High-Quality Single-Source White-Light Emission in Double Perovskite Cs2NaGdCl6: Sb3+/Tb3+/Mn2+ for Ratiometric Optical Thermometry and Anti-Counterfeiting Applications

In lead-free halide double perovskites, doping and alloying are key strategies for optical performance enhancement. However, achieving stable full-spectrum white emission in single-component phosphors remains challenging. Herein, high-quality single-phase white emission is realized in Cs2NaGdCl6 via Sb3+/Tb3+/Mn2+ codoping. Pristine Cs2NaGdCl6 shows a weak sky-blue emission, while Sb3+ doping significantly enhances the self-trapped exciton (STE) intensity. Density functional theory calculations show that Sb3+ doping creates new sensitizing states, boosting the STE emission. Under Sb3+ sensitization, STE energy transfers to Tb3+ and Mn2+. The incorporation of Tb3+ elevates the photoluminescence quantum yield from 0.66% (pristine) to 68.38% (CNGC: Sb3+, Tb3+), while the broadband red emission from Mn2+ plays a critical role in achieving full-spectrum coverage. Tunable white-light emission is achieved via a dopant ratio optimization. Combining the phosphor with a 340 nm commercial light-emitting diode chip produces a white light-emitting diode with a correlated color temperature of 4658 K and a color rendering index of 91.9. Temperature-dependent photoluminescence spectra confirm ratiometric sensing capability, yielding maximum relative sensitivities (Sr) of 1.06%/K (I463 nm/I608 nm) and 1.12%/K (I548 nm/I608 nm) at 298 K. These results demonstrate that this material exhibits multifunctional application potential in the fields of white-light illumination, optical thermometry, and fluorescent anticounterfeiting.