Anomalous photoluminescence inBaS:Eu

The photoluminescence emission properties of $\mathrm{BaS}:\mathrm{Eu}$ 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 $878\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and is related to ${\mathrm{Eu}}^{2+}$ centers. From the viewpoint of the large Stokes shift, the large emission bandwidth, and the temperature quenching profile, the ${\mathrm{Eu}}^{2+}$ emission in $\mathrm{BaS}:\mathrm{Eu}$ is totally different than what can be expected from the analogy with the similar materials $\mathrm{CaS}:\mathrm{Eu}$ and $\mathrm{SrS}:\mathrm{Eu}$. Hence the emission in $\mathrm{BaS}:\mathrm{Eu}$ is anomalous. Photoluminescence excitation and electron paramagnetic resonance spectra show that the incorporation of ${\mathrm{Eu}}^{2+}$ in $\mathrm{BaS}$ is not radically different from $\mathrm{SrS}:\mathrm{Eu}$ or $\mathrm{CaS}:\mathrm{Eu}$. The appearance of infrared emission is related to the position of the $5d$ excited level of ${\mathrm{Eu}}^{2+}$ relative to the conduction band of $\mathrm{BaS}$, which leads to autoionization of the ${\mathrm{Eu}}^{2+}$ centers upon $4{f}^{7}\text{\ensuremath{-}}4{f}^{6}5d$ 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 $0.51\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ was found. The thermal quenching behavior was evaluated as well.