Yb/Er Co‐Doped Double Perovskite Titanate Phosphors: Harnessing Upconversion for Advanced Optical Thermometry and NIR‐Triggered Green LEDs

To meet the demand for upconversion materials for non‐contact temperature sensing and optoelectronic devices, the development of durable and highly responsive phosphors is essential. Herein, we report a new strontium (Sr)‐rich double perovskite titanate, Na 0.5 La 0.5 SrTi 2 O 6 , co‐doped with Yb 3+ /Er 3+ to demonstrate optical thermometers based on thermally coupled ( 2 H 11/2 / 4 S 3/2 ) and non‐thermally coupled ( 2 H 11/2 / 4 F 9/2 ) energy levels. Under a 980 nm excitation, the optimized Na 0.5 La 0.41 SrTi 2 O 6 :7%Yb 3+ , 2%Er 3+ (NLSTO:7%Yb 3+ , 2%Er 3+ ) phosphors showed outstanding temperature sensing performance at 296 K, achieving record sensitivities of 0.48% absolute sensitivity (S a ) and 1.28% relative sensitivity (S r ) for thermally coupled levels (TCLs), together with 1.79% S a and 1.06% S r for non‐thermally coupled levels (NTCLs) and a repeatability deviation of ±0.24°C at ambient temperature. The phosphors also show remarkable chemical stability, retaining 98%, 96.9%, and 95.6% of their upconversion intensities after 30 days of immersion in acidic and aqueous media containing nitric acid, hydrochloric acid, and water, respectively. Moreover, NLSTO:7%Yb 3+ , 2%Er 3+ was successfully integrated onto a 980 nm LED chip using a simple PDMS‐assisted coating, yielding uniform and intense green emission. These findings highlight that Sr‐rich double perovskite titanates can serve as reliable materials for advanced optical thermometry and durable upconversion‐based devices in challenging chemical and thermal environments.