In this paper, Mn2+ and Ln3+ (Ln = Eu, Yb) co-doped MgGeO3 phosphors were prepared using a solid state reaction technique, and their optical properties were investigated. Mn2+-doped samples exhibit persistent luminescence in the red region, peaking at 677 nm, because of the 4T1 → 6A1 transition of the Mn2+ ions under ultraviolet (UV) excitation. Based on the charge transfer (CT) transition of Eu3+ and the band-gap energy, energy level diagrams with divalent lanthanide ground states relative to the conduction and valence band edges were constructed. ΔE(Ln), (Ln = Eu, Yb), which represents the energy gaps between the divalent lanthanide ground states and the bottom of the conduction band, were found to be 0.95 and 0.52 eV, respectively. Compared to a Mn2+ singly-doped sample, the thermoluminescence (TL) glow curves of the Mn2+–Eu3+ co-doped sample and the Mn2+–Yb3+ co-doped sample showed an additional TL glow peak at approximately 502 and 332 K with trap depths (Etrap) of 1.49 and 0.99 eV, respectively. The correspondence of Etrap with ΔE(Ln) suggests that Eu3+ and Yb3+ themselves work as electron traps in the MgGeO3:Mn2+ phosphors. We have also demonstrated that the Mn2+–Eu3+ co-doped material could be a good probe with photo-stimulated functions for long-term in vivo imaging owing to its deeper trap depth.

We investigated thermally activated ionization and thermally activated crossover as the two possibilities of quenching of 5d luminescence in Pr3+-doped Y3Al5-xGaxO12. Varying the Ga content x gives the control over the relative energy level location of the 5d and 4f2:PJ3 states of Pr3+ and the host conduction band (CB). Temperature-dependent luminescence lifetime measurements show that the 5d luminescence quenching temperature T50% increases up to x=2 and decreases with further increasing Ga content. This peculiar behavior is explained by a unique transition between the two quenching mechanisms which have an opposite dependence of thermal quenching on Ga content. For low Ga content, thermally activated crossover from the 4f5d state to the 4f2(PJ3) states is the operative quenching mechanism. With increasing Ga content, the activation energy for thermally activated crossover becomes larger, as derived from the configuration coordinate diagram, while from the vacuum referred binding energy diagram the activation energy of thermal ionization becomes smaller. Based on these results, we demonstrated that the thermal quenching of Pr3+:5d1-4f luminescence in Y3Al5-xGaxO12 with x=0,1,2 is a thermally activated crossover while for x=3,4,5 it results from the thermal ionization.

The Y₃Al_5−xGa_xO₁₂(YAGG):Ce³⁺-Cr³⁺ persistent phosphor is one of the materials in which Cr³⁺ ions act as electron traps. The possibility of electron traps by other transition metal ions (TM³⁺, TM = Sc, Ti, V, Cr, Fe) was investigated and those electron trap depth was compared with each other. In the thermoluminescence (TL) glow curves, the YAGG:Ce³⁺ samples co-doped with different TM³⁺ ions show different TL glow peak temperatures (i.e. different electron trap depth). The estimated vacuum referred binding energy of TM²⁺ from the trap depth shows a zig-zag curve, which is found to be originated from the different 3d electron energies affected by a number of d electron, nuclear charge and crystal field splitting. Utilizing the obtained TM²⁺ zig-zag curve, a new persistent phosphor of Y₃Al₂Ga₃O₁₂:Ce³⁺-Sc³⁺ and a photostimulated phosphor of Y₃Al₂Ga₃O₁₂:Ce³⁺-V³⁺ were successfully developed.

Y₃Al₅O₁₂(YAG):Ce³⁺ is the most widely applied phosphor in white LEDs (w-LEDs) because of strong blue absorption and efficient yellow luminescence combined with a high stability and thermal quenching temperature, required for the extreme operating conditions in high-power w-LEDs. The high luminescence quenching temperature (∼600 K) has been well established, but surprisingly, the mechanism for temperature quenching has not been elucidated yet. In this report we investigate the possibility of thermal ionization as a cause of this quenching process by measuring thermoluminescence (TL) excitation spectra at various temperatures. In the TL excitation (TLE) spectrum at room temperature there is no Ce³⁺:5d₁ band (the lowest excited 5d level). However, in the TLE spectrum at 573 K, which corresponds to the onset temperature of luminescence quenching, a TLE band due to the Ce³⁺:5d₁ excitation was observed at around 450 nm. On the basis of our observations we conclude that the luminescence quenching of YAG:Ce³⁺ at high temperatures is caused by the thermal ionization and not by the thermally activated cross over to the 4f ground state. The conclusion is confirmed by analysis of the positions of the 5d states of Ce³⁺ relative to the conduction band in the energy band diagram of YAG:Ce³⁺.