Energetic structure of Sm³⁺ luminescence centers in Sr₂TiO₄

A luminescent material based on the strontium orthotitanate (Sr₂TiO₄) matrix doped with 1% of a mole of samarium was obtained using the typical solid-state synthesis method under a neutral atmosphere of nitrogen. The sample was investigated using powder X-ray diffraction (XRD) and several luminescence techniques to study the phase composition, luminescence properties as well as to determine the position of the energetic states of Sm³⁺ in relation to the valence and conduction bands of Sr₂TiO₄. The XRD result shows that the product of the synthesis is pure Sr₂TiO₄. From the PL spectra, it can be seen that the phosphor can be effectively excited at 409 and 342 nm to emit significantly different emission spectra. The luminescence obtained under 409 nm excitation is typical of Sm³⁺ in Sr₂TiO₄ and attributed to the nonsymmetrical luminescent center (A-center). In contrast, the luminescence obtained under excitation at 342 nm originates from the symmetrical center (B-center) and has not been reported in the literature so far. The presence of the two emission centers related to Sm³⁺ ions in the Sr₂TiO₄ matrix characterized by only one crystallographic site of Sr²⁺ was explained by considering the different ways of charge compensation: Sm³⁺ in the A-center via strontium vacancy (V''_sr), and Sm³⁺ in the B-center via Ti³⁺ (Ti'_Ti).