The Impact of Coordination Environment on the Thermodynamic Stability of Uranium Oxides

Amorphous
uranium oxides are known to arise via industrial processes
relevant to the nuclear fuel cycle yet evade rigorous structural characterization.
A promising approach is to develop statistical relationships between
uranium–oxygen coordination environments and thermodynamic
stability from which general statements about the likelihood of observing
particular U–O arrangements can be made. The number of known
crystalline uranium oxides is insufficient to build statistical relationships.
We have developed a database of theoretical compounds using genetic
algorithms with the density functional theory as a foundation to analyze
coordination geometries in the uranium–oxygen phase space.
We draw fundamental insights into the nature of uranium–oxygen
interactions by correlating total energy with the coordination environment.
The most stable configuration of U cations with O anions is in an
environment with coordination numbers 5–8 in a cubic configuration.
Higher and lower coordination numbers are observed only in metastable
phases. General trends are observable in the relationships between
the coordination number, density, and uranium fraction in each structure.
The new insight into uranium coordination enabled by these analyses
is foundational for further studies into the characteristic properties
of individual uranium oxide materials and for elucidation of potential
oxidation pathways for uranium metal.