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

The near-infrared luminescence of Ca6Ba(PO4)(4)O: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 E-1 -> (3)A(2) transition with a 37.5% internal quantum efficiency and an excited-state lifetime of about 350 ps. We derived the crystal field and Racah parameters and calculated the appropriate Tanabe-Sugano diagram for this phosphor. We found that E-1 emission quenches due to the thermally-assisted cross-over with the T-3(2) state and that the relatively high Debye temperature of 783 K of Ca6Ba(PO4)(4)O facilitates efficient emission. Since Ca6Ba(PO4)(4)O 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.

Automated summary

The near-infrared luminescence of Ca6Ba(PO4)(4)O:Mn5+ is studied and explained in this paper. The phosphor exhibits a narrow emission band and high internal quantum efficiency. The crystal field and Racah parameters are derived, and the appropriate Tanabe-Sugano diagram is calculated to explain the luminescence mechanism. Additionally, the electronic band structure, density of states, and vibrational spectra of the Eu2+ dopant in Ca6Ba(PO4)(4)O are investigated. Finally, the phosphor is applied in luminescence intensity ratio thermometry, showing potential for physiological temperature measurements.