Structural, electronic and optical properties of prominent M2Si5N8:Eu2+ phosphors (M = Mg, Ca, Sr, Ba) from the ground–state and excited–state first principles calculations

In this paper, the Heyd–Scuseria–Ernzerhof (HSE06) hybrid functional is employed to explore the structural, electronic, and optical properties of M₂Si₅N₈:Eu²⁺ phosphors (M = Mg, Ca, Sr, Ba) on the ground–state and excited–state levels. From the detailed study of electronic structures of pure M₂Si₅N₈, the top of the valence band is predominantly of N p character and the calculated band gaps are consistent with the reported experimental values. By Eu doping, the occupied 4f band is located directly in the band gap and results in the optical transition of the 4f–5d character. In order to confirm the location of the Eu 5d band below the conduction band, the excited–state calculation was executed. Using the position of the lowest excited–state 5d band below the conduction band, the calculated 4f–5d energy gaps agrees well with the experimental data. The excited–state calculation confirms that the Eu²⁺ d–f emission will be predominant in these phosphors. Furthermore, in order to understand and explain the excitation and emission observations, the energy level scheme was constructed from the calculated electronic structures. The findings in this article could well suggest a new Mg₂Si₅N₈:Eu²⁺ phosphor with a stable Eu²⁺ d–f emission and can also be applied to select another rare-earth dopant.