Designing Luminescent Materials and Band Gaps: A Soft X-ray Spectroscopy and Density Functional Theory Study of Li₂Ca₂[Mg₂Si₂N₆]:Eu²⁺ and Ba[Li₂(Al₂Si₂)N₆]:Eu²⁺

Abstract

A large band gap is a prerequisite for efficient emissions from a rare earth doped phosphor and is consequently a prerequisite for its application in high-quality lighting. We present a detailed characterization of luminescent materials Li2Ca2[Mg2Si2N6]:Eu2+ and Ba[Li2(Al2Si2)N6]:Eu2+ using soft X-ray spectroscopy and density functional theory calculations, including a rigorous experimental determination, and theory-based elucidation, of their band gaps. The band gap of Li2Ca2[Mg2Si2N6]:Eu2+ is determined to be 4.84 ± 0.20 eV, while that of Ba[Li2(Al2Si2)N6]:Eu2+ is 4.82 ± 0.20 eV. The origin of the band gaps is discussed in the context of the calculated DOS of each material and compared to benchmark luminescent materials Sr[LiAl3N4]:Eu2+ and Sr[Mg3SiN4]:Eu2+. Critically, the elements determining the band gaps are identified using the calculated density of states, as well as experimental resonant X-ray emission measurements. This allows for predictive power when searching for new nitridosilicates and related host structures, which upon doping with rare earth elements, may find application in the next-generation of phosphor converted light emitting diodes.