Using the upconversion luminescence of the CaWO₄:Yb³⁺–X³⁺ (X = Er/Ho/Tm) phosphors for ratiometric thermal sensing

Based on the luminescence ratiometric method, here we systematically investigate the relative sensitivity (Sr) of three pairs of typical thermally coupled energy levels (TCELs), that are, the ²H_11/2/⁴S_3/2 levels of Er³⁺, the two sub-Stark ⁵F₅ levels of Ho³⁺, and the ³F₃/³H₄ levels of Tm³⁺, embedded in the CaWO₄ host. On the basis of the emission spectra of the CaWO₄:Yb³⁺–X³⁺ (X = Er/Ho/Tm) phosphors, the gaps that separate the ²H_11/2/⁴S_3/2, the two sub-Stark ⁵F₅, and the ³F₃/³H₄ TCELs are calculated to be 719, 402, and 1785 cm⁻¹, respectively. According to the expression Sr = ΔE/kT² (where ΔE is the energy gap, k is the Boltzmann constant and T is the absolute temperature), the maximum relative sensitivities of these three pairs of TCELs are expected to be 1.12, 0.63, and 2.79% K⁻¹ at room temperature 303 K. However, the practical relative sensitivities 1.03, 0.23, and 0.22% K⁻¹ are achieved at 303 K for these three pairs of TCELs accordingly. It is then demonstrated that not only at 303 K but over the whole experimental temperature range, the relative sensitivities of these three pairs of TCELs are confirmed to be overestimated to some degree. The work not only presents the practical relative sensitivities of these three pairs of TCELs, but also probably discloses a quite common phenomenon that the real relative sensitivity of TCELs is often overestimated. Therefore, it is expected to provide guidance for the practical applications of the luminescence ratiometric thermometry in the future.