Energy Transfer between Tb³⁺ and Eu³⁺ in LaPO₄: Pulsed versus Switched-off Continuous Wave Excitation

The energy transfer (ET) between Tb³⁺ and Eu³⁺ is investigated experimentally and with available theoretical models in the regime of high Tb³⁺ concentrations in ≈30 nm LaPO₄ nanoparticles at room temperature. The ET efficiency approaches 100% even for lightly Eu³⁺-doped materials. The major conclusion from the use of pulsed laser excitation and switched-off continuous wave laser diode excitation is that the energy migration between Tb³⁺ ions, situated on La³⁺ sites with ≈4 Å separation, is not fast. The quenching of Tb³⁺ emission in singly doped LaPO₄ only reduces the luminescence lifetime by ≈50% in heavily doped samples. Various theoretical models are applied to simulate the luminescence decays of Tb³⁺ and Tb³⁺, Eu³⁺-doped LaPO₄ samples of various concentrations and the transfer mechanism is identified as forced electric dipole at each ion.