Formation of UO2(BO3)4 clusters in B2O3 glass due to charge transfer and vibronic effects

Laser-induced excitation and fluorescence spectra and X-ray absorption fine structure (EXAFS) of the U6+ L3 edge (2p–5d) transitions indicate that uranyl ions in B2O3 glass have ordered local structure. Based on molecular dynamics (MD) simulation, four oxygen ions bind with each linear uranyl in the equatorial plane to form a stable crystalline UO6 cluster that binds with four groups of BO2. A charge transfer–lattice interaction model for the UO6 cluster formation in B2O3 glass is developed to explain the strong local lattice ordering and extraordinary stability of this cluster. The model is based on self-consistent oxygen–oxygen and oxygen–uranium charge transfer accompanied by lattice distortion both in the framework of pseudo-Jahn–Teller scheme.