Photoluminescence and tunable emissions of La2(GeO4)O:Bi3+, Eu3+ phosphor for ultraviolet converted light emitting diodes

Highlights: • La₂(GeO₄)O: Bi³⁺/Eu³⁺ phosphors were synthesized by a solid-state method. • Bi³⁺ doped La₂(GeO₄)O presented a high QE and excellent thermal stability. • A white-emitting phosphor La₂(GeO₄)O:0.03Bi³⁺, 0.03Eu³⁺ was obtained. • La₂(GeO₄)O: Bi³⁺/Eu³⁺ exhibited energy transfer and small thermal quenching. In the past decades, a great variety of single-phase white-emitting phosphors have been developed, however most them can't satisfy us due to their low quantum efficiency and poor thermal stability. Herein, we reported a promising candidate of a white-emitting La₂(GeO₄)O:Bi³⁺/Eu³⁺ phosphor, exhibiting an efficient energy transfer, good thermal stability and high quantum efficiency. Rietveld structure refinement of La₂(GeO₄)O:Bi³⁺ was performed to confirm Bi³⁺ occupation of La sites in La₂(GeO₄)O. An analysis on the red-shift of both excitation and emission spectra of La₂(GeO₄)O:Bi³⁺ was made by combining the crystal structure and their spectroscopy. Thermal quenching temperature (T_50%) of La₂(GeO₄)O:Bi³⁺ was approximately 532 K which is higher than that (473 K) of the LED chips under high power current driving and La₂(GeO₄)O:Bi³⁺ presented a spectral blue-shift as an increase in temperature. Furthermore, thermal quenching of La₂(GeO₄)O:Bi³⁺ was explained in detail by the configurational coordinate. More importantly, single Bi³⁺ doped La₂(GeO₄)O presented a blue-greenish broad band at 480 nm with a high quantum efficiency (48%) and excellent thermal stability (ΔE = 0.353 eV). Energy transfer from Bi³⁺ to Eu³⁺ in La₂(GeO₄)O efficiently occurred and energy transfer mechanism of Bi³⁺→Eu³⁺ was demonstrated to be dipole-quadrupole interaction. By codoping Eu³⁺ in La₂(GeO₄)O:Bi³⁺, La₂(GeO₄)O:Bi³⁺, Eu³⁺ phosphor exhibited the blue-green and red dual emissions. Fortunately, a single-phase white-emitting phosphor La_1.94(GeO₄)O:0.03Bi³⁺, 0.03Eu³⁺ was obtained with excellent resistance against thermal quenching and a quantum efficiency of 37%.