Anomalous dispersion and band gap reduction in UO2+ and its possible coupling to the coherent polaronic quantum state

Abstract

Hypervalent UO 2 , UO 2(+x) formed by both addition of excess O and photoexcitation, exhibits a number of unusual or often unique properties that point to it hosting a polaronic Bose-Einstein(-Mott) condensate. A more thorough analysis of the O x-ray absorption spectra of UO 2 , U 4 O 9 , and U 3 O 7 shows that the anomalous increase in the width of the spectra features assigned to predominantly U 5f and 6d final states that points to increased dispersion of these bands occurs on the low energy side corresponding to the upper edge of the gap bordered by the conduction or upper Hubbard band. The closing of the gap by 1.5 eV is more than twice as much as predicted by calculations, consistent with the dynamical polaron found by structural measurements. In addition to fostering the excitation that is the proposed mechanism for the coherence, the likely mirroring of this effect on the occupied, valence side of the gap below the Fermi level points to increased complexity of the electronic structure that could be associated with the Fermi topology of BEC-BCS crossover and two band superconductivity.

Chemical State of U in U–N–O Ternary System from First-Principles Calculations

“…Importantly, according to the fingerprint peaks (U–O 5 VV), the band gaps of UN 1.66 , UO 2 , and UN x O y were estimated to be 0.65, 1.8, and 1.4 eV, respectively. The band gap of UO 2 determined in this method is a little smaller from 2.1 eV obtained in the optical spectrum and 1.5 eV obtained by X-ray absorption spectra . Moreover, the thermal stability of uranium nitride and oxynitride film in an ultrahigh vacuum environment using Auger electron spectroscopy (AES) revealed that UN 1.42 O 0.23 film exhibited better thermal stability than UN 1.66 film below 573 K .…”

“…The band gap of UO 2 determined in this method is a little smaller from 2.1 eV obtained in the optical spectrum 6 and 1.5 eV obtained by X-ray absorption spectra. 7 Moreover, the thermal stability of uranium nitride and oxynitride film in an ultrahigh vacuum environment using Auger electron spectroscopy (AES) revealed that UN 1.42 O 0.23 film exhibited better thermal stability than UN 1.66 film below 573 K. 8 Interestingly, when the temperature exceeded 573 K, O in UN 1.66 and UN 1.42 O 0.23 films decreased significantly, whereas N increased due to the diffusion of N from the inner to outer layer. These findings suggest a diversified chemical formula of UN x O y , as evidenced by electron-energy-loss spectroscopy 9 and first-principles calculations.…”

Evolution of the Uranium Chemical State in Mixed-Valence Oxides

“…9 Uranium oxides show exciting interdependence between their physicochemical properties and complex electronic structures. 10,11 In particular, the presence of delocalized valence electrons can be seen to stimulate oxidation of UO 2 . Accommodation of excess oxygen atoms onto specific sites in the fluorite (Fm3̅ m) lattice has the potential to increase hybridization between O 2p and U 5f/6d orbitals.…”

Uranium Oxide Nanocrystals by Microwave‐Assisted Thermal Decomposition: Electronic and Structural Properties

“…As previously reported, the spectral features strongly depend on the preparation method. [26] In case of the UO 2 powder, the signals corresponding to the hybridized uranium 5f 3 states were more pronounced than in the dispersed sample. In Figure 6 and the reference spectrum by Jollet et al, the 5f 3 states appear as "shoulders" towards lower energies of the 5f 2 6de g 1 signal, whereas in the UO 2 powder both signals are distinguishable and exhibit comparable intensities ( Figure 7).…”