NaGd(MoO 4 ) 2 :Er 3+ /Tm 3+ Upconversion Phosphors: Tunable Energy Migration Under 1550 nm Excitation for Ratiometric Temperature Sensing

ABSTRACT Lanthanide‐ion‐doped upconversion phosphors for temperature sensing continue to attract considerable fundamental and practical interests. In this work, a NaGd(MoO 4 ) 2 :Er 3+ ,Tm 3+ phosphor, characterized by efficient electron storage at the 3 F 4 (Tm 3+ ) and 4 I 11/2 (Er 3+ ) levels, is designed to reconfigure energy migration pathways under 1550 nm excitation. The results demonstrate that a competitive interplay between backward energy transfer (BET) and forward energy transfer (FET) governs energy migration. The observed upconversion luminescence (UCL) exhibits intense 795 nm near‐infrared emission attributed to the Tm 3+ : 3 H 4 → 3 H 6 transition, which is further significantly enhanced upon heating due to phonon‐assisted energy transfer and thermally induced lattice expansion that facilitate population of the Tm 3+ : 3 H 4 level, leading to a 2.41‐fold enhancement, while the Er 3+ red emission ( 4 F 9/2 → 4 I 15/2 ) undergoes conventional thermal quenching. Leveraging these contrasting thermal quenching behaviors and a high‐thermal‐conductivity phosphor‐on‐ceramic architecture, reliable UCL‐based temperature sensing is demonstrated. This study highlights the controllable nature of excitation energy migration in Er 3+ /Tm 3+ co‐doped systems and provides valuable design principles for high‐performance upconversion thermometry.