Enhanced photoluminescence and thermal properties due to size mismatch in Mg2TixGe1−xO4:Mn4+ deep-red phosphors

Abstract

In this study, a warm red phosphor based on the substitution of Ge⁴⁺ for Ti⁴⁺ in the lattice of Mg₂Ti_xGe_1−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., Mg₂Ti_0.65Ge_0.35O4: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, Mg₂Ti_0.65Ge_0.35O₄: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 Mg₂Ti_0.65Ge_0.35O₄: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 Mg₂Ti_0.65Ge_0.35O4:Mn⁴⁺ is a promising candidate as a warm red phosphor for near-UV and blue light emitting diodes.