Concentration and temperature dependent luminescence properties of the SrI₂-TmI₂ system

The concentration dependent luminescence of the SrI₂-TmI₂ system was investigated. For Tm²⁺ concentrations up to 5 mol %, the quantum efficiency (QE) of the ²F_5/2→²F_7/2 emission exhibits a constant value above 50%. The QE drops for higher Tm²⁺ concentrations, partly due to concentration quenching, as evidenced by a decreasing luminescence lifetime of the ²F_5/2→²F_7/2 emission, and partly due to the formation of a second crystal phase with CdCl₂ structure, in which the ²F_5/2→²F_7/2 emission is quenched. The temperature and time dependent relaxation dynamics were studied to identify the origin of the limited QE for Tm²⁺-doping levels below 5 mol %. An anti-correlation between the 5d-4f (³H₆,t_2g)_S=3/2→²F_7/2 and 4f-4f ²F_5/2→²F_7/2 emission intensities was found and rationalised by non-radiative, thermally stimulated, inter-configurational 5d-4f relaxation to the emitting ²F_5/2 level of Tm²⁺. Both, the rise time of the 4f-4f and the decay time of the 5d-4f emission become shorter with increasing temperature. We suggest a similar non-radiative relaxation from the 5d level towards the ²F_7/2 ground state to limit the QE below unity. This route becomes more efficient when the 5d (³H₆,t_2g)_S=3/2 state moves closer to the 4f ²F_5/2 and ²F_7/2 states, which is the case for the CdCl₂ phase with a QE close to zero.