Photoluminescence properties and energy level locations of RE3+ (RE = Pr, Sm, Tb, Tb/Ce) in CaAlSiN3 phosphors

Abstract

RE³⁺ (RE = Pr, Sm, Tb, Tb/Ce)-activated CaAlSiN₃ samples were prepared by a solid-state reaction method at high temperature, and their photoluminescence properties were investigated. An interesting observation is that the 5d bands of Pr³⁺ and Tb³⁺ are at rather low energy in CaAlSiN₃ compared to oxides. Pr³⁺-doped CaAlSiN₃ shows strong 4f¹5d¹ → 4f² emission bands (334–480 nm) as well as typical 4f² → 4f² emission lines (480–800 nm) of Pr³⁺ under 4f² → 4f¹5d¹ excitation. Sm³⁺-doped CaAlSiN₃ exhibits bright red emission originating from ⁴G_5/2 → ⁶H_J (J = 5/2, 7/2 and 9/2) transitions, and the charge transfer band of Sm³⁺ was observed at an unusually low energy of 3.91 eV. In Tb³⁺-doped samples, the direct Tb³⁺ 4f⁸ → 4f⁷5d¹ excitation leads to ⁵D₃ → ⁷F_J (J = 6, 5, 4, 3, 2, 1) (blue) and ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) (green) line emissions. The bands at about 252 nm in the excitation spectra are attributed to the host lattice absorption. It is observed that there is energy transfer from the host lattice to the luminescent activators (Pr³⁺, Sm³⁺, Tb³⁺). In Tb³⁺/Ce³⁺-codoped CaAlSiN₃, an effective energy transfer process exists from Tb³⁺ to Ce³⁺. A detailed energy level diagram that contains the position of the 4f and 5d levels of all divalent and trivalent lanthanide ions with respect to the valence and conduction bands of CaAlSiN₃ has been constructed and explained.