Interplay of spin-allowed and spin-forbidden 5d-4f luminescence from rare earth ions

The conditions for co-existence of emissions from spin-allowed and spin-forbidden 5d-4f transitions in rare earth ions and for thermal quenching of these emissions have been analyzed by taking Tm3+ 5d-4f luminescence in LiYF4:Tm3+ and Lu3+ 5d-4f luminescence in LuF3 as examples. It is shown that temperature behavior of 5d-4f luminescence agrees with the common trends in decreasing energy splitting between the lowest high-spin and low-spin 5d levels as well as decreasing energy gap between the lowest 5d level and the bottom of the conduction band of the host crystal towards heavier rare earth ions (from Er3+ to Lu3+)