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

•Improved up-conversion luminescence of NaBiF4: Tm3+, Yb3+ is realized by Al3+ substitution.•Lattice shrink caused by the introduction of Al3+ is responsible for the enhancement.•The optimized sample exhibits excellent temperature sensing characteristics from 301 K to 501 K. 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 3F2, 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.