubic ice in nature," said Bai Xuedong. Bai Xuedong said, however, in the microscopic world, cubic ice maintains considerable stability during observation time, both during growth and under electron beam excitation, without any signs of transition to hexagonal ice. "The stability of this structure proves that cubic ice has considerable competitiveness in the process of water freezing, and therefore may play a crucial role in this process." "Now we have confirmed that snowflakes do not always' six out '." Bai Xuedong said, "In situ transmission electron microscopy technology has great potential in the study of ice. I believe that every progress in experimental technology will bring us new insights and uncover more mysteries about ice."