Genetic Algorithms and DFT in the Search for Novel Stable and Metastable Crystal Structures of the Uranium Oxides

The existence of non-stoichiometric uranium oxides is well known. However, many of these possible structures are not well-characterized experimentally because traditional analysis tools fail to easily characterize the structure of systems with low or no translational symmetry. Even stoichiometric uranium oxides such as U₃O₈, U₄O₉, and UO₃ exhibit complex crystal phases when varying temperature and pressure. Using a combination of genetic algorithms and DFT calculations, we have searched the uranium–oxygen system for novel stable and metastable UO_x states. Several structure prediction codes already exist that implement genetic algorithms for this purpose, such as the Genetic Algorithm for Structure Prediction and the Universal Structure Predictor: Evolutionary Xtallography (USPEX) codes. USPEX can do additional searches for metastable states from a well-converged starting structure using an evolutionary metadynamics algorithm. By further calculating experimental observables such as the vibrational spectra of low-energy predicted structures, direct comparisons to experimentally obtained data of unknown compounds can be made, and unknown experimental samples can potentially be assigned to a predicted crystal structure.