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]3d(2) electronic configuration and always experience a strong crystal field due to a high electric charge; therefore, their lower electronic states have the (3)A(2) < E-1 < (1)A(1) < T-3(2) < T-3(1) 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 (3)A(2) -> T-3(1)(F-3) 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 E-1 -> (3)A(2) transition, whose energy is independent of the crystal field strength and determined by the nephelauxetic effect. We demonstrate the linear relationship between E-1 state energy and the nephelauxetic parameter beta(1) using Racah parameter literature data for Mn5+ phosphors. Lastly, we address the recent applications of these Mn5+ phosphors in luminescence thermometry.