Pressure-induced phase transition, its mechanism and compressibility studies on nanocrystalline and bulk U3O8

Abstract

High Pressure X-Ray Diffraction (HPXRD) studies were conducted on powder samples of U3O8 with crystallite sizes of 54 nm, 82 nm, as well as bulk U3O8. The investigation revealed that U3O8 undergoes a phase transition to a fluorite phase through an intermediate tetragonal structure, with the (200) plane of the orthorhombic structure playing a pivotal role. This transition occurs through the movement of uranium atoms while maintaining the overall integrity of the lattice. The phase transition is sluggish, and an extensive coexistence regime of the ambient and High-Pressure (HP) fluorite phase is observed in nano form as compared to bulk- U3O8. Moreover, the ambient phase of nano- U3O8 exhibits an elevated bulk modulus of 92 GPa, in contrast to the bulk sample's modulus of 78 GPa. Similarly, for the high-pressure phase, the bulk modulus is 419 GPa for nano- U3O8 and 367 GPa for bulk- U3O8. These findings provide valuable insights into the phase behavior and compressibility of U3O8, thereby carrying potential implications for nuclear technologists.