Enhanced photoluminescence and thermal properties due to size mismatch in Mg2TixGe1−xO4:Mn4+ deep-red phosphors
In this study, a warm red phosphor based on the substitution of Ge4+ for Ti4+ in the lattice of Mg2TixGe1−xO4:Mn4+ was prepared by a solid-state reaction route. Ge4+ acts to distort the structure of Mg2TiO4 and enhances the photoluminescence, thermal stability, quantum yield and lifetime decay significantly. The sample with the replacement of Ti by 0.35 mol of Ge, i.e., Mg2Ti0.65Ge0.35O4:Mn4+, shows efficient excitation and emission; its excitation and photoluminescence intensities are over 2.5 and 2.7 times higher than those of the Mg2TiO4:Mn4+ phosphor and its CIE coordinates moved to a warm color. Moreover, Mg2Ti0.65Ge0.35O4:Mn4+ shows better thermal stability and higher quantum yield. The integrated PL intensity at 150 °C is still 80.2% of that measured at room temperature under 330 nm light excitation. The internal quantum yield of Mg2Ti0.65Ge0.35O4:Mn4+ was 77.8%, while that of the sample without Ge4+ was 32.3%. The underlying mechanisms behind the optical changes on adjusting the cation composition of the phosphors are revealed, and the crystal field strength and the Racah parameters are estimated for Mn4+ in the Mg2TiO4:Mn4+ lattice. The luminescence mechanism was explained by the Tanabe–Sugano energy level diagram of the Mn4+ ion. These results imply that Mn4+-doped Mg2Ti0.65Ge0.35O4:Mn4+ is a promising candidate as a warm red phosphor for near-UV and blue light emitting diodes.