Significant enhancement of green upconversion emission and achieving record FIR sensitivity in double molybdates via in Ca2+-modified NaBi(MoO4)2

In view of the demand for high sensitivity in optical thermometry based on the fluorescence intensity ratio of thermally coupled levels of rare-earth ions, in this work Ca 2+ -modified double molybdate NaBi(MoO 4 ) 2 co-doped with Yb 3+ and Er 3+ was employed as the host to systematically investigate its upconversion (UC) luminescence behavior and optical temperature-sensing performance. Appropriate substitution of Ca 2+ induces lattice distortion and optimizes the local crystal field and energy-level matching, thereby significantly enhancing the energy transfer efficiency between the activator ions. As a result, the green UC emission intensity in the Ca 2+ -doped samples is markedly increased compared with that of the undoped host, and the green-to-red emission intensity ratio is enhanced by 1.7 times, reaching a value of 62, with the corresponding green color purity exceeding 96%. Meanwhile, Ca 2+ doping effectively tunes the thermally coupled levels of Er 3+ , leading to a 2.05-fold improvement in the absolute optical temperature sensing sensitivity relative to the undoped sample, with a maximum absolute sensitivity of 1.793 %·K −1 , which is much higher than that of other reported UC phosphors of similar type. Cyclic temperature sensing measurements confirm that Ca 2+ -modified Y 3+ ,Er 3+ :NaBi(MoO 4 ) 2 exhibits excellent repeatability in the temperature range of 300-460 K, indicating promising application prospects. This study provides a new design strategy and experimental basis for constructing high-performance double molybdate materials that integrate strong UC luminescence with highly sensitive optical thermometry.