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

Abstract A comparative study on luminescence properties and their nature in relation to different substitutions at the rare-earth site of Pr 3+ -doped layered perovskite Na 2 Ln 2 Ti 3 O 10 (Ln = La, Gd) micro-crystals has been reported. At room temperature, the sample for Ln = La exhibits intense greenish-blue ( 3 P 0 → 3 H 4 ) and red ( 1 D 2 → 3 H 4 ) emissions under bandgap excitation, whereas for Ln = Gd the red emission can be only observed and all 3 P 0 -related emissions were completely quenched. It turns out that 3 P 0 → 1 D 2 nonradiative relaxation in Na 2 Ln 2 Ti 3 O 10 (Ln = La, Gd) critically depends on the energy location of Pr 3+ -Ti 4+ intervalence charge transfer state and thus on the location of Pr 3+ ground state 3 H 4 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 Pr 3+ luminescence, which is ascribed to an efficient thermally-activated energy transfer process from host to Pr 3+ 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 3 P 0 → 1 D 2 nonradiative relaxation of Pr 3+ 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.