On the Mn⁴⁺ R-line emission intensity and its tunability in solids

The spectroscopic properties of Mn⁴⁺ ions in solids are analyzed to establish key parameters that are responsible for the energy and the intensity of the ²E_g→⁴A_2g zero-phonon emission transition (R-line). These two factors of the emission spectrum are critical in determining the phosphor luminosity or brightness. The emission energy is chiefly determined by the “Mn⁴⁺-ligand” bonding covalence. In fluorides, the Mn-F bonding covalence is weak, and the R-line occur at higher energy. In oxides, the stronger Mn-O bonding covalence reduces the energy of the ²E_g state. In oxides, the R-line shifts to higher energy with increasing local distortion of the MnO₆ octahedral moiety. Those host crystals that locate the Mn⁴⁺ ion at a noncentrosymmetric site (absence of inversion symmetry) relax the parity section rule, which enhances the R-line intensity. Increased R-line intensity in the emission spectrum produces color changes that increases phosphor luminosity. The influence of second coordination sphere (next-nearest neighbors) on the R-line intensity is considered. This article establishes guidelines for the development of host crystals that meet the requirements of high luminosity phosphors for general illumination applications.