The materials of the nuclear fuel cycle can be exposed to extreme environments, including elevated pressures. Therefore, understanding the behavior of these materials under pressure is critical. As in situ monitoring of high-pressure experiments via Raman or infrared spectroscopy is often used to track pressure-induced changes in materials, a computational understanding of predicted changes supports the interpretation of experimental results. Here, the high-pressure (up to 45 GPa) behavior of four phases of UO3 was predicted using Density Functional Theory as implemented in VASP. Structural rearrangements in the beta and gamma (orthorhombic) and gamma (tetragonal) phases occur between 16 and 24 GPa, leading to a change in coordination environment around the uranium cations. At each pressure studied in the range up to 45 GPa, the vibrational modes of each pressurized structure were predicted using Density Functional Perturbation Theory. Unexpected and unusually large softening of several phonon modes was predicted in multiple phases, highlighting a potentially distinctive feature of UO3 pressurized materials. For phases that undergo a rearrangement of the uranium coordination environments, this transition was also observed in a corresponding shift of the vibrational properties.

Under pressure: Predicting the pressure-induced structural changes of UO3 phases

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

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