Multiple freezing–melting pathways of high-density ice at room temperature

Various metastable ice phases and their complicated transition pathways have been found by pressurization at low temperatures, where slow kinetics and high metastability can be easily achieved. In contrast, such diversity is less expected at room or elevated temperatures. Here, using a dynamic diamond anvil cell and X-ray free electron laser techniques, we demonstrate that supercompressed water transforms into ice VI through multiple freezing–melting pathways at room temperature, hidden within the pressure region of ice VI. The multiple transition pathways occur via a new metastable ice and a metastable ice VII in the supercompressed water. We found that the structural evolution of supercompressed water from high density to very high density underlies the multiple transition pathways. These findings provide new insights to find more metastable ice phases and their transition pathways at room or elevated temperatures on icy planets.