Temperature dependent relaxation dynamics of luminescent NaX:Tm²⁺ (X=Cl, Br, I)

In recent years, Thulium in its 2 + oxidation state has been identified as candidate dopant in halide hosts for luminescent solar concentrators. Yet, some of its luminescent properties with regard to these applications remain unexplored. In this study we report on the temperature dependent photo-luminescent behaviour of NaCl:Tm²⁺, NaBr:Tm²⁺, and NaI:Tm²⁺. These monohalide materials demonstrate up to five distinct emission peaks which can be attributed to the 4f¹²→4f¹² and 4f¹¹5d¹→4f¹² transitions of Tm²⁺. Their time- and temperature dependent luminescence intensity behaviours are explained by a qualitative model describing the thermally stimulated radiative- and non-radiative relaxation dynamics. The behaviour of Tm²⁺ in these monohalides proves to be similar to earlier reported findings on Tm²⁺-doped trihalide perovskites of the form CsCaX₃ (X = Cl, Br, I), however, the 4f-4f emission is by far the most dominant emission between 10 and 300 K.