Synthesis by ammonolysis and luminescence properties of cerium or europium-doped oxynitrides with the apatite structure
Rare-earth doped oxynitrides have attracted much attention as phosphors for white LEDs. A new family of Ce3+ or Eu2+-doped oxynitride phosphors with the apatite structure has been studied: La8+xSr2-x(Si/Ge)6NyO26+x/2-3/2y. The ammonolysis of an apatite oxide precursor has been used as a general method of synthesis, allowing decreasing the nitriding temperature respective to the classical solid state reaction in N2/H2 atmosphere. The luminescence properties of the obtained phosphors have been studied and relationships with the crystalline structure have been drawn.The apatite structure shows several crystallographic sites available for nitrogen as well as two cationic sites for optically active rare-earth ions. Several structural characterization technique have been used (IR, Raman, NMR, Neutron diffraction) and important information has been obtained concerning the nitrogen distribution in the available positions of the crystal structure. Comparison of the luminescent properties with those of compounds reduced under Ar/H2 allowed attributing emissions to the different crystallographic sites available in the structure.Quantum efficiency measurements have been carried out as well as trials for improving the intensity of the luminescent properties.