³P₀ → ¹D₂ non-radiative relaxation control via IVCT state in Pr³⁺-doped Na₂Ln₂Ti₃O₁₀ (Ln=La, Gd) micro-crystals with triple-layered perovskite structure

A comparative study on luminescence properties and their nature in relation to different substitutions at the rare-earth site of Pr³⁺-doped layered perovskite Na₂Ln₂Ti₃O₁₀ (Ln = La, Gd) micro-crystals has been reported. At room temperature, the sample for Ln = La exhibits intense greenish-blue (³P₀ → ³H₄) and red (¹D₂ → ³H₄) emissions under bandgap excitation, whereas for Ln = Gd the red emission can be only observed and all ³P₀-related emissions were completely quenched. It turns out that ³P₀→ ¹D₂ nonradiative relaxation in Na₂Ln₂Ti₃O₁₀ (Ln = La, Gd) critically depends on the energy location of Pr³⁺-Ti⁴⁺ intervalence charge transfer state and thus on the location of Pr³⁺ ground state ³H₄ with respect to the top of the valence band. Temperature-dependent photoluminescence spectra in the 4.5–300 K range reveals a significant increase of Pr³⁺ luminescence, which is ascribed to an efficient thermally-activated energy transfer process from host to Pr³⁺ ions. Lower energy of self-trapped exciton state relative to the bottom of conduction band for Ln = Gd is responsible for thermoluminescence observed at higher temperatures than that for Ln = La sample. In the case of Ruddlesden-Popper type layered perovskite oxide compounds our results show a possibility of controlling the ³P₀ → ¹D₂ nonradiative relaxation of Pr³⁺ ions and the energy distance of the relatively shallow traps in relation to the bottom of conduction band, giving a way for specific band-gap engineering in these materials.