Structure identification at extreme conditions*

We are investigating low-crystallinity, amorphous uranium oxides. One compound has stoichiometry of ~UO3.5 (a-UO3). Powder XRD shows broad peaks commensurate with extreme finite-size broadening or amorphous character. To understand the a-UO3 structure, we use density functional theory with a genetic algorithm structure prediction mechanism to identify stable structures of non-stoichiometric uranium oxides. The lowest energy predicted structure with stoichiometry UO3.5 contains a peroxide bridge. The presence of peroxide bridges is compatible with a study of comparable material using neutron pair-distribution function techniques. We attempted to crystallize a-UO3 using a diamond anvil cell and Raman spectroscopy. We observed highly anharmonic responses to pressure, but no long range order. Computational studies on the low-energy structure from the genetic algorithm show commensurate responses to the dynamical observations, along with anomalous changes in local bonding character. Here we discuss anharmonic responses to pressure coupled with computational modeling to shed light on the use of extreme conditions for structure identification.