Complex Phase Behavior of U₃O₈

Oxidation of UO₂ produces U₃O₈. During oxidation, the unit cell undergoes a significant expansion (approx. 17%). This reaction must be considered in technological applications of UO₂ (such as nuclear fuel). U₃O₈ itself has multiple crystal phases, including an orthorhombic Amm2 phase that occurs below 300 ^oC, and a hexagonal P-62m phase above 300 ^oC. Although these phases are nearly isomorphic, one fact is that the Amm2 phase has two crystallographic uranium sites, and the oxidation configuration is 2U(V) + U(VI). But, the P-62m phase has only one crystallographic site, and so each of the three uranium atoms must carry 16/3 charge. We present preliminary investigations into possible resolutions of this observation, including temperature dependent Raman and x-ray diffraction measurements. Coupled with density functional theory calculations, we show an intimate relationship between phonon softening and local U-O structural re-arrangements. In particular, softening of one strong Raman-active mode near 420 cm^-1 appears to be the driving effect in the orthorhombic-to-hexagonal phase transition. We also explore the possibility of coupling between this phonon and the electronic structure and whether this phonon facilitates an electron-hopping mechanism.