Ce3+-Activated γ-Ca2SiO4 and Other Olivine-Type ABXO4 Phosphors for Solid-State Lighting

Yellow-emitting phosphors activated by Ce3+ are key components of white light-emitting diodes (LEDs) based on the blue (Ga,In)N LED. In these phosphors, the electronic environment around Ce3+ determines its photoluminescence wavelengths. Placing Ce3+ in octahedral sites in oxides can potentially lead to yellow or orange phosphors that can compete with the important commercial phosphor, Y3Al5O12:Ce. However, there are very few examples of such materials. In this article, we explore the photoluminescent behavior of Ce3+ in oxides with the olivine structure, whereby two distorted octahedral doping sites exist. We demonstrate that the promising new yellow phosphor γ-Ca2SiO4:Ce requires a second dopant, Al3+, to avoid the formation of the undesired β polymorph. Our results indicate that the yellow emission is indirectly caused by the complex polymorphism of Ca2SiO4, particularly the low temperature formation of the γ-phase. The dramatic shift in emission from yellow to light-blue when this phosphor is heated to 800 °C is explained in terms of the redistribution of Ce3+ in the lattice. A similar effect is observed when Si4+ is substituted by Ge4+. The light-blue emission of Ca2GeO4:Ce,Al is reported as well as the photoluminescent properties of the Ca2(Ge,Si)O4:Ce,Al solid solution.