Photoluminescence properties of Pr³⁺, Sm³⁺ and Tb³⁺ doped SrAlSi₄N₇ and energy level locations of rare-earth ions in SrAlSi₄N₇

RE³⁺ (RE = Pr, Sm, and Tb)-doped SrAlSi₄N₇ samples were synthesized by a solid-state reaction method at high temperature, and their photoluminescence properties were investigated. It is noticeable that the 5d bands of Pr³⁺ and Tb³⁺ are at rather low energy in SrAlSi₄N₇ compared to oxides. Typical 4f² → 4f² emission lines (480-800 nm) of Pr³⁺ under 4f² → 4f¹5d¹ excitation were observed in Pr³⁺-doped SrAlSi₄N₇. Sm³⁺-doped SrAlSi₄N₇ shows 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 3.98 eV. The Tb³⁺-doped sample exhibits ⁵D₃ → ⁷F_J (J = 6, 5, 4, 3, 2, 1) (blue) and ⁵D₄ → ⁷F_J (J = 6, 5, 4, 3) (green) line emissions in the wavelength range of 375-650 nm under the direct Tb³⁺ 4f⁸ → 4f⁷5d¹ excitation. The bands at about 256 nm in the excitation spectra are attributed to the host lattice absorption. In addition, there is energy transfer from the host lattice to the luminescent activators (Pr³⁺, Sm³⁺, and Tb³⁺). The energy level diagram containing the position of the 4f and 5d levels of all divalent and trivalent lanthanide ions relative to the valence and conduction band of SrAlSi₄N₇ has been constructed and discussed.