The influence of crystal structure and chemical composition on the energy transfer processes in uranates

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Abstract
The luminescence properties of several uranates are reported. The compounds that have been investigated are Y6UO12, Li4UO5, Li2UO4, and MgUO4, all of which contain distorted uranate octahedra. Emission and excitation spectra and the quantum efficiency of the luminescence were measured at 4.2 K. Furthermore, the temperature dependence of the luminescence was investigated. The results show that the excitation energy migrates through the lattice and that the luminescence occurs from uranate centers near defects. These centers act as traps for the migrating excitation energy. The quantum efficiency of the luminescence varies strongly within this series of compounds. This is ascribed to a variation in the concentratio ratio of traps and killers (U5+ ions). When the energy migration process is described with a diffusion model, the diffusion coefficient appears to be ≳5×10−8 cm2 s−1. Finally, some more specific results are presented for Y6UO12, Li4UO5, and Li2UO4.