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 E-2(g)->(4)A(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 E-2(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.