Enhancement of ⁴T₂ → ⁴A₂ with temperature via energy transfer of the optical thermometer Mg₅Al₇O₁₆:Cr³⁺

The luminescence properties and temperature sensing performances of Mg₅Al₇O₁₆:Cr³⁺ are systematically studied in this paper, synthesized using the solid-state sintering method. Based on X-ray diffraction and first-principles calculations, Cr³⁺ is verified to occupy the octahedral Mg²⁺ and Al³⁺ sites in the host Mg₅Al₇O₁₆. From the Tanable-Sugano diagram, Cr³⁺ is in a strong crystal environment to induce sharp line emissions of Cr³⁺ around 700 nm. As the temperature rises, the excitation and emission ranging from 303 to 543 K are recorded, and overlapping of emissions from ²E → ⁴A₂ and ⁴T₂ → ⁴A₂ is observed. The emissions in the 303-543 K range are separated using deconvolution. The emission from ²E → ⁴A₂ decreases with increasing temperature, which is a normal thermal quenching behavior. However, the emission from ⁴T₂ → ⁴A₂ increases with temperature until 523 K. Energy transfer through phonon assistance from ²E to ⁴T₂ is verified using temperature-dependent excitation and decay curves. It is used for the temperature-reading method based on the luminescence intensity ratio (LIR). The highest relative sensitivity is 1.80% K⁻¹ at 303 K and is retained at above 1.00% K⁻¹ in the 303-403 K range.