Luminescence properties of La2W2−xMoxO9 (x = 0–2):Eu3+ materials and their Judd–Ofelt analysis: novel red line emitting phosphors for pcLEDs

Abstract

A series of novel red emitting La_1.95Eu_0.05W_2−xMo_xO₉ (x = 0–2, in steps of 0.1) and La_2−yEu_yW_1.6Mo_0.4O₉ (y = 0–2, insteps of 0.2) phosphors were prepared by a conventional solid state reaction. Powder XRD analysis of La_1.95Eu_0.05W_2−xMo_xO₉ (x = 0–2, in steps of 0.1) reveals a phase transition from a triclinic to a cubic structure with increasing Mo⁶⁺ concentration (x ≥ 0.2). All the compositions show a broad charge transfer (CT) band due to CT from O to W/Mo and red emission (due to Eu³⁺ ions). In order to obtain the strongest red emission, concentration variation of Eu³⁺ was performed in La_2−yEu_yW_1.6Mo_0.4O₉ (x = 0–2, insteps of 0.2). The Eu_1.6La_0.4W_1.6Mo_0.4O₉ composition shows a high red emission intensity (∼8.2 times) compared to a commercial red phosphor with very good CIE chromaticity coordinates (x = 0.67, y = 0.33). A selected composition has also been synthesized by simple co-precipitation method. The emission intensity of the Eu_1.6La_0.4W_1.6Mo_0.4O₉ phosphor synthesized by co-precipitation technique is ∼1.78 times higher than that of the phosphor synthesized by the solid state reaction. Phonon sideband spectrum analysis has been carried out for Eu³⁺ activated La₂M₂O₉ (M = W and Mo). The temperature dependent photoluminescence studies reveal that the Eu_1.4La_0.6W_1.6Mo_0.4O₉ compositions loses 30% of its efficiency up to 400 K (compared to ∼80% for CaS:Eu²⁺) and the Judd–Ofelt parameters were also calculated for the Eu³⁺ ions.