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Temperature-sensing media based on the fluorescence intensity ratio (FIR) of upconversion materials that suffer from low sensitivity owing to the small energy gap still have a need for new compounds with strong upconversion luminescence (UCL). In this work, a series of MSc2O4:Er3+/Yb3+ (M = Mg, Ca, Sr, and Ba) nanocrystals were prepared by a hydrothermal method using NaOH alkaline solution. The structure, morphology, and UCL characteristics of materials were investigated, and the red UCL of the CaSc2O4:Er3+/Yb3+ sample was dramatically enhanced by a factor of ∼12, ∼23, and ∼2000 compared with SrSc2O4, MgSc2O4, and BaSc2O4 samples, respectively. By adjusting alkali ions (Li+, Na+, K+), the UCL intensities of CaSc2O4:Er3+/Yb3+ and SrSc2O4:Er3+/Yb3+ samples were further improved, especially in the presence of Li+ ions. Excellent temperature-sensing behaviors are realized for CaSc2O4:Er3+/Yb3+ and SrSc2O4:Er3+/Yb3+ samples in the presence of Li+ ions, in which the maximum absolute sensitivity S A values are about 0.0041 and 0.0036 K–1 at 600 K and the corresponding relative sensitivity S R values are expressed as 1197/T2 and 1129/T2 (the current optimal S R = 1289/T2), respectively. The intense UCL and excellent S A and S R values indicate that CaSc2O4:Er3+/Yb3+ and SrSc2O4:Er3+/Yb3+ materials are promising candidates for application in high-temperature sensors working under 980 nm excitation.

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