<p>Here, we study and compare the luminescent properties and thermal sensitivity of calcium nanofluorides (CaF₂) doped with trivalent erbium ions, CaF₂:xEr³⁺, where x is 1.0 or 2.0&#xa0;mol%. The samples were synthesized via the hydrothermal and coprecipitation routes. X-ray diffraction and scanning electron microscopy techniques were used for structural and morphological analysis. The results show the formation of the face-centered cubic crystalline structure of CaF₂, where, through the Rietveld refinement method, the average crystallite size is estimated to be around 70 and 90&#xa0;nm for samples prepared via coprecipitation and hydrothermal routes, respectively. In terms of morphology, a surface with little roughness and porosity, but not uniform, was found for all samples. For the luminescence measurements under excitation wavelength at 980&#xa0;nm, upconversion emissions from the ²H_11/2, ⁴S_3/2, and ⁴F_9/2 energy levels of Er³⁺ were observed in NPs prepared by coprecipitation. The intensity of emissions is lower for the sample with a higher concentration of Er³⁺ ions, which could be attributed to the phenomenon of luminescence concentration quenching. For optical thermometry and relative thermal sensitivity (S_r) measurements, the luminescence intensity ratio technique of thermally coupled levels was used. For nanoparticles with x = 1.0% and 2.0% of Er³⁺, a maximum Sr of 1.11% and 1.13% was found for the samples prepared by coprecipitation and the hydrothermal routes, respectively.</p>