The dual-model up/down-conversion green luminescence of NaSrGd(MoO 4 ) 3 : Er 3+ and its application for temperature sensing.

Er 3+ -doped phosphors are widely used as dual-functional optical thermometers due to their distinctive up/down-conversion luminescence and the thermally coupled energy states ( 2 H 11/2 and 4 S 3/2 ) of Er 3+ . The development of high-performance Er 3+ -activated optical thermometers is both an intriguing subject and a formidable challenge in the field. This article investigates the up/down-conversion (UC and DC) photoluminescence properties of NaSrGd(MoO 4 ) 3 (NSGM): Er 3+ . When excited at 375 and 975 nm, the phosphors emit peaks at 530, 550, and 657 nm, corresponding to the 2 H 11/2 , 4 S 3/2 , and 4 F 9/2 → 4 I 15/2 transitions of Er 3+ , with the 4 S 3/2 → 4 I 15/2 transition displaying the highest intensity. The optical properties are comprehensively studied through UV-visible absorption, PL spectroscopy, and PLE spectroscopy. Optimal luminescence intensity is achieved at an Er 3+ concentration of 4% mol. The resulting chromatic coordinates ( x , y ) and high correlated color temperature (CCT) values of the doped phosphors yield thermally stable cold emissions in the green region, boasting color purities of approximately 98.76% and 80.74% for DC and UC conversion, respectively. The optical temperature sensing properties of thermally coupled energetic states are explored based on the fluorescence intensity ratio principle. NSGM: 0.04Er 3+ , under 375 nm light excitation, demonstrates the maximum relative sensitivity of 0.87%/K -1 at 298.15 K, spanning a wide temperature range from 298.15 to 488.15 K. Conversely, under 975 nm light excitation, NSGM: 0.04Er 3+ exhibits the maximum relative sensitivity of 0.63%/K -1 over the same temperature range, with temperature uncertainty ( δT ) less than 0.50 K and repeatability ( R ) (more than 98%). These findings position this material as a promising candidate for optical thermometer applications. The optical heating capacity of the synthesised phosphor is also determined using optical thermometry results, and heat generation up to approximately 457 K is found, indicating that NSGM: 0.04Er 3+ could be useful for photo-thermal therapy.