Tuneable persistent luminescence of novel Mg$_3$Y$_2$Ge$_3$O$_{12}$ garnet

In this work novel garnet persistent luminescence phosphor Mg$_3$Y$_2$Ge$_3$O$_{12}$:Tb$^{3+}$ (MYG:Tb$^{3+}$) with bright blue to green tuneable emission is reported.MYG:Tb$^{3+}$ samples with Tb$^{3+}$ content ranging from 0% to 100% were analysed. X-ray absorption spectroscopy analysis showed that Tb$^{3+}$ ions incorporate in the garnet structure by replacing Y$^{3+}$ ions. Optimal dopant content was determined to achieve bright afterglow lasting more than ten hours. Tuneability of persistent luminescence was attributed to the deviations in populations of $^5$D$_4$ and $^5$D$_3$ emitting states of Tb$^{3+}$ due to multiphonon relaxation and cross-relaxation processes.Photoluminescence, thermostimulated luminescence (TSL) and electron paramagnetic resonance (EPR) methods were used to determine the dominant persistent luminescence processes in the investigated material. The analysis of intrinsic defects in the material using EPR spectroscopy showed the presence of three distinct paramagnetic V-type centres and at least one F-type centre. The role of Tb$^{3+}$ as a charge trap was discussed. MYG:Tb$^{3+}$ exhibited luminescence tuneability by doping and temperature, which is promising for advanced anti-counterfeiting applications