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 (S_r) of 1.06%/K (I_463 nm/I_608 nm) and 1.12%/K (I_548 nm/I_608 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.