Alkali metal doping-induced luminescence enhancement and color tuning in Na_xLi_yK_zGd(MoO₄)₂

Na_xLi_yK_zGd(MoO₄)₂:Er³⁺/Yb³⁺ (x + y + z = 1) phosphors were synthesized via a sol-gel approach, with a focus on the synergistic effects of multi-alkali-metal co-doping (Li⁺, Na⁺, K⁺) on upconversion luminescence. Remarkably, the introduction of dual/triple alkali metal ions achieved a 3.92-fold luminescence enhancement compared to single-alkali-doped systems, surpassing the performance of rare-earth-only doping systems. Systematic structural characterization revealed that controlled variation of alkali metal ratios effectively modulates lattice parameters, establishing a direct correlation between lattice contraction and luminescence enhancement mechanisms. Additionally, Na_xLi_yK_zGd(MoO₄)₂:Tm³⁺/Yb³⁺ powders were also prepared, and the luminescence variation trend with alkali metal ratios differed from that observed in Er³⁺/Yb³⁺ systems. By co-doping with rare earth ions (Er³⁺, Yb³⁺, and Tm³⁺) and adjusting alkali metal ratios, a continuous linear color transition was achieved, with CIE coordinates distributed approximately along the line y = 9.32x-1.43 (R² = 0.94). Furthermore, two-dimensional optical encoding was realized by mapping emission color against rare-earth concentration and alkali ratios. This multi-alkali co-doping strategy overcomes the limitations of single-ion approaches, providing a generalizable route to design tunable luminescent materials for anti-counterfeiting, displays, and optical data storage.