Achieving high-sensitivity dual-mode optical thermometry via phonon-assisted cross-relaxation in a double-perovskite structured up-conversion phosphor

High sensitivity is still a challenge for achieving optical thermometry applications. In this work, we effectively regulated the phonon energy of matrix crystals by substituting the A-site ions in the tellurate double perovskite up-conversion (UC) phosphors of ALaLiTeO6:5%Yb3+,0.2%Tm3+ with Ca2+, Sr2+, and Ba2+. On this basis, a dual-mode thermometer based on the fluorescence intensity ratio of FIR1 = I694/I649 and FIR2 = I694/I475 was constructed by utilizing significant differences in the response to temperature changes between 649 nm/475 nm emissions (from both Yb3+ and Tm3+) and 694 nm UC emission (from Yb3+ → Tm3+). As the temperature increases, involving the assistance of matrix crystal phonon energy, the collaborative action of two non-radiative transitions (Tm3+) and three cross-relaxation processes (Tm3+–Tm3+) can degrade the emission intensity at 475 and 649 nm, but enhance the UC emission intensity at 694 nm, thus effectively improving the thermometry sensitivity. Finally, we achieved the maximum thermometry sensitivity values of Sr = 4.69% K−1 and Sa = 10.05% K−1. These results provide new insights into exploring and constructing ultra-sensitive optical temperature sensors.