Highly Efficient pc-LED Phosphors Sr1-xBaxSi2O2N2:Eu2+ (0 x 1)-Crystal Structures and Luminescence Properties Revisited

Due to their excellent luminescence properties in the blue-green to green-yellow spectral region, oxonitridosilicates Sr1-xBaxSi2O2N2:Eu2+ (0 x 1) are promising conversion materials for application in phosphor-converted high-power LED devices. In order to understand the properties and thus to fully exploit the potential of these materials, detailed knowledge of corresponding (local) crystal structures is indispensable. Detailed insights into real structures have been achieved by combining X-ray diffraction and electron-microscopy methods. A major reason for the excellent luminescence properties of the phases Sr1-xBaxSi2O2N2:Eu2+ (0 x 1) is the rigid silicate substructure built up of two-dimensionally condensed SiON3 tetrahedra. The general topology of these layers is analogous for all members. However, there is no complete solid-solution series. Crystal-structure determination was frequently not straightforward because several real-structure effects had to be considered. The relative orientation of the silicate layers and the metal-atom layers inserted between them can differ without changing the chemical composition. As a consequence, polytypes are formed. The differentiation between such closely related structures was only possible by a thorough analysis of crystallographic data. The same applies for phases which differ in their composition as all Sr1-xBaxSi2O2N2:Eu2+ (0 x 1) phases are very similar. The literature on these compounds is critically discussed with respect to phase analysis and structure determination. Different synthesis routes are reviewed and the results of luminescence investigations are discussed in this contribution. Beyond thermal as well as chemical stability and high transparency, electron-phonon coupling is effectively suppressed in Sr1-xBaxSi2O2N2:Eu2+ (0 x 1) phases. Therefore, primary UV to blue light (GaN based semiconductor LEDs) is efficiently converted into visible components of the spectrum. Sr1-xBaxSi2O2N2:Eu2+ (0 x 1) phases are therefore promising oxonitridosilicate phosphors for application in LED industry.