M_xLa_{1- x}SiO_2-yN_z(M = Ca/Sr/Ba): Elucidating and Tuning the Structure and Eu²⁺ Local Environments to Develop Full-Visible Spectrum Phosphors

The local environments of rare-earth activators have profound effects on the luminescent properties of phosphors. Here, we elucidate the crystal structure of the LaSiO₂N phosphor host using a combination of density functional theory calculations and synchrotron X-ray diffraction. We determine that LaSiO₂N crystallizes in the monoclinic C2/c instead of the hexagonal P6̅c2 space group. To improve the luminescence performance, divalent cations M (M = Ca/Sr/Ba) were introduced into LaSiO₂N to eliminate Eu³⁺. A family of apatite M_1+xLa_4-xSi₃O_13-x/2:Eu²⁺ (x ∼1.5, M = Ca/Sr/Ba) phosphors was further developed with unprecedented ultra-broadband (290 nm) emission spectra and excellent thermal stability. Detailed local environment investigations reveal that the formation of oxygen vacancies within and beyond the first-shell environment of Eu²⁺ is responsible for the redshift and broadening of the emission spectra via geometrical alteration of the Eu²⁺ local environment. This work provides new insights for understanding and optimizing the luminescence of rare-earth phosphors. © 2022 American Chemical Society.