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

Various metastable ice phases and their complicated transition pathwayshave been found by pressurization at low temperatures at which slowkinetics and high metastability are easily achieved. By contrast, suchdiversity is less expected at room or elevated temperatures. Here, using acombination of a dynamic diamond anvil cell and X-ray free electron lasertechniques, we demonstrate t ha t sup erco mp ressed water transforms intoice VI through multiple freezing–melting pathways at room temperature,hidden within the pressure region of ice VI. These multiple transitionpathways occur via a metastable ice (more specifically, ice XXI withbody-centred tetragonal structure (Ī 42d)) discovered in this study and ametastable ice VII that exists within the pressure range of ice VI. We find thatsupercompressed water structurally evolves from high-density water tovery-high-density water, causing multiple transition pathways. Thesefindings provide an insight to find more metastable ice phases and theirtransition pathways at elevated temperatures.