Mn 5+ -activated Ca 6 Ba(PO 4 ) 4 O near-infrared phosphor and its application in luminescence thermometry.

The near-infrared luminescence of Ca 6 Ba(PO 4 ) 4 O:Mn 5+ 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 1 E →  3 A 2 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 1 E emission quenches due to the thermally-assisted cross-over with the 3 T 2 state and that the relatively high Debye temperature of 783 K of Ca 6 Ba(PO 4 ) 4 O facilitates efficient emission. Since Ca 6 Ba(PO 4 ) 4 O also provides efficient yellow emission of the Eu 2+ 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.