Discovery of a Quaternary Sulfide, Ba_2–xLiAlS₄:Eu²⁺, and Its Potential as a Fast-Decaying LED Phosphor

The inorganic crystal structure database (ICSD) lists no quaternary sulfide in its Ba–Li–Al–S compositional space. We discovered a novel quaternary sulfide, Ba2LiAlS4, by screening the Ba–Li–Al–S compositional space (BaS–Li2S–Al2S3 ternary space) with the assistance of a particle swarm optimization (PSO) algorithm. The PSO algorithm significantly alleviated the experimental burdens in searching the BaS–Li2S–Al2S3 ternary space. The structure of Ba2LiAlS4 (P21/m) was determined by direct methods and Rietveld refinement and finally verified via density functional theory (DFT) calculations. The Eu2+ activation gave rise to a green light emission for the Ba-deficient Ba2–xLiAlS4 host. The photoluminescence was activated by the enhanced band gap caused by the Ba desertion, and no interactivator energy transfer was confirmed since the direct quenching to anionic redox was active in the Ba-deficient Ba2–xLiAlS4 host, which had a positive impact on the fast decay.