New luminescent rare earth activated oxynitridosilicates and oxynitridogermanates with the apatite structure

Abstract

New luminescent oxynitrides based on the apatite structure doped with Eu²⁺ and Ce³⁺ have been prepared by ammonolysis of oxysilicate or oxygermanate precursors. The host compounds have the general formula La_10−xSr_x(Si/Ge)₆O_{27−x/2−3y/2}N_y. When doped with divalent europium or trivalent cerium they present blue, green, yellowish green or orange luminescence under UV/blue excitation. Remarkable long wavelength emissions centred beyond 500 nm are observed for the Ce³⁺ activated oxynitridosilicates and oxynitridogermanates, with compositions La_8.9Ce_0.1SrSi₆N_0.37O_25.95, La_8.9Ce_0.1SrSi₆N_1.06O_24.91 (λ_em = 545 nm) and La_9.9Ce_0.1Ge₆N_2.46O_23.31 (λ_em = 590 nm). The treatment of precursor oxides with the required cationic ratios in NH₃ gas was used as a convenient general method to prepare the oxynitride materials at moderate temperatures. This allowed decreasing the nitriding temperature from 1600–1400 °C to 1050–600 °C with respect to the method generally used for the synthesis of luminescent (oxy)nitride silicates, the solid state reaction between oxides, nitrides and carbonates under a N₂ or N₂–H₂ flow.