Bandshift Luminescence Thermometry Using Mn⁴⁺:Na₄Mg(WO₄)₃ Phosphors

The feasibility of employing Mn⁴⁺-activated phosphors for bandshift luminesence thermometry is demonstrated for the first time using Mn⁴⁺:Na₄Mg­(WO₄)₃. Substitution of Mn⁴⁺ for Mg²⁺ in Na₄Mg­(WO₄)₃ yields phosphors in which Mn⁴⁺ activators are distributed over (Na/Mg)₂O₁₀ dimers. Neutron powder diffraction reveals that metal–oxygen bond distances in these dimers are significantly longer than those typically observed in Mn⁴⁺-activated phosphors. Multisite distribution of Mn⁴⁺ activators coupled to long metal–oxygen distances impart thermometric functionality to Mn⁴⁺:Na₄Mg­(WO₄)₃. Red emission from two distinct Mn⁴⁺ emitters leads to a broad band extending from 560 to 850 nm. Differential thermal quenching of these emitters drives a sigmoidal blueshift of the emission maximum upon increasing temperature from 100 to 400 K (ca. 30 nm); bandshift luminescence thermometry is thus realized. Quantitative assessment of the thermometric performance of Na₄Mg_0.940Mn_0.030(WO₄)₃ yields a sensitivity of 0.127 nm K^–1, a repeatability greater than 99%, and a temperature resolution of 2.5 K at 300 K. Findings presented in this article expand the scope of applicability of Mn⁴⁺-activated thermosensitive phosphors beyond ratiometric bandshape and lifetime thermometry. More importantly from the perspective of developing new phosphors, our results provide structural guidelines for discovering and screening novel oxide hosts for Mn⁴⁺ activators.