Energy Transfer between Tb ³⁺ and Eu ³⁺ in LaPO ₄: Pulsed versus Switched-off Continuous Wave Excitation

The energy transfer (ET) between Tb ³⁺ and Eu ³⁺ is investigated experimentally and with available theoretical models in the regime of high Tb ³⁺ concentrations in ≈30 nm LaPO ₄ nanoparticles at room temperature. The ET efficiency approaches 100% even for lightly Eu ³⁺ -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 Tb ³⁺ ions, situated on La ³⁺ sites with ≈4 Å separation, is not fast. The quenching of Tb ³⁺ emission in singly doped LaPO ₄ only reduces the luminescence lifetime by ≈50% in heavily doped samples. Various theoretical models are applied to simulate the luminescence decays of Tb ³⁺ and Tb ³⁺ , Eu ³⁺ -doped LaPO ₄ samples of various concentrations and the transfer mechanism is identified as forced electric dipole at each ion.