(Invited) Mn4+ r-Line Emission Intensity: The Ways of Its Enhancement

Abstract

Mn4+-based red phosphors are excellent materials to be used for making white LEDs with warm white light. They can be excited by the blue or ultraviolet light to produce red photoluminescence (PL), which is due to the parity and spin-forbidden 2Eg→4A2g transition. However, in many host materials, the intensity of the 2Eg→4A2g zero-phonon line (R-line) is very weak due to the presence of the inversion center of the crystallographic site occupied by the manganese ions. As a result, the red-shifted Stokes components dominate in the emission spectra of such hosts, but this leads to the decreased overlap of the PL spectra with human eye sensitivity curve and decreased intensity of the phosphor (lower lumen/Watt value). Therefore, finding the recipes of enhancement of the R-line intensity and production of brighter phosphors is an important scientific and technological task. In the present work we establish the key parameters responsible for the energy and the intensity of the Mn4+ R-line. Thorough consideration and analysis of the optical spectra for a large number of the Mn4+-based phosphors allowed to draw the following conclusions: The energetic position of the R-line is determined by the “Mn4+-ligand” bonding covalence. Most phosphors for the solid state lighting can be conditionally divided into fluorides and oxides (where the fluorine or oxygen ions are the Mn4+ nearest neighbors, respectively). In ionic fluorides, the Mn-F bonding covalence is weak, which shifts the R-line to higher energies, with the emission wavelength at about 620 nm. In covalent oxides, the stronger Mn-O bonding covalence lowers the energy of the 2Eg state, moving the emission to about 650 nm.In the host crystals, where the Mn4+ ions are located at the sites without inversion center, the R-line intensity is higher compared with the crystals possessing inversion symmetry at the Mn4+ Absence of the inversion center relaxes the parity section rule, which enhances the R-line intensity. As a result, the phosphor luminosity is increased considerably. The inversion center can be removed by cation co-doping, when different cations in the second coordination sphere around the Mn4+ ion remove the inversion center. As an example, a figure below shows the PL spectra of K2SiF6:Mn4+ and NaKSiF6:Mn4+. In the former case the ideal MnF6 octahedron with the inversion center does not allow for high R-line intensity. It the latter case structural disorder and mixed occupation at the K/Na site results in the presence of different cations in the second coordination sphere. As a result, the intensity of the R-line is increased by several times. Therefore, to get the phosphors with increased luminosity one has to focus on the host materials without inversion center at the dopant sites. If, however, the inversion symmetry exists, it should be broken by additional co-doping that would produce structural disorder around the dopant. Figure 1