4f→5d and anomalous emission in Yb²⁺ doped NaI, SrI₂ and LaI₃ powders prepared by rapid melting and quenching in vacuum

The temperature dependent luminescence properties of Yb2+ doped in NaI, SrI2 and LaI3 powders, prepared by a fast synthesis method involving the rapid melting and quenching in vacuum are reported. The 4f→5d excitation and emission spectra obtained for SrI2:Yb2+ were found to be in accordance with earlier work validating the synthesis methods. In both NaI and SrI2, doping with Yb2+ leads to 5d→4f emission, whereas in LaI3 no 5d→4f emission was found but instead anomalous emission is observed. In order to understand the observed differences, vacuum referred binding energy (VRBE) schemes were constructed for NaI and LaI3. For this purpose a NaI:Tm3+ sample was prepared, establishing the Tm3+ charge transfer energy at 3,5 eV. The VRBE energy scheme of NaI clearly reveals that the 5d states of Yb2+ and the other divalent lanthanides are well below the conduction band making 5d→4f emission possible. In LaI3 the 5d states are almost 1 eV inside the conduction band, which explains the absence of 5d→4f emission and the presence of the anomalous emission.