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-5d(1) 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(Sr/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--Eu3+/2+ and O2--Ce4+/3+ charge transfer as well as bandgap were estimated and the electronic structure of Ce3+ were constructed. A larger energy barrier E-dC between the Ce3+ 5d(1) 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.