Medium-entropy configuration boosting upconversion luminescence in sesquioxide for optical temperature sensing

Rare earth-doped sesquioxides have emerged as promising candidates for upconversion luminescence (UCL) materials, yet their practical applications remain constrained by suboptimal emission intensity. Herein, we propose an entropy engineering strategy to enhance UCL in sesquioxides by systematically introducing La³⁺ and Y³⁺ dopants into the cubic (Gd_0.78Yb_0.2)₂O₃: 0.02Er host, to form medium-entropy configurations. The configuration entropy serves as an effective strategy for enhancing the UCL intensity. Notably, the engineered medium-entropy sesquioxide preserves cubic phase integrity while achieving remarkable UCL enhancement factors of 5.80× for Er³⁺ and 6.53× for Ho³⁺ compared to conventional binary counterparts. Through spectroscopic analysis and Eu³⁺ probe characterization, we reveal that the medium-entropy configuration reduces the lattice symmetry of the matrix. This symmetry-breaking effect may alter the crystal field environment around Er³⁺/Ho³⁺ activators, promoting radiative transitions and thereby improving UCL intensity. Moreover, the medium-entropy sesquioxide exhibits pronounced temperature-dependent UCL behavior. Utilizing the non-thermally coupled levels of Er³⁺ ions, we achieve outstanding temperature sensing performance with a maximum absolute sensitivity of 91.97%/K and relative sensitivity of 0.78%/K at 313 K through the luminescence intensity ratio technique. This work presents an innovative entropy engineering strategy for enhancing UCL and opens new possibilities for designing high-performance optical thermometric materials.