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 Eu 2+ centers. From the viewpoint of the large Stokes shift, the large emission bandwidth, and the temperature quenching profile, the Eu 2+ 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 Eu 2+ 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 Eu 2+ relative to the conduction band of BaS, which leads to autoionization of the Eu 2+ 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.

The photoluminescence of Ca(2)SiS(4):Eu powders was investigated in detail as a function of europium concentration (from 0.1% Ca substitution to the fully substituted Eu(2)SiS(4)). At low europium dopant concentration (<10%) the powders crystallize in an orthorhombic structure and the emission spectrum is dominated by two broad emission bands, at 564 and 660 nm. The emission can be tuned from yellow (CIE x = 0.46,y = 0.53) to red (CIE x = 0.65,y = 0.35) by variation of the Eu concentration. An energetic coupling exists between both bands, leading to a broad excitation wavelength range. Powders with high europium concentration (>40%) crystallize in a monoclinic structure, details of which were determined by Rietveld refinement of x-ray diffraction data. For the composition CaEuSiS(4) (i.e. 50% substitution), the luminescence peaks at 614 nm, shifting to shorter wavelengths upon further substitution of Ca by Eu. Although considerable thermal quenching is present at room temperature in the fully Eu-substituted compound, Eu(2)SiS(4) is still photoluminescent, with a peak emission wavelength of 577 nm. A strong correlation is found between the crystallographic and luminescent properties of the (Ca,Eu)(2)SiS(4) powders. The broad emission and excitation bands make this phosphor a good candidate for use in phosphor-converted light-emitting diodes (pcLEDs).

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