Luminescence of ytterbium in CaS and SrS

CaS and SrS, in particular when doped with divalent europium and/or trivalent cerium, have been extensively studied as phosphor materials. In contrast, surprisingly little is known on the behavior of divalent Yb in both hosts. Therefore we report on the luminescence of divalent ytterbium in calcium sulfide and strontium sulfide. In CaS, an emission peak at 760 nm is found, while in SrS ytterbium emission peaks at a strongly red-shifted 950 nm are found. It is motivated that the former is typical for the common 5d-4f luminescence, while the latter is typical for anomalous divalent ytterbium luminescence. Depending on the excitation wavelength, trivalent ytterbium luminescence can also be observed in both hosts.