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 Mn4+ 2Eg -> (4)A(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 beta(1)=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 T-4(2) 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.