Bi2O3 boosts brightness, biocompatibility and stability of Mn-doped Ba3(VO4)2 as NIR-II contrast agent

Pigments and Near-Infrared Phosphors Based on Mn5

The optical properties of Mn5+ ions, which are responsible for the intense green-turquoise-blue coloration of Mn5+-based pigments and the near-infrared emission of phosphors, are the focus of this article. Mn5+ ions enter crystalline matrices in four-fold coordinated positions and can maintain their 5+ valence state when crystalline hosts meet the conditions described in this work. Mn5+ ions have [Ar]3d2 electronic configuration and always experience a strong crystal field due to a high electric charge; therefore, their lower electronic states have the 3A2 < 1E < 1A1 < 3T2 < 3T1 progression in energy. We present the properties of several Mn5+-based pigments and discuss the electronic transitions responsible for their coloration. Specifically, we show that the color is determined by the spin-allowed 3A2 → 3T1(3F) absorption, which extends across the orange-red-deep red spectral region and is strongly influenced by crystal field strength. The narrow-band emission Mn5+-activated near-infrared phosphors arise from the spin-forbidden 1E → 3A2 transition, whose energy is independent of the crystal field strength and determined by the nephelauxetic effect. We demonstrate the linear relationship between 1E state energy and the nephelauxetic parameter β1 using Racah parameter literature data for Mn5+ phosphors. Lastly, we address the recent applications of these Mn5+ phosphors in luminescence thermometry.

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.