Abstract
The optical properties of Mn(5+) ions, which are responsible for the intense green-turquoise-blue coloration of Mn(5+)-based pigments and the near-infrared emission of phosphors, are the focus of this article. Mn(5+) 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. Mn(5+) 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) < (1)E < (1)A(1) < (3)T(2) < (3)T(1) progression in energy. We present the properties of several Mn(5+)-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) → (3)T(1)((3)F) absorption, which extends across the orange-red-deep red spectral region and is strongly influenced by crystal field strength. The narrow-band emission Mn(5+)-activated near-infrared phosphors arise from the spin-forbidden (1)E → (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 (1)E state energy and the nephelauxetic parameter β(1) using Racah parameter literature data for Mn(5+) phosphors. Lastly, we address the recent applications of these Mn(5+) phosphors in luminescence thermometry.