Improved upconversion luminescence of NaBiF4: Tm3+/Yb3+/Al3+ as a ratio thermometer

NaBiF₄: 0.5 %Tm³⁺/20 %Yb³⁺/x%Al³⁺ upconversion luminescence materials were synthesized by co-precipitation method. The crystal structure, upconversion luminescence properties and temperature measurement properties systematically studied. Under 980 nm laser excitation, the characteristic transitions of Tm³⁺ were observed, corresponding to ¹G₄ → ³H₆ (475 nm), ¹G₄ → ³F₄ (650 nm), ³F₂, ₃ → ³H₆ (700 nm) and ³H₄ → ³H₆ (800 nm), respectively. Al³⁺ substitution significantly increases the upconversion luminescence intensity, and the fluorescence lifetime of ¹G₄ level shortens from 277.8 to 179.1 μs. An anomalous thermal enhancement behavior is observed. The optical thermometry properties of Tm³⁺ based on the non-thermally coupled energy levels ³F_{2, 3} and ³H₄ have been studied using the fluorescence intensity ratio technique. Relative sensitivity and absolute sensitivity show maximum values at 316 K, which are 1.1 % K⁻¹ and 0.21 % K⁻¹, respectively. The above results demonstrate that this material is a promising optical ratio thermometer.