Luminescence properties of terbium-, cerium-, or europium- doped alpha-sialon materials

New interesting luminescent -sialon (M(m/val+)val+ Si12-(m+n) Al(m+n)O n N(16-n)) (M=Ca, Y) materials doped with Ce, Tb, or Eu have been prepared and their luminescence properties studied. These show that Tb and Ce are in the 3+ and Eu in the 2+ state. Low-energy 4f 5d transitions are observed as compared to the luminescence of these ions doped in oxidic host-lattices. This is partially explained by the nitrogen-rich coordination of the rare-earth ion and partially by the narrow size of the lattice site. The latter gives rise to a strong crystal-field splitting of the 5d band and a rather large Stokes shift for Ce3+ and Eu2+ (6500-7500 and 7000-8000& cm-1, respectively). For (Y,Tb)--sialon the Tb3+ 4f 5d excitation band (~260 nm) is in the low-energy host-lattice absorption band ( 290 nm), giving rise to a strong absorption for 254-nm excitation, but a low quantum efficiency. The latter is due to photoionization processes or selective excitation of Tb3+ at the defect-rich surface, resulting in radiationless transitions. Ce- and Eu-doped Ca--sialon show bright long-wavelength luminescence (maxima at 515-540 and 560-580 nm for Ce and Eu, respectively) with a high quantum efficiency and high absorption for 365- and 254-nm excitation. The Eu2+ emission intensity and absorption increases for increasing m, which is explained by the Eu2+ richer -sialon composition. The position of the Eu2 emission does not shift with changing composition of the host-lattice (m, n values), indicating that the local coordination of the Eu2+ ion is hardly dependent on the matrix composition. © 2002 Elsevier Science (USA)

Luminescence properties of red-emitting M2Si5N8:Eu2+ (M = Ca, Sr, Ba) LED conversion phosphors

The influence of the type of the alkaline-earth ion and the Eu2+ concentration on the luminescence properties of Eu2+-doped M2Si5N8 (M = Ca, Sr, Ba) has been investigated. XRD analysis shows that Eu2+-doped Ca2Si5N8 forms a limited solid solution with a maximum solubility of about 7 mol% having a monoclinic lattice. The Eu2+ ion can be totally incorporated into Sr2Si5N8 and Ba2Si5N8 forming complete solid solutions with orthorhombic lattices. M2Si5N8:Eu2+ (M = Ca, Sr) shows typical broad band emission in orange to red spectral range (600-680 nm) depending on the type of M and the europium concentration. Ba2Si5N8:Eu2+ shows yellow to red emission with maxima from 580 to 680 nm with increasing Eu2+ content. The long-wavelength excitation and emission is attributed to the effect of a high covalency and a large crystal-field splitting on the 5d band of Eu2+ in the nitrogen environment. With increasing the europium concentration, the emission band shows a red-shift for all M2Si5N8:Eu2+ compounds due to changing Stokes shift and the reabsorption by Eu2+. The conversion (i.e. quantum) efficiency increases going from Ca to Ba and Sr under excitation at 465 nm. In particular, Sr2Si5N8:Eu2+ with a quantum efficiency of 75-80% and a thermal quenching of only a few percent at 150 ¿C, demonstrates to be a highly promising red-emitting conversion phosphor for white-LED application