Abstract
The study investigates the structural transition pathways of U3O8 under varying temperature and pressure conditions, focusing on both bulk and nanocrystalline forms. The bulk U3O8 is known to undergo reversible phase transitions to a hexagonal structure at temperatures above 150°C and irreversible transitions to a fluorite structure under pressure at ambient temperature. High-pressure XRD experiments reveal a significant decrease in transition pressure for the hexagonal to fluorite phase transition in bulk U3O8, with the HT-hex phase exhibiting a bulk modulus of 155 ± 21 GPa at 450°C. In 82 nm nanocrystalline U3O8, the transition from orthorhombic to hexagonal phase is reversible up to 1.0 GPa and 500°C but becomes irreversible above 1.7 GPa. The bulk modulus of the hexagonal phase of nanocrystalline U3O8 is estimated to be 168±12 GPa and 146±16 GPa at 400°C and 500°C, respectively. The findings highlight the complexity of structural transformations in U3O8 and their dependence on temperature and pressure, offering insights valuable for materials science and nuclear reactor engineering.
Published Version
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