Promising oxonitridosilicate phosphor host Sr _3Si _2O _4N _2: Synthesis, structure, and luminescence properties activated by Eu ~(2+) and Ce ~(3+)/Li ~+ for pc-LEDs

摘要

A novel oxonitridosilicate phosphor host Sr _3Si _2O _4N _2 was synthesized in N _2/H _2 (6%) atmosphere by solid state reaction at high temperature using SrCO _3, SiO _2, and Si _3N _4 as starting materials. The crystal structure was determined by a Rietveld analysis on powder X-ray and neutron diffraction data. Sr _3Si _2O _4N _2 crystallizes in cubic symmetry with space group Pa3?, Z = 24, and cell parameter a = 15.6593(1) ?. The structure of Sr _3Si _2O _4N _2 is constructed by isolated and highly corrugated 12 rings which are composed of 12 vertex-sharing [SiO _2N _2] tetrahedra with bridging N and terminal O to form three-dimensional tunnels to accommodate the Sr ~(2+) ions. The calculated band structure shows that Sr _3Si _2O _4N _2 is an indirect semiconductor with a band gap ≈ 2.84 eV, which is close to the experimental value ≈ 2.71 eV from linear extrapolation of the diffuse reflection spectrum. Sr _(3-x)Si _2O _4N _2:xEu ~(2+) shows a typical emission band peaking at ～600 nm under 460 nm excitation, which perfectly matches the emission of blue InGaN light-emitting diodes. For Ce ~(3+)/Li ~+-codoped Sr _3Si _2O _4N _2, one excitation band is in the UV range (280-350 nm) and the other in the UV blue range (380-420 nm), which matches emission of near-UV light-emitting diodes. Emission of Sr _(3-2x)Si _2O _4N _2:xCe ~(3+),xLi ~+ shows a asymmetric broad band peaking at ～520 nm. The long-wavelength excitation and emission of Eu ~(2+) and Ce ~(3+)/Li ~+-doped Sr _3Si _2O _4N _2 make them attractive for applications in phosphor-converted white light-emitting diodes.