Synthesis, crystal and electronic structure of Y2OSe2

Y₂OSe₂, a novel phosphor matrix, was first synthesized via sintering/melt crystallization of Y₂Se₃ and Y₂O₂Se precursors. Guided by LLM predictions (accurately forecasting synthesis at 800–1000 °C and incongruent melting ∼1450 °C), it crystallizes orthorhombically (Pnma, Gd₂OSe₂-type; a = 15.9748 (2), b = 3.89420 (5), c = 6.96804 (9) Å, V = 433.476 (10) ų, Z = 4), contrasting the LLM's initial monoclinic (P2₁/c) prediction. Particles are oval with layered granular morphology (microhardness 290 ± 8 HV). The phase is stable under standard conditions and melts incongruently at 1470 ± 8 °C (forming melt + Y₂O₂Se), validating LLM thermal forecasts. Polycrystalline Y₂OSe₂ coexists in equilibrium with Y₂Se₃ or Y₂O₂Se. Raman spectra analysis, combining experimental data and DFT calculations, was performed. It has been shown that in the crystal structure of Y₂OSe₂, ion vibrations exhibit collective behavior in the low-frequency region, while the intense peaks above 110 cm^–1 are mainly due to vibrations of specific types of ions. The simulation of the band structure and of the absorption spectrum reveals the origin of the onset of fundamental absorption, namely, the onset of direct transition at 1.76 eV into highly dispersive part of conduction band that contributes to the indirect bandgap, then the onset of more pronounced absorption above 3 eV at transitions to less dispersive part of conduction band. Four of five experimentally observed bands in the absorption spectrum are present in the simulated spectrum. Observed features are common to a variety of chalcogenides that were investigated in last two years.