Anomalous photoluminescence in BaS: Eu

The photoluminescence emission properties of BaS:Eu powders have been studied. At room temperature, several weak emission bands related to impurities or intrinsic defects are observed in the visible part of the spectrum. The strongest emission band is situated mainly in the infrared with a peak emission wavelength of 878 nm and is related to Eu2+ centers. From the viewpoint of the large Stokes shift, the large emission bandwidth, and the temperature quenching profile, the Eu2+ emission in BaS:Eu is totally different than what can be expected from the analogy with the similar materials CaS:Eu and SrS:Eu. Hence the emission in BaS:Eu is anomalous. Photoluminescence excitation and electron paramagnetic resonance spectra show that the incorporation of Eu2+ in BaS is not radically different from SrS:Eu or CaS:Eu. The appearance of infrared emission is related to the position of the 5d excited level of Eu2+ relative to the conduction band of BaS, which leads to autoionization of the Eu2+ centers upon 4f(7)-4f(6)5d excitation and the formation of impurity trapped excitons. The influence of trap levels was studied by thermoluminescence and a major trap with an activation energy of 0.51 eV was found. The thermal quenching behavior was evaluated as well.