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

Abstract

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 → 4A2g 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 T1/2=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.

Quantitative Determination of the Effective Mn4+ Concentration in a Li2TiO3:Mn4+ Phosphor and Its Effect on the Photoluminescence Efficiency of Deep Red Emission

“…Dramićanin et al reported an investigation of the detailed luminescence properties of Li 2 TiO 3 :Mn 4+ . 28 According to their report, the 2 E → 4 A 2 PL emission is located at 688 nm and the PLE bands due to the 4 A 2 → 4 T 1 and 4 A 2 → 4 T 2 transitions are located at approximately 28 000 (∼350 nm) and 18 000–20 000 cm –1 (500–550 nm), respectively. The PL and PLE positions in the LTO:Mn phosphors in this study were similar to those reported in LTO:Mn phosphors.…”

A Theoretical Framework for Ratiometric Single Ion Luminescent Thermometers—Thermodynamic and Kinetic Guidelines for Optimized Performance

“…If those states couple more strongly to local odd‐parity vibrational modes or phonons, the transition can become vibronically allowed. A very prominent example for that is the excited 2 E state (in cubically symmetric fields) of the 3d 3 configuration like in Mn 4+ or Cr 3+ , [ 64–77 ] but also the lowest excited (dominantly) triplet states 3 T 2u and 3 E u of the 4f 13 5d 1 configuration of Yb 2+ . [ 189 ]…”

“…[ 258,259 ] The temperature dependence is the foundation of the lifetime‐based thermometric performance of Mn 4+ ‐ or Cr 3+ ‐activated phosphors at cryogenic temperatures due to the stronger electron‐phonon coupling in d orbitals. [ 63,70,72–75 ] Since coth( x ) → 1 for x ≫ 1 (), while coth( x ) ≫ 1 for x ≪ 1 (), the radiative decay rate becomes faster at low temperatures already.…”

Making Nd3+ a Sensitive Luminescent Thermometer for Physiological Temperatures—An Account of Pitfalls in Boltzmann Thermometry

“…Among the various temperature-dependent optical parameters that can be used for temperature calibration, the luminescence intensity ratio (LIR) of two emissive transitions from thermally coupled excited states has emerged as an especially robust and yet easily measurable representative to extract information about the local temperature of a medium in contact with luminescent nanocrystals. Also, other methodological approaches such as lifetime thermometry [15][16][17][18][19][20][21], or the concept of excited state absorption (ESA) [22][23][24] have been developed for that purpose while organic framework-based thermometers increasingly attract attention [25,26].…”