Mn⁵⁺-activated Ca₆Ba(PO₄)₄O near-infrared phosphor and its application in luminescence thermometry

The near-infrared luminescence of Ca₆Ba(PO₄)₄O:Mn⁵⁺ is demonstrated and explained. When excited into the broad and strong absorption band that spans the 500–1000 nm spectral range, this phosphor provides an ultranarrow (FWHM = 5 nm) emission centered at 1140 nm that originates from a spin-forbidden ¹E → ³A₂ transition with a 37.5% internal quantum efficiency and an excited-state lifetime of about 350 μs. We derived the crystal field and Racah parameters and calculated the appropriate Tanabe–Sugano diagram for this phosphor. We found that ¹E emission quenches due to the thermally-assisted cross-over with the ³T₂ state and that the relatively high Debye temperature of 783 K of Ca₆Ba(PO₄)₄O facilitates efficient emission. Since Ca₆Ba(PO₄)₄O also provides efficient yellow emission of the Eu²⁺ dopant, we calculated and explained its electronic band structure, the partial and total density of states, effective Mulliken charges of all ions, elastic constants, Debye temperature, and vibrational spectra. Finally, we demonstrated the application of phosphor in a luminescence intensity ratio thermometry and obtained a relative sensitivity of 1.92%K⁻¹ and a temperature resolution of 0.2 K in the range of physiological temperatures.