Photoluminescence and energy transfer behavior of narrow band red light emitting Li3Ba2Tb3(MoO4)8:Eu3+

Abstract

This work concerns the polycrystalline red emitting solid state compound Li₃Ba₂(Tb_1−xEu_x)₃(MoO₄)₈, from which a series of powder samples was prepared by a conventional solid-state reaction. The phase formation of the samples was investigated by X-ray diffraction which revealed the formation of a solid solution without a miscibility gap. Photoluminescence (PL) spectra and decay curves were recorded as a function of Tb³⁺ and Eu³⁺ concentration and temperature. It turned out that the external quantum efficiency of Eu³⁺ photoluminescence is between 30 and 80%, while the highest quantum yield is achieved for about 60% Tb³⁺. An increase of emission intensity can be realized by co-doping of Eu³⁺ and Tb³⁺. Moreover, the emission has a luminous efficacy of 275 lm W_opt⁻¹ which is a distinct advantage over the widely applied Mn⁴⁺ activated fluorides. The time dependent photoluminescence as a function of Tb³⁺ concentration demonstrates the presence of an efficient energy transfer from Tb³⁺ to Eu³⁺. Temperature dependent PL measurements revealed that Li₃Ba₂(Tb_1−xEu_x)₃(MoO₄)₈ loses just 20% of PL efficiency up to 400 K. Therefore, the investigated phosphors are attractive for application in pcLEDs. It is finally demonstrated that the application of a Eu³⁺/Tb³⁺ co-doped ceramic disc is useful for the colour conversion of a blue emitting LED with a higher conversion rate compared to Li₃Ba₂(La_1−xEu_x)₃(MoO₄)₈.