Fig. 7

(a) Temperature-dependent upconversion emission spectra of MgAl₂O₄: 0.01Er³⁺, 0.05Yb³⁺ at different temperatures; (b) The relationship between the integrated emission peak intensities (I₅₂₄ and I₅₄₅) and temperatures; (c) The FIR ratio as a function of temperature; (d) Temperature dependence of sensitivities for the TCLs.