Discovering a Novel Cyan‐Light‐Emitting Thiosilicate Phosphor, Li ₂ BaSiS ₄ :Eu ²⁺ , via Experimental Active Learning

Abstract There are likely numerous known and yet‐to‐be‐discovered alkaline earth (AE)‐alkali metal (A) thiosilicates, thioaluminates, and thiophosphates that can serve as suitable hosts for the Eu ²⁺ 5d‐4f emission. These structures offer an infinite number of possible compositions, with the potential for multi‐elemental occupancy at the AE and A sites. Screening such an extensive exploratory space through conventional experimentation poses a significant challenge due to the so‐called combinatorial explosion issue. To address this challenge, an artificial intelligence, more specifically a particle swarm optimization (PSO) algorithm within an experimental active learning framework, that enables to explore the effectively infinite search space and ultimately identify a novel, promising host structure candidate for Eu ² ⁺ 5d‐4f emission is employed. Starting with completely random compositions and no preset structures in the vast exploratory space, the PSO‐driven experimental active learning process converges on a single‐phase Li ₂ BaSiS ₄ :Eu ²⁺ phosphor, featuring a tetragonal structure with I‐42 m symmetry ( a = 6.5825 Å, c = 8.0015 Å). The Li ₂ BaSiS ₄ :Eu ²⁺ phosphor exhibits cyan‐blue light emission with a peak at 480 nm, a full width at half maximum of 35 nm (1423 cm ⁻¹ ), and a broad excitation range from 325 to 425 nm, indicating its potential for application in light‐emitting diodes (LEDs).