Li₂TiO₃:Mn⁴⁺ Deep-Red Phosphor for the Lifetime-Based Luminescence Thermometry

Luminescence of monoclinic lithium metatitanate (Li₂TiO₃) powders activated with different quantities of Mn⁴⁺ is studied in detail. Its strong deep-red emission arising from the Mn^{4+ 2}E_g → ⁴A_2g spin forbidden transition is centered at around 688 nm and is suitable for luminescence thermometry. Structural and electron paramagnetic resonance analyses show that Mn⁴⁺ ions are equally distributed in two almost identical Ti⁴⁺ sites in which they are octahedrally coordinated by six oxygen ions. Calculations based on the exchange charge model of the crystal field provided values of Racah parameters (B=760 cm⁻¹, C= 2993 cm⁻¹), crystal-field splitting Dq= 2043 cm⁻¹, and the nephelauxetic parameter β₁=0.9775. The maximal quantum efficiency of 24.1% at room temperature is found for 0.126% Mn⁴⁺ concentration. Temperature quenching of emission occurs by a cross-over via ⁴T₂ excited state of the Mn⁴⁺ ions with T_1/2=262 K and is quite favorable for the application in the lifetime-based luminescence thermometry since relative changes in emission decay values are exceptionally-large (around 3.21% at room temperature). We derived theoretical expressions for the temperature dependence of the absolute and relative sensitivities and discuss the influence of host material properties on lifetime sensitivities.