Uranyl sensitized Eu3+ luminescence in Ln(UO2)3(PO4)2O(OH)·6H2O phosphors (Ln = Y, Eu, La) for warm-white light emitting diodes

This work concerns the red line emitting phosphors Eu(UO2)3(PO4)2O(OH)·6H2O, (Y0.95Eu0.05)(UO2)3(PO4)2O(OH)·6H2O, and (La0.95Eu0.05)(UO2)3(PO4)2O(OH)·6H2O, which are isostructural to the mineral Françoisite. These materials were prepared via a precipitation method for the first time. All materials show strong absorption in the spectral range from 250 nm to 550 nm as a result of the quantum mechanically allowed ligand-to-metal charge transfer transitions centered at the uranyl cation (UO2)2+. A highly efficient energy transfer from (UO2)2+ to Eu3+ takes place, resulting in the material showing solely Eu3+ line emission upon excitation at 450 nm. This brands Eu(UO2)3(PO4)2O(OH)·6H2O as an interesting candidate for solid state light sources with a low color temperature. The color point of the emission could be shifted by partially substituting Eu3+ with Y3+ and La3+, respectively.