Effects of bandgap and defects on the photoluminescence of Li6La3MZrO12:Tb3 + (M = Ta, Nb) phosphors

Garnet-type Li 6 (La 1- x Tb x ) 3 MZrO 12 (M = Ta, Nb; x  = 0.02–0.06) phosphors were synthesized through solid-state reaction method. XPS and UV-vis spectroscopy analyses revealed that the Nb or Ta component exerted a profound influence on the bandgap energies and Nb/Ta valence states in the host lattice, with which the systematically varying excitation and emission behaviors of the two systems were comprehensively deciphered. It was found that the higher Nb Nb ′ and V O · · defects density and the narrower bandgap in Li 6 (La 1- x Tb x ) 3 NbZrO 12 were detrimental to the quantum efficiency and thermal stability of Tb 3+ luminescence due to the increased process of nonradiative recombination at Nb Nb ′ / V O · · defects and thermal ionization in conduction band. The Li 6 (La 0.95 Tb 0.05 ) 3 TaZrO 12 optimal phosphor showed quantum yields of ∼41 % under 257 nm excitation, a fluorescence lifetime of ∼1.15 ms, color coordinates of around (0.297, 0.586), and can retain as high as ∼79 % of its room-temperature emission intensity at 423 K. The investigation of the impacts of bandgap energies and defects on the luminescence properties provides insights into the fundamental material parameters of Li-stuffed garnet phosphors and offers a pathway for enhancing their luminescence properties.