Enhanced and tunable white luminescence in NaGdGeO₄: Bi³⁺, Dy³⁺ phosphors featuring anomalous thermal quenching

A novel series of NaGdGeO₄ phosphors co-doped with Bi³⁺ and Dy³⁺ ions were successfully synthesized via a conventional high-temperature solid-state reaction method. By systematically varying the Dy³⁺/Bi³⁺ doping concentrations, the emission color under 347 nm excitation could be effectively tuned from blue to cool white. Remarkably, these phosphors exhibit a distinct anomalous thermal quenching behavior. Specifically, for the NGGO: 0.01Bi³⁺, 0.05Dy³⁺ phosphor, the luminescence intensity at 577 nm measured at 573 K reached 163.2% of its initial value at 293 K, demonstrating exceptional thermal stability for high-temperature applications. The energy transfer process from Bi³⁺ to Dy³⁺ was comprehensively investigated through photoluminescence spectroscopy and decay lifetime measurements. This anomalous thermal quenching phenomenon is attributed to the synergistic effect of temperature-dependent modulation of the energy transfer efficiency from Bi³⁺ (³P₁ → ¹S₀) to Dy³⁺ (⁴F_9/2 → ⁶H_13/2) and the direct excitation of Dy³⁺ at 347 nm. This work highlights the significant potential of NaGdGeO₄: Bi³⁺, Dy³⁺ phosphors for application in white light-emitting diodes, particularly where thermal management is critical.