The optical properties of BaAl 2 S 4 doped with Eu 2+ were investigated. The synthesis was performed in evacuated and sealed quartz ampul at 1000°C during 1 h, then at 840°C during 20 h. With absorption, excitation, and emission spectra we have determined the crystal-field splitting, the redshift, the influence of the lattice vibration properties, and the thermal quenching of the luminescence. The crystal-field splitting of Eu 2+ in BaAl 2 S 4 was estimated on the order of 8000 to 10 000 cm −1 and the Huang-Rhys parameter S = 5 ± 1 corresponds to an intermediate electron-phonon coupling regime. The mean phonon energy for BaAl 2 S 4 was evaluated by optical and Raman methods, and the obtained values were 32 ± 5 and 40 meV, respectively. These results confirm the very interesting properties of this blue BaAl 2 S 4 :Eu 2+ phosphor, the most important being the very slight thermal quenching of the luminescence.Inorganic thin-film electroluminescent ͑iEL͒ devices are expected to be very interesting candidates for full-color flat display panels. The lack of a blue phosphor with suitable color coordinates, high enough luminance and efficiency was one of the main difficulties to commercialize full color devices. These last years, binary and ternary sulfide phosphors have been investigated, like SrS:Ce, 1 ͑Ca,Sr͒Ga 2 S 4 :Ce, 2 SrS:Cu, 3 SrS:Cu,Ag, 4 and CaS:Pb. 5 Unfortunately, not one of these phosphors provides suitable performances for full-color display application.A breakthrough was made by Miura et al. in 1999, who have reported on a new thin-film blue EL phosphor, BaAl 2 S 4 doped with Eu 2+ , which gives a higher brightness of 65 cd/m 2 at 50 Hz and good color coordinates, x = 0.12 and y = 0.10. 6 Greater brightness than 700 cd/m 2 at 120 Hz has been obtained using the thick dielectric technology at iFire Company. 7 This performance allows iFire Company to announce a full-color 34-in. iEL HDTV screen for the 2006 timeframe using the Color-By-Blue ͑CBB͒ technique. 8 The first report concerning this BaAl 2 S 4 :Eu 2+ phosphor was presented by Donohue and Hanlon. 9 The cubic structure with a lattice parameter of 1.2588 nm was established. At 300 K, a blue band emission peaked at 475 nm was obtained with a decay time on the order of 0.45 s. In this pioneering paper, it was pointed that this phosphor will be efficient due to a wide bandgap leading to a low interaction between the excited states of Eu 2+ and fundamental edge. This cubic structure was summarized by Eisenmann et al. 10 and some luminescence properties were presented by Le Thi et al. 11 The radiative mechanisms of this BaAl 2 S 4 :Eu 2+ phosphor are not completely understood and their fundamental knowledge needs to be enlarged. In this paper diffuse reflectance, excitation, and emission spectra, and photoluminescence ͑PL͒ decay curves of powder samples were analyzed and the influence of temperature on the luminescence process was investigated. The results are discussed in regards to the results recently presented for BaGa 2 S 4 :Eu 2+ phosphor, 12 which h...