Ratiometric Thermometry Using Thermochromic Tb³⁺:Mn⁴⁺:Na₄Mg(WO₄)₃ Phosphors

The utility of Tb³⁺:Mn⁴⁺:Na₄Mg(WO₄)₃ phosphors as ratiometric luminescent thermometers in the 200-450 K temperature range is demonstrated. Targeted substitution of Tb³⁺ for Na⁺ and Mn⁴⁺ for Mg²⁺ yields phosphors of formula Na_4-12Tb₄Mg_1-2Mn(WO₄)₃. UV excitation of the Na₄Mg(WO₄)₃ host results in green emission from terbium (544 nm) and red emission from manganese (682 nm). Differential thermal quenching of these emissions renders Na_3.976Tb_0.008Mg_0.990Mn_0.005(WO₄)₃ thermochromic between 78 and 450 K, with an orange-to-green color change that is particularly noticeable above 200 K. As a result, this phosphor serves as a ratiometric and colorimetric thermometer between 200 and 450 K. Quantitative assessment of its performance yields a maximum relative sensitivity of 2.5 × 10^-2 K^-1 at 375 K, along with a resolution of 0.5 K and a repeatability of 98%. These findings highlight the versatility of Na₄Mg(WO₄)₃ as a platform to realize luminescent thermometers via targeted aliovalent substitutions.