Local coordination, electronic structure, and thermal quenching of Ce³⁺ in isostructural Sr₂GdAlO₅ and Sr₃AlO₄F phosphors

Sr₂GdAlO₅:Ce and Sr₃AlO₄F:Ce are isostructural phosphors in which the Ce³⁺ 4f-5d₁ transition can be efficiently excited by a photon with energy lower than 3.1 eV. Herein, we analyze the crystal chemistry of the Ce³⁺ local coordination, compare the thermal quenching behavior and construct the electronic structure of Ce³⁺ in them. The Rietveld refinement on two occupancy models suggests that Gd³⁺ only occupies the 8h site in Sr₂GdAlO₅; this provides a hint on the preferred occupancy of dopant Ce³⁺ in this site. The large crystal filed splitting of Ce_8h is mainly due to the fact that the 8h site is bonded to two oxygen with relatively short d_{S r/Gd-O} and forms a quasi-square antiprism which experiences a large distortion. The Ce³⁺ 5d-4f luminescence in Sr₃AlO₄F is much more stable against thermal quenching than that in Sr₂GdAlO₅, as evidenced by the temperature-dependent luminescence intensity and luminescence decay studies. The energy of the O²⁻-Eu^3+/2+ and O²⁻-Ce^4+/3+ charge transfer as well as bandgap were estimated and the electronic structure of Ce³⁺ were constructed. A larger energy barrier ΔE_dC between the Ce³⁺ 5d₁ level and the conduction band bottom in Sr₃AlO₄F is seen from the Vacuum Referred Binding Energy (VRBE) diagrams which explains the higher thermal quenching temperature by thermal ionization model.