Abnormal improvement in emission of lanthanum oxysulfide phosphor La₂O₂S:Tb³⁺ synthesized by a novel method, thermal decomposition in eutectic molten salt

The lanthanum oxysulfide phosphor La₂O₂S:Tb³⁺ is generally synthesized by the direct sulfidation of La₂O₃ using H₂S or CS₂ gas. However, these gases are known to be highly toxic to humans. In this paper, a new synthesis method that uses 0.6Li₂SO₄-0.4Na₂SO₄ as the flux is proposed for obtaining La₂O₂S:Tb³⁺ phosphor, without the need for H₂S or CS₂ gas. The proposed method consists of two steps: 1) oxidation to obtain La₂O₂SO₄ and 2) reduction to reduce La₂O₂SO₄ to La₂O₂S. Using this novel method, (La_1-xTb_x)₂O₂S (0.01≤x≤0.13) were successfully synthesized in a single-phase form. The synthesized phosphors exhibited extraordinary luminescence behavior. Further, despite their low Tb³⁺ ion content, the phosphors showed high emission intensities. The relative emission intensity of (La_0.95Tb_0.05)₂O₂S (λ_ex=270 nm) was 132% that of the commercial green-emitting phosphor (La_0.52Ce_0.31Tb_0.17)PO₄ (λ_ex=285 nm), which shows the highest excitation absorption.