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

<jats:title>Abstract</jats:title><jats:p>Luminescence of monoclinic lithium metatitanate (Li<jats:sub>2</jats:sub>TiO<jats:sub>3</jats:sub>) powders activated with different quantities of Mn<jats:sup>4+</jats:sup> is studied in detail. Its strong deep‐red emission arising from the Mn<jats:sup>4+ 2</jats:sup>E<jats:sub>g</jats:sub> → <jats:sup>4</jats:sup>A<jats:sub>2g</jats:sub> spin forbidden transition is centered at around 688 nm and is suitable for luminescence thermometry. Structural and electron paramagnetic resonance analyses show that Mn<jats:sup>4+</jats:sup> ions are equally distributed in two almost identical Ti<jats:sup>4+</jats:sup> 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<jats:sup>−1</jats:sup>, C= 2993 cm<jats:sup>−1</jats:sup>), crystal‐field splitting Dq= 2043 cm<jats:sup>−1</jats:sup>, and the nephelauxetic parameter β<jats:sub>1</jats:sub>=0.9775. The maximal quantum efficiency of 24.1% at room temperature is found for 0.126% Mn<jats:sup>4+</jats:sup> concentration. Temperature quenching of emission occurs by a cross‐over via <jats:sup>4</jats:sup>T<jats:sub>2</jats:sub> excited state of the Mn<jats:sup>4+</jats:sup> ions with <jats:italic>T</jats:italic><jats:sub>1/2</jats:sub>=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.</jats:p>