6d5f and 5f2 configurations of U4+ doped into LiYF4 and YF3 crystals

The properties of U4+ 6d5f→5f2 luminescence in LiYF4 and 5f2→5f2 luminescence in YF3 are very similar to properties of 5d4f→4f2 and 4f2→4f2 luminescence in the iso-electronic Pr3+ ion embedded into the same crystals. In LiYF4 the lowest level of the 6d5f electronic configuration of U4+ is located below the highest 1S0 level of the 5f2 electronic configuration whereas in YF3 the relative energy position of these levels is opposite. Accordingly, the interconfigurational broad-band luminescence from the lowest 6d5f level is observed from the U4+:LiYF4 crystal, but the intraconfigurational narrow-line luminescence from the 1S0 level of the U4+ ion is observed in the U4+:YF3 crystal when it is excited into the 6d5f levels of U4+. The U4+ doped crystals can be considered as more attractive phosphor systems based on cascade luminescence compared with those of Pr3+ doped because the main 1S0 emission in U4+ occurs in the visible range (around 500 nm). However, no definite evidence of the second step in cascade luminescence originating from the lower 5f levels populated by the radiative decay of the 1S0 level of U4+ was identified in the emission spectrum of the U4+:YF3 crystal.