Structure and luminescence of Ca 2 Si 5 N 8 :Eu 2+ phosphor for warm white light-emitting diodes

We have synthesized Ca 2 Si 5 N 8 :Eu 2+ phosphor through a solid-state reaction and investigated its structural and luminescent properties. Our Rietveld refinement of the crystal structure of Ca 1.9 Eu 0.1 Si 5 N 8 reveals that Eu atoms substituting for Ca atoms occupy two crystallographic positions. Between 10 K and 300 K, Ca 2 Si 5 N 8 :Eu 2+ phosphor shows a broad red emission band centred at ∼1.97 eV–2.01 eV. The gravity centre of the excitation band is located at 3.0 eV–3.31 eV. The centroid shift of the 5 d levels of Eu 2+ is determined to be ∼1.17 eV, and the red-shift of the lowest absorption band to be ∼0.54 eV due to the crystal field splitting. We have analysed the temperature dependence of PL by using a configuration coordinate model. The Huang–Rhys parameter S = 6.0, the phonon energy hv = 52 meV, and the Stokes shift Δ S = 0.57 eV are obtained. The emission intensity maximum occurring at ∼200 K can be explained by a trapping effect. Both photoluminescence (PL) emission intensity and decay time decrease with temperature increasing beyond 200 K due to the non-radiative process.