The Influence of Defects on the Luminescence of Trivalent Terbium in Nanocrystalline Yttrium Orthovanadate

Terbium-doped YVO₄ has been considered a nonluminescent solid since the first classic studies on rare-earth-doped phosphors in the 1960s. However, we demonstrate that defect engineering of YVO₄:Tb³⁺ nanoparticles overcomes the metal-metal charge transfer (MMCT) process which is responsible for the quenching of the Tb³⁺ luminescence. Tetragonal (Y_1-Tb)VO₄ nanoparticles obtained by colloidal precipitation showed expanded unit cells, high defect densities, and intimately mixed carbonates and hydroxides, which contribute to a shift of the MMCT states to higher energies. Consequently, we demonstrate unambiguously for the first time that Tb³⁺ luminescence can be excited by VO₄^3- → Tb³⁺ energy transfer and by direct population of the ⁵D₄ state in YVO₄. We also discuss how thermal treatment removes these effects and shifts the quenching MMCT state to lower energies, thus highlighting the major consequences of defect density and microstructure in nanosized phosphors. Therefore, our findings ultimately show nanostructured YVO₄:Tb³⁺ can be reclassified as a UV-excitable luminescent material.